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The Southwest Technology Development Institute web site maintains a PV Systems Inspector/Installer Checklist and all copies of the previous “Perspectives on PV” articles for easy downloading. A color copy of the latest version (1.93) of the 150-page, Photovoltaic Power Systems and the 2005 National Electrical Code: Suggested Practices, written by the author, may be downloaded from this web site: http://www.nmsu.edu/~tdi/Photovoltaics/Codes-Stds/Codes-Stds.html

 

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Ungrounded Electrical Systems! Ungrounded photovoltaic (PV) systems? What is the world coming to?

Posted By John Wiles, Wednesday, September 01, 2010
Updated: Friday, January 18, 2013

A Little History

Actually the United States is catching up to the rest of the world, which has, for the most part, been using ungrounded electrical systems for aslong as the U. S. has been using grounded electricalsystems. More than 100 years ago, the debate on grounded vs. ungrounded electrical systems began and the U. S. went grounded while many other countries went ungrounded. When we discuss grounded vs. ungrounded electrical systems, we are addressing whether one of the circuit conductors, like our ac neutral conductor, is grounded or not. Except for ungrounded three-phase delta-connected transmission and distribution systems, most of our electrical systems in the U. S. have a grounded circuit conductor. In Europe and elsewhere, ungrounded electrical systems are common and, in fact, in Germany, ungrounded three-phase ac power at 230 volts comes directly into the dwellings.

No Grounding

No grounding

To some extent, most electrical systems in the developed counties use a system of equipment-grounding conductors, called protective earth (PE) in Europe, to provide an outer layer of defense against electrical shocks from exposed conductive surfaces that could become energized. Of course, as in the U. S., double-insulated appliances and tools can be found that do not require an equipment-grounding system.

No Transformers

No transformers

There will be no attempt in this article to further the ages-old debate of the safety of ungrounded vs. grounded electrical systems. Given the history, equipment, training, and experiences on both sides of the issue, it appears that either system can provide equal levels of safety. As the world grows smaller, IEC standards in Europe are being harmonized with the standards developed by Underwriters Laboratories (UL) here in the U. S. and the codes are slowly adopting similar requirements and allowances.

Impact on PV System Design

Since the U. S. uses grounded electrical systems, PV systems installed in the U. S. have been required to have a grounded circuit conductor since 1984 when PV requirements first appeared in theNational Electrical Code (NEC).From the beginning, PV systems with a maximum systems voltage of 50 volts or below have not required a grounded circuit conductor and inNEC-2005, Section 690.35 was added to theCodeto permit the use of ungrounded PV arrays with few voltage restrictions.

Photo 1. A 9 kW transformerless inverter by SMA Solar Technologies AG

Photo 1. A 9 kW transformerless inverter by SMA Solar Technologies AG

In utility-interactive PV systems, the inverter can be greatly simplified to a conceptual switching device and a filter with other added control components. Of course, how the utility-interactive inverter actually works is far more complex. The switch reverses the polarity of the dc output from the PV array 120 times per second to generate a 60 Hz waveform that is shaped into a sine wave by the filter. In Europe, they use 100 switches per second to get 50 Hz. Because the European PV arrays and the electrical system are ungrounded, the PV utility-interactive inverter can be relatively simple compared to what is required in the United States. In the U. S., with a grounded circuit conductor from the PV array and a grounded circuit conductor in the ac inverter output circuit, it is not possible to use a direct switching device because the switch would be shorted as it tried to reverse the polarity of the dc circuit into an ac signal. A transformer is required in inverters used in the U. S. to isolate the grounded dc circuits from the grounded ac circuits. The transformer is usually a heavy, costly, and bulky device that decreases efficiency, increases the size, and increases the shipping costs of the inverter.

U. S. inverter manufacturers and inverter manufacturers in the rest of the world can now sell transformerless inverters in the U. S. Those inverters must be used with an ungrounded PV array, and theNECallows such ungrounded PV arrays (see 690.35). Several inverters are on the market now (see photos 1, 2, and 3). What are these systems going to look like to the PV installer and the inspector?

The Ungrounded PV System

Photo 2. A 5 kW transformerless inverter by Power One

Photo 2. A 5 kW transformerless inverter by Power One

These ungrounded systems are not going to be significantly different from the PV systems that we have been installing and inspecting for many years. They will continue to have a system of equipment-grounding conductors that will connect the module frames, racks, enclosures of combiners, disconnects and inverters together and to ground (earth in Euro-speak).

According to NEC 690.35(B), dc overcurrent protection (when required for three strings of modules or more) will be required in both of the now-ungrounded circuit conductors. PV source circuit combiners for multiple strings of modules will have overcurrent protection in both the positive and negative dc inputs from each string of modules.

The PV dc disconnecting means will be required in both of the ungrounded conductors [690.35(A)]. With disconnects required in each ungrounded circuit conductor, external and internal disconnects will have a switch pole in each of the conductors coming from the PV array.

Ampacity calculations will be the same for grounded and ungrounded systems, and the calculations for maximum system voltage will be the same.

The color code of white for a grounded conductorwill no longer be used; and it is logical that the color code of red for a positive conductor and black for a negative conductor be used, but there is noCoderequirement that these colors be used. As before, the module interconnecting cable and other short-runs of exposed single conductor cables will usually have black insulation (for superior UV resistance) with colored markings used for identification. As an exercise, look at photo 4 and determine what sort of system is shown.

Photo 2. A 5 kW transformerless inverter by Power One

Photo 3. A 3.6 kW transformerless inverter by Power One

All exposed single-conductor cables including those attached directly to the module must be the new PV Wire or PV Cable made and listed to UL Standard 4703 [690.35(D)(3)]. Installers and inspectors should be aware that some of the European PV Cables, PV Wires or other cables with similar names made for the European market (and even made to UL Standard 4703) may use fine-stranded, flexible conductors and it will be difficult to obtain lugs and terminals suitable for use with these cables where they transition to a conduit wiring method. (See NEC 690. 31(F) and "Perspectives on PV” in the January/February 2005, IAEI News).

The inverter must be listed and clearly marked for use with ungrounded PV arrays, and it must have an appropriate internal ground-fault detection and indication system [690.35(C)]. That circuit will not be required to interrupt the ground-fault current (as is required on grounded PV arrays) because on the first ground fault on an ungrounded system, there will be no ground-fault currents. The inverter or charge controller will be required to shut down and indicate that a ground fault has occurred.

Summary

Ungrounded PV arrays, permitted by theNEC, will allow the use of the new transformerless inverters. Color codes will no longer require the white conductor. Disconnects will have poles in both the negative and positive conductors; and overcurrent devices, where required, will be in both conductors too.

Photo 4. Is it an ungrounded PV source circuit or an improperly color-coded grounded source circuit?

Photo 4. Is it an ungrounded PV source circuit or an improperly color-coded grounded source circuit?

For Additional Information

If this article has raised questions, do not hesitate to contact the author by phone or e-mail. E-mail: jwiles@nmsu.edu Phone: 575-646-6105

See the web site below for a schedule of presentations on PV and theCode.

A color copy of the latest version (1.91) of the 150-page,Photovoltaic Power Systems and the 2005 National Electrical Code: Suggested Practices, written by the author, may be downloaded from this web site:http://www.nmsu.edu/~tdi/Photovoltaics/Codes-Stds/Codes-Stds.html

The Southwest Technology Development Institute web site maintains a PV Systems Inspector/Installer Checklist and all copies of the previous "Perspectives on PV” articles for easy downloading. Copies of "Code Corner” written by the author and published inHome Power Magazineover the last 10 years are also available on this web site:http://www.nmsu.edu/~tdi/Photovoltaics/Codes-Stds/Codes-Stds.html

The author makes 6–8 hour presentations on "PV Systems and theNEC” to groups of 60 or more inspectors, electricians, electrical contractors, and PV professionals for a very nominal cost on an as-requested basis. A schedule of future presentations can be found on the IEE/SWTDI web site.


An Update on Microinverters and AC PV Modules

As the microinverters, combinations of microinverters attached to PV modules, and the AC PV modules come to market, there will be and already has been some confusion about the code requirements for various products.

Both microinverters and microinverters attached to PV modules in the field or in the factory that have any exposed dc single conductor cables are required to meet all of the dc wiring requirements in theNEC. These may include 690.5 ground-fault detector requirements, dc and ac disconnect requirements (potentially handled by connectors listed as disconnects), and inverter dc grounding electrode requirements. Confusion arises when these are calledAC modules. They are not AC PV modules.

TrueAC PV modules, as defined inNEC690.2 and 690.6, have a module and inverter assembled as one environmentally protected unit in the factory, and there is no accessible dc wiring. None of the dc wiring requirements in theCodeapplies, because there is no dc wiring outside the listed unit. A single equipment-grounding connection will usually be the only requirement to properly ground the combined module/inverter assembly.


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Tags:  Featured  Perspectives on PV  September-October 2010 

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Odds and Ends

Posted By John Wiles, Thursday, July 01, 2010
Updated: Friday, January 18, 2013

In the course of daily business, I get some questions repeated many times. I try to address these areas of common and frequent interest in this series of articles, but there are always a few that need clarification or repeating.

Inverter DC Grounding Electrode Conductor

In the "Perspectives on PV” in the September-October 2009 IAEI News, we covered 690.47(C) in both the 2005 and the 2008 NEC and discussed that since this section is permissive in bothCodes, that either the 2008 or 2005 requirements may be applied in jurisdictions using either edition of theNEC. It should be clarified that the combined conductor permitted by 690.47(C) inNEC-2008 originates at the inverter and runs to the first grounding bar in a panel where a grounding electrode conductor (connected to a grounding electrode) is attached. It should be noted that this combined dc inverter grounding electrode conductor/ac inverter equipment-grounding conductor does not originate at the PV array. The PV array is normally grounded with an equipment grounding conductor routed with the dc circuit conductors per 690.45. Additional grounding of the PV array may be required by 690.47(D) when the array is ground mounted or mounted on a separate structure from the PV inverter.

Main AC Service Disconnect Ground-Fault Protection

NEC 230.95 requires that solidly grounded wye services with a line-to-ground voltage of 150 to 600 volts be provided with ground-fault protection. This protection is generally provided by a main disconnect consisting of a circuit breaker with an attached or included ground-fault protection device (GFPD). How should the PV designer, installer or inspector proceed where a utility-interactive PV system connection could backfeed this GFPD breaker? The answer: With a great deal of caution.

First, we need to know about one of those hidden-meanings, UL Standards that says if a circuit breaker is not marked "line” and "load,” it has been evaluated for current/power flow in both directions and is suitable for backfeeding. Most of the newer, smaller molded-case circuit breakers that we deal with are not marked "line” and "load” and are suitable for backfeeding. However, in retro-fit situations we may be dealing with main disconnect circuit breakers that are 40 or 50 years old or more and may have "line” and "load” markings. With those markings, the breaker should not be backfed.

Photo 1. GFP Breaker. To feed back or not.

Photo 1. GFP Breaker. To feed back or not.

Let’s assume that we have a main disconnect breaker that is suitable for backfeeding and it is also equipped with a GFPD as required by NEC-2008 and earlier Codes. Discussions with engineers at UL and with the circuit breaker manufacturers reveal that the GFPD may not have been tested for backfeeding in a method that duplicates the utility-interactive PV situation. When a ground fault trips a GFPD breaker that is being backfed by a PV inverter, both the line and load terminals may be energized at the same time for up to 2 seconds as the inverter shuts down. Many older GFPD devices could be damaged when this happens. Some of the newer GFPD breakers are not susceptible to this kind of damage, but no one seems to have a good universal answer to all GFPD breakers in all installations. So, the first hurdle is to get the design engineer at the breaker/GFPD manufacturers to provide written statements that the GFPD device will not be damaged when tripped while being backfed by a utility-interactive inverter.

The second hurdle is posed by meeting the Exception to 690.64(B)(3). How are the load circuits protected from ground-fault currents from the inverter? An analysis of the various impedances involved (inverter output source circuits vs. utility source circuits) to determine how currents would be shared between the inverter and the utility would not be simple. It may be possible that the inverter can source sufficient fault currents so that the GFPD does not trip. Then there is the fact that the GFPD has adjustable trip points, and the NEC provides no guidance on how they should be set in a non-PV installation, let alone in a PV installation. When the adjustment ranges over several hundred amps on a 1000-amp GFPD amp breaker, it is not clear how this adjustment should be made. Then if we try to put a GFPD on the output of the inverter, there is a question of how it should be connected and would it provide the desired protection?

At this point, I feel that when the existing installation has a main breaker or any breaker (or any fused disconnect) with a GFPD function, then that device should not have a utility-interactive inverter attached to any circuits that feed the load terminals of the GFPD. Supply side connections [690.64(A)] are the way to make these PV installations and avoid the issues until they are resolved.

690.64(B) All the Way

The "Perspectives on PV” in the November-December 2009 issue of the IAEI News dealt with supply side connections and the article assumed hope for the future code in this area. Unfortunately, NEC 690.64(B) and 705.12(D) will be with us for a long time since it appears that proposed changes for NEC-2011 were rejected. This code requirement applies to any bus bar or conductor that has multiple sources of supply (utility and PV inverter outputs) with each supply protected by an overcurrent device (fuse or circuit breaker). Load breakers are not counted in this requirement. In a typical utility interactive PV system, the requirement would apply to all busbars and conductors from the service disconnect (breaker or fused disconnect) to the first dedicated overcurrent device/disconnect on the inverter output circuit. Although the number of subpanels and conductors between the service disconnect and the PV inverter output may be numerous, and the load on the building large compared with the rating of the PV system, there is always the possibility that any conductor in this path may be subjected to backfeed currents from the PV system. Each of those panel busbars and conductors between them must be sized to meet the requirements of 690.64(B) / 705.12(D).

If the PV inverter output connection cannot be made at the very last breaker position in the most distant panel from the service disconnect as required by 690.64(B)(7), then the calculations for ampacity and busbars become more complex. Without this opposite breaker configuration, it may be possible to overload portions of the busbar or some conductors with current from both the utility and the PV system. The 120% allowance in 690.64(B)(2) cannot be applied, nor can just that first dedicated breaker connected to the PV inverter output be used in the calculations for each conductor and busbar. The designer/installer/inspector must look at each panel busbar and each conductor segment and determine which breakers are limiting current to that specific bus bar or conductor. These areusually the main breaker on the panel and the single backfed breaker in that particular panel that is handling backfed current from the possibly distant PV inverter. It is not the dedicated breaker connected directly to the inverter. Unfortunately, we have lost the 120% allowance and frequently a main breaker and the panel rating are the same. Therefore it is not possible to have breaker carrying backfed PV currents connected to this panel or conductor.

In some cases the main breaker for a panel may be reduced below the rating of the bus bar, and this can allow a backfed breaker to be connected anywhere on that panel. Load calculations determine if the breaker can be reduced. If so, then the sum of the rating of the main breaker (supplying utility power) and the rating of the backfed breaker in that panel may not exceed 100% of the bus bar rating for that panel. And, upstream panels and circuits toward the service entrance must still be analyzed to see if the 100% rule can be met. In many cases, a supply side connection is then the only option available.

Summary

Details are the meat and potatoes of the Code. By looking into theCoderequirements in detail, we see how those requirements are to be implemented. Sometimes the results of these inspections are not what we expect, but the end result is safer electrical systems.

For Additional Information

If this article has raised questions, do not hesitate to contact the author by phone or e-mail. E-mail: jwiles@nmsu.edu. Phone: 575-646-6105

A color copy of the latest version (1.9) of the 150-page, Photovoltaic Power Systems and the 2005 National Electrical Code: Suggested Practices, written by the author, may be downloaded from this web site: http://www.nmsu.edu/~tdi/Photovoltaics/Codes-Stds/Codes-Stds.html

The Southwest Technology Development Institute web site maintains a PV Systems Inspector/Installer Checklist and all copies of the previous "Perspectives on PV” articles for easy downloading. Copies of "Code Corner” written by the author and published in Home Power Magazine over the last 10 years are also available on this web site: http://www.nmsu.edu/~tdi/Photovoltaics/Codes-Stds/Codes-Stds.html

The author makes 6–8 hour presentations on "PV Systems and the NEC” to groups of 60 or more inspectors, electricians, electrical contractors, and PV professionals for a very nominal cost on an as-requested basis. A schedule of future presentations can be found on the IEE/SWTDI web site.

This work was supported by the United States Department of Energy under Contract DE-FC 36-05-G015149


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Tags:  Featured  July-August 2010  Perspectives on PV 

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Connecting to Mother Earth

Posted By John Wiles, Saturday, May 01, 2010
Updated: Friday, January 18, 2013

When buying real estate, conventional wisdom dictates the three most important elements are—Location, Location, and Location.

Based on my twenty-six years of working with PV systems, including the school of hard knocks, I strongly feel that the three most important elements to long- and short-term PV safety are—Grounding, Grounding, and Grounding.

Utility-interactive residential (dwelling unit), commercial, and megawatt PV systems operate with dc voltages from 50 volts to 600 volts and higher. AC voltages start at 120 and go to 23 kV on some of the larger systems. Underwriters Laboratories (UL) has determined that there is a shock hazard in exposed circuits operating at over 30 volts (ac or dc) in wet locations. SeeNEC690.31(A) and 690.33(C).

Operating currents range from less than 10 amps dc and ac to about 2200 amps dc on some of the larger inverters. An arc at currents around 1 amp can start a fire in the right material. Module power can be as low as 20 watts, but ranges upward to 320 watts. Consider the small 7-watt night-light or Christmas tree bulb (before LEDs). Seven watts can start a fire.

Photo 1. Improper module grounding: Plated steel thread cutting screw is not corrosion resistant; THHN conductors and nylon lug are not UV-rated.

Photo 1. Improper module grounding: Plated steel thread cutting screw is not corrosion resistant; THHN conductors and nylon lug are not UV-rated.

PV modules and wiring as well as outdoor-mounted inverters are subjected to severe environmental conditions. Rain, sleet, snow, hail, sand, wind, and sunlight coupled with low and high temperatures would wear down the most stalwart postal worker over a 40–50 year span—the life expectancy of a PV module for producing dangerous amounts of voltage and current. USE-2 cables and the new PV cables are some of the toughest generally available cables, and we have seen USE-2 holding up well after 25 years when properly installed; but what about a less-than-outstanding installation after 30 or 40 years?

The environmental conditions, the use of copper conductors to ground aluminum module frames, and the daily thermal cycling that terminals, combiners, and modules are subjected to will eventually cause a break down in the insulations involved or in the electrical connections.

Proper grounding is a must, even when the NEC and the UL Standards do not fully addressthe issue.

Grounding Problems Are Being Observed and Reported

Tens of thousands of PV systems are being installed annually with financial incentives available at the federal and state levels (http://www.dsireusa.org/). Payments for net energy generated and for all energy produced from renewable sources are being made by utility companies in some states.

Photo 1. Improper module grounding: Plated steel thread cutting screw is not corrosion resistant; THHN conductors and nylon lug are not UV-rated.

 

Photo 2. Improper lugs and conductors. Not securely fastened.

 

Unfortunately, getting the PV modules and racks grounded in a manner that will yield a low-resistance connection to the grounding system that will last for 50 or more years appears to be difficult. Inspectors are seeing improper grounding techniques being used (see photos 1, 2 and 3.) Improper grounding instructions are even appearing in the instruction manuals for listed PV modules (see photo 4). Inspections and tests of installed PV systems have found that in some cases, module-grounding connections have deteriorated in as little as three years and sooner in some areas (see photos 5 and 6).

There is significant confusion among module manufacturers, PV installers, and inspectors concerning how to properly ground a PV module; that confusion is becoming more and more apparent as numerous PV systems are being installed. A little history may highlight the cause of this confusion.

A Look at UL Standard 1703

The first edition (1986) and the current edition (2002) of Underwriters Laboratories (UL) Standard 1703, PV Flat Plate Modules, have a single section devoted to grounding and bonding. Bonding refers to the factory-made electrical connections between the four or more aluminum sections of the module frame. Grounding refers to the field-installed electrical connection between the aluminum module frame and the equipment-grounding system (usually copper conductors).
 
Photo 3. THHN conductor, thread cutting screw and copper are in contact with aluminum.

Photo 3. THHN conductor, thread cutting screw and copper are in contact with aluminum.

Bonding the frame pieces together in the factory using very specific materials and methods results in a durable electrical connection between the frame pieces so that any failure in the module insulation or external conductor insulation will result in all pieces of the frame receiving equal voltage. The factory bonding also insures that when the module frame is properly field-grounded at one of the marked and tested points, the entire module frame is maintained at the ground potential under fault conditions.

During the bonding process, all screw fasteners are precisely torqued to the specified value by automated equipment or by trained technicians using torque screwdrivers. The factory bonding materials and methods are evaluated for low resistance and durability during the listing process. Subsequent to the listing, if the manufacturer changes any of the bonding materials or methods, the changes must be reevaluated by the listing agency. The materials (including any screws or washers) are not specified generically; they are specified to the original equipment manufacturers (OEM) and must always be obtained and used from those sources unless any change is reevaluated by the listing agency.

Photo 4. Installed per instruction manual—but copper touching aluminum?

Photo 4. Installed per instruction manual—but copper touching aluminum?

Contrast this precisely controlled and evaluated factory bonding system with the field-installed grounding techniques used to connect a copper equipment-grounding conductor to the aluminum module frame. Grounding PV modules is haphazard at best for a number of reasons. The first is that the module manufacturers do not realize the importance of this connection to the overall safety of the system. Second and possibly the most critical is that the Bonding/Grounding section in UL 1703 does not clearly distinguish the differences between bonding and grounding. The manufacturers have the impression that the bonding techniques and materials used in the factory may be applied to the grounding connections made by the installer in the field. Instruction manuals and hardware (sometimes supplied) show techniques which are not consistent with good electrical connections (see photo 4). Field-made connections using a threaded fastener are rarely torqued to the specified value, even when that value is given in the module instruction manual, because few PV installers have or carry torque screwdrivers. The field grounding connection may or may not be inspected by the AHJ, and they are never tested for overall continuity. Also, since the PV system can operate without trouble for many years, there is little motivation to inspect these connections after the original installation.

In late 2007, UL issued an "Interpretation” of UL 1703 which focused on module field grounding. This interpretation was to be used by module manufacturers and the module testing/certification/listing laboratories (UL, CAS, TUV and ETL) to evaluate and possibly revise the grounding methods, hardware (if any) and instructions supplied with the modules. Unfortunately, it is not possible for the laboratories to review all existing modules and supposedly modules are reevaluated every five years when the listing must be renewed. A few module manufacturers have revised their grounding instructions, but it would appear that these revised instructions in some cases may have not been carefully evaluated or even reviewed by the certification/listing laboratories.

Grounding Instructions Not Consistent

For example, some instructions list lock washers, star washers and other critical grounding hardware that is distributed by major national hardware stores that maintain no source control over their suppliers. These are not OEM vendors. Others continue to use or recommend thread cutting or thread forming screws when the UL Interpretation says that all threaded fasteners must be installed and removed ten times without damage to any threads. This requirement is nearly impossible to meet with the soft aluminum used for module frames.
 
Photo 5. Improper use of module bonding screw and copper in braid touching aluminum.
 
Photo 5. Improper use of module bonding screw and copper in braid touching aluminum.

The UL Interpretation of UL 1703 has very specific information about not putting dissimilar metals into contact and gives a chart that shows the compatibility of various metals. Copper and aluminum may not come in contact and if they do, the aluminum at the contact point will be removed by galvanic corrosion destroying the connection. Inadvertent contact between the bare copper equipment-grounding conductor and an aluminum module frame or rack does not pose problems because the small amount of aluminum that may disappear is not involved in a specific electrical contact.

Photo 6. Tinned copper braid offers no protection for aluminum module frame.

Photo 6. Tinned copper braid offers no protection for aluminum module frame.

In some cases, the instructions specify the use of a stainless-steel washer to isolate the copper conductor from the aluminum frame, but no surface preparation of the oxidized, anodized, and/or clear-coated aluminum module frame is specified. If this method were to be done properly with surface preparation, then the presumption is that the mechanical fastener (screw and nut) and the stainless steel washer would carry the fault currents. But these devices are generic in nature and have not been evaluated for carrying current.

A casual examination of any common electrical device such as a circuit breaker, a receptacle outlet, or a wall switch will show that the mechanical fastener provides only pressure to push the two electrical conductors together. Those mechanical fasteners (screws) are not normally designed or specified to carry currents, unless they have been specifically tested and evaluated to do so during the listing of the device. An example of a device where the provided screw has been evaluated to carry current is the neutral-to-ground bonding screw used in many service-entrance panels.

To further confuse the situation, it appears that the high currents, steel plates, and test methods used in UL Standard 467 for evaluating and listing grounding devices may not be applicable to evaluating grounding devices used to ground PV modules and racks where the currents are low and the aluminum surfaces are oxidized, anodized or clear coated.

Help Is Coming

Underwriters Laboratories has a group developing specific requirements for PV module grounding that will appear in UL 1703, the PV module standard. The requirements will cover methods and hardware supplied by the module manufacturers as well as the existing and new grounding devices being used for the purpose.

Photo 7. One method of grounding a PV module when all else fails

Photo 7. One method of grounding a PV module when all else fails

AHJ comments on poor grounding and confusing grounding instructions to the UL AHJ reporting web site may speed the process, as UL is made more aware of the pressing problem.

http://www.ul.com/global/eng/pages/offerings/perspectives/regulator/electrical/productreport/

Although not presently on the market, some modules have been built with plastic frames—maybe they will return.

When the grounding instructions furnished by the module manufacturer are inadequate or contradict NEC or UL requirements, the PV installer and the inspector must come to some agreement on what is an acceptable module grounding method and hardware.

One method used by utility companies for many years to connect copper conductors to aluminum busbars in an outdoor environment uses surface preparation and a tin-plated copper lay-in lug listed for direct burial. A description of this method was presented in The "Perspectives on PV” column in the IAEI News for September-October 2008. It may also be found in the Burndy instructions for installing lay-in lugs and in Appendix G of the NEC/PV Suggested Practices manual. Both may be found on my web site—below. (See photo 7).

Summary

Grounding is critical to the short- and long-term public safety of PV systems. These systems may be producing power 50 years from the installation date with possibly deteriorating electrical connections and insulations. Grounding all exposed metal surfaces for the life of the systems is mandatory, and the techniques used may have to exceed existing Code and UL requirements.

What are you waiting for?

For Additional Information

If this article has raised questions, do not hesitate to contact the author by phone or e-mail. E-mail: jwiles@nmsu.edu Phone: 575-646-6105

A color copy of the latest version (1.9) of the 150-page,Photovoltaic Power Systems and the 2005 National Electrical Code: Suggested Practices, written by the author, may be downloaded from this web site: http://www.nmsu.edu/~tdi/Photovoltaics/Codes-Stds/Codes-Stds.html

The Southwest Technology Development Institute web site maintains a PV Systems Inspector/Installer Checklist and all copies of the previous "Perspectives on PV” articles for easy downloading. Copies of "Code Corner” written by the author and published inHome Power Magazineover the last 10 years are also available on this web site: http://www.nmsu.edu/~tdi/Photovoltaics/Codes-Stds/Codes-Stds.html

For an intensive 7–8 hour training session on PV and the NEC, see the web site above for a schedule of presentations made to inspectors, electricians, electrical contractors, and PV professionals. The hosting organization usually charges a very nominal fee and controls registration and attendance


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Tags:  Featured  May-June 2010  Perspectives on PV 

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The Microinverter and the AC PV Module

Posted By John Wiles, Monday, March 01, 2010
Updated: Friday, January 18, 2013

No discussion of PV systems would be complete without a look at the newest inverter technologies that the installer and inspector will face. These new technologies include the microinverter and the AC PV module.

Microinverters

The inverters that have been covered in the past several issues are known as string inverters because they operate with a string of series connected PV modules. These inverters range in power from one megawatt down to about 700 watts. DC maximum system voltages can get as low as about 125 volts.

Photo 1. Enphase microinverter

Photo 1. Enphase microinverter

The new Enphase microinverter (photos 1 and 2) is a small inverter (hence the name) that is designed to work with a single PV module and operate at a maximum of about 70 volts dc. The inverter is connected directly to the PV module using the existing conductors and connectors (now locking in most cases) attached to both the module and the inverter. Available units are rated in the 170–210 watt range, but as with other PV products, ratings and specifications change continually.

Photo 2. Pair of Enphase microinverters showing ac and dc cables

Photo 2. Pair of Enphase microinverters showing ac and dc cables

The microinverter is a utility-interactive inverter with dc ground-fault protection (690.5) in the current offering. The Enphase microinverter has been on the market since early 2009 and it internally grounds the positive dc module conductor. That internal grounding bond (via the dc ground-fault protection circuits,NEC690.5) requires that the inverter have a dc grounding electrode terminal and that terminal is on the outside of the Enphase microinverter case. Other types and brands of microinverters may accomplish grounding differently or go to an ungrounded configuration using modules with the new "PV cable” required byNEC690.35 for such systems.

Photo 3. Microinverter AC output connector

Photo 3. Microinverter AC output connector

The microinverter has ac input and output cables and connectors and has been listed in a manner that will allow multiple inverters to be connected with up to about 15 units on a single output cable. See photo 3. With a power output in the 170–210 watt range (depending on model), the rated ac output current at 240 volts will range from 0.71 amps to 0.79 amps. On the 14 AWG cable with a 15-amp overcurrent device, the rated current for that circuit is limited to a maximum of 12 amps. This rating will allow 1–15 inverters to be installed on the same ac output cable.

AC PV Modules

In the factory, take a normal dc PV module and connect a microinverter to it, fasten the microinverter to the back of the module and cover the dc, exposed conductors so none of them are accessible and you have an AC PV module (photo 4). By the time this article is published, at least one AC PV module should be on the market. It is the Andalay AC PV module by Akeena Solar and it has a unique frame that is also the module mounting rack. The lead-in photo shows a typical Andalay PV system using dc PV modules. Since the dc wiring between the module and inverter is no longer accessible and has become an integral part of the product, dc requirements in the Code no longer apply to the AC PV module. The AC PV module is a utility-interactive device and has a similar ac output cabling system to the microinverter addressed above.

Photo 4. Almost an AC PV module—just make the dc wiring not accessible


Photo 4. Almost an AC PV module—just make the dc wiring not accessible

 

DC Connections

In the standard PV module/microinverter combination, the microinverter dc connection to the PV module may have to be disconnected to replace the microinverter should it or the module fail (say once in every 20–30 years). While the voltage will be a maximum of about 70 volts with current inverter designs, the current may be in 3–7 amp range and the connectors could possibly be damaged at this voltage and current, posing a possible safety hazard. While a very few inspectors may request a costly and impractical load-break rated disconnect, the code-compliant solution is really quite simple. The back of the PV module must be accessed to reach these dc connections and this generally requires that the module be unfastened from the mounting system. Since the module is accessible and is being accessed, just putting a blanket or other opaque material over it per 690.18 will reduce the dc output voltage and current (and the ac current) to near zero, allowing the module/inverter dc connectors to be safely opened. Opening this connection with the module blacked out will, in all likelihood, be safer than opening the same connectors on a module in a high-voltage string of modules. Of course, the AC PV module has no accessible dc connections.

AC Connections

Each microinverter or AC PV module will have an ac input/output cable to allow the multiple inverter parallel connections. This cable may carry currents in bright sunlight of 0.7 amps at 240 volts from the first module/inverter in the set to as much as 12 amps at 240 volts through the last connector of the set that has multiple devices. Servicing the single AC PV module or utility-interactive microinverter could be accomplished by covering the module to reduce the dc and hence the ac current to zero. However, not covering all modules in the set would allow current from other, non-covered, modules/inverter to flow through the cable and, at 240 volts, could damage the connector and possibly pose a shock hazard when opening these ac connections under load. To some extent, the hazard is minimized because the inverter anti-islanding circuits shut down very rapidly, reducing any arcing when the ac connector is opened.

Photo 5. Numerous parts are required for a string inverter PV system

Photo 5. Numerous parts are required for a string inverter PV system

Opening the ac circuit at the PV backfed breaker in the building service entrance panel would be safe solution if that breaker could be locked open, but breaker locks are few and far between and lock-out/tag-out procedures are not generally used in residential and commercial electrical systems.

NEC690.14(D) addresses the situation and it would appear that the installation of a separate ac disconnect on the roof near the AC PV modules or microinverters will meetCoderequirements and enhance safety. A common 60-amp unfused, pull-out air conditioning disconnect costs less than $10 at the building supply centers. It provides the disconnect, a place to terminate the ac output cable from a set of microinverters or AC PV modules, a place to originate the field-installed wiring system to the ac load center in the house, and is usually cheaper than a separate junction box and cover.

Advantages

The use of microinverters and AC PV modules will proliferate due to several advantages they offer over the conventional string inverters.

The first is a simplified set of installation requirements and a reduced number of separate parts. See photos 5 and 6 for some quantitative differences in the amount and types of equipment involved in installing an AC PV Module system vs. a conventional string inverter system.

Photo 6. Far fewer parts are needed to install the Andalay PV/ microinverter system

Photo 6. Far fewer parts are needed to install the Andalay PV/ microinverter system

In a dc series-connected string of PV modules, module mismatch is sometimes an issue that affects the string performance. Modules come out of the factory with slight (up to 10%) variations in specifications. The string of modules in a dc system cannot deliver current above the current delivered by the weakest module in the string. The mismatch between module currents results in some lost power compared to a dc string of modules that are equal in every specification. The PV modules near the top of an array on a sloped roof may operate hotter (and at reduced power) than modules lower down on the roof due to hot air rising behind the modules. Depending on how each string of modules is connected, some loss of power may occur if hot modules are connected in series with cooler modules.

Shading is also a problem in a conventional string-inverter configuration. The shading of a single module will result in a power loss from that module, but may also reduce power from the other, non-shaded modules in the string.

The microinverter and the AC PV module work at the individual module level. Each inverter extracts the maximum power from that module no matter what the other modules in the PV array are doing. The output of each is independent of the other modules/inverters in the set. The outputs of the microinverters or AC PV modules are connected in parallel, rather than in series, and this isolates one from another.

The outputs are at 240 volts ac and these ac output circuits act much like ac branch circuits. They go dead when the ac utility power is removed at any disconnect in the circuit so they do not pose the safety hazards associated with the daytime "always-energized” dc circuits operating at hundreds of volts between the modules and the inverter. If a short circuit or a ground fault were to occur in these ac output circuits, the dedicated branch-circuit breaker would open and the circuit would go dead. Opening the main service disconnect or the backfed PV breaker will de-energize those PV ac output circuits—a boon to fire fighters.

Disadvantages

There may be some cost impact of using AC PV modules or microinverters on each module when compared to the use of the single string inverter. However, two factors must be considered. The cost of the dc switchgear and the required conduit (or other appropriate wiring method) for the dc conductors inside the building plus the cost of the single inverter must be compared to the added cost of multiple small inverters or AC PV modules with an inverter on each module.

Then there are the life cycle costs. Modules are guaranteed for power production for 25 years, but can be expected to produce power for as long as 50 years. Large inverter manufacturers do not seem to be interested in or able to extend the average longevity past about 15 years at reasonable costs. The microinverter manufacturers, using different construction methods and topologies, are predicting significantly longer lives for their products. Time will reveal all.

A Word of Warning

The microinverter or AC PV module output must be connected on a dedicated circuit per 690.64. See the "Perspectives on PV” in recent editions of theIAEI Newsfor details on how to connect multiple sets of these devices. They should never be connected to a circuit protected by a GFCI or AFCI, because neither of these devices has been tested or listed for backfeeding.

Summary

2010 will see numerous microinverters and AC PV modules being installed. They are being sold in the home improvement centers and building supply houses as well as in local electrical supply houses, and the general public will be buying them. Inspectors must become familiar with these devices and the Code requirements that apply to them. See the author’s web site below for a white paper on connecting and grounding the Enphase microinverter.

For Additional Information

If this article has raised questions, do not hesitate to contact the author by phone or e-mail. E-mail: jwiles@nmsu.edu Phone: 575-646-6105

A color copy of the latest version (1.9) of the 150-page, Photovoltaic Power Systems and the 2005 National Electrical Code: Suggested Practices, written by the author, may be downloaded from this web site: http://www.nmsu.edu/~tdi/Photovoltaics/Codes-Stds/Codes-Stds.html

The Southwest Technology Development Institute web site maintains a PV Systems Inspector/Installer Checklist and all copies of the previous "Perspectives on PV” articles for easy downloading. Copies of "Code Corner” written by the author and published in Home Power Magazine over the last 10 years are also available on this web site: http://www.nmsu.edu/~tdi/Photovoltaics/Codes-Stds/Codes-Stds.html

The author makes 6–8 hour presentations on "PV Systems and the NEC” to groups of 60 or more inspectors, electricians, electrical contractors, and PV professionals for a very nominal cost on an as-requested basis. A schedule of future presentations can be found on the IEE/SWTDI web site.


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Tags:  Featured  March-April 2010  Perspectives on PV 

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Supply-side PV Utility Connections

Posted By John Wiles, Friday, January 01, 2010
Updated: Friday, January 18, 2013

Many larger PV systems cannot meet the requirements for a load-side (of the service disconnect) connection to the premises wiring system and a supply-side connection must be considered.


Code Considerations

The supply-side connection (also known as a service-entrance tap) is allowed by theNational Electrical Code(NEC) and is addressed in a number of sections in theCode.

Section 690.64(A) {moving to 705.12(A) in the 2008-2011NEC} allows a supply (utility) side connection as permitted in 230.82(6). Section 230.82(6) indicates that solar photovoltaic equipment is permitted to be connected to the supply side of the service disconnect.

Photo 1. A breaker as a supply-side tap. But is it Code legal?

It is evident that the connection of a utility-interactive PV inverter to the supply-side of a service disconnect is essentially connecting a second service-entrance disconnect to the existing service and many, if not all, of the rules for service-entrance equipment must be followed. Many years ago, the National Fire Protection Association (NFPA) made an informal interpretation that these supply-side taps were essentially a second service entrance on the building or structure and should be treated as such.

Section 240.21(D) allows the service conductors to be tapped and refers to 230.91. In general, the other "Tap Rules” of Section 240 do not apply because they were not developed to address two sources of power in a tap circuit, nor were they developed to assure safe operation when one source is an unprotected utility power source.

Photo 2. Utility-required ac disconnects. Could have been combined into one.

Section 230.91 requires that the service overcurrent device be co-located with the service disconnect. A circuit breaker or a fused disconnect would meet these requirements. See photo 1. A utility-accessible, visible break, lockable (open) fused disconnect (aka safety switch) used as the new PV service disconnect may also meet utility requirements for an external PV ac disconnect in areas where utilities require such an additional disconnect. See photo 2.

Section 230.71 specifies that the service disconnecting means for each set of service-entrance conductors shall be a combination of no more than six switches and sets of circuit breakers mounted in a single enclosure or in a group of enclosures. The PV system may be counted as a separate service (230.2) and could have up to six disconnects of its own.

Photo 3. Directory for PV system

Location and Directory

Section 230.70(A) establishes the location requirements for the service disconnect. Section 705.10 requires that a directory be placed at each service equipment location, showing the location of all power sources for a building. See photo 3. Locating the PV ac disconnect adjacent to or near the existing service disconnect may facilitate the installation, inspection, and operation of the system. See photo 4.

Size Matters

Obviously the size of the new PV service disconnect is important. It will normally be sized at 125% of the rated output current from the PV inverter(s). But in small systems, a question arises; how small can it be? Section 230.79 addresses the rating issue. Some inspectors have looked at 230.79(A) and say that it can be as low as 15 amps if that value is at or above the rating of the inverter output circuit. The connection of other allowed loads at this level is common.

I would suggest caution here, since the tap is to service-entrance conductors rated at 100 amps and above. The typical 15-amp circuit breaker with 10,000 amps of interrupt capability, in this application, may not be able to withstand the available fault current, since it is not protected and coordinated with any main breaker typically rated at 22,000 amps. Of course, Section 110.9 should be followed and fault current calculated. Also a service entrance rated 30-amp fused disconnect with 15-amp fuses could be used.

Photo 4. PV ac disconnect above closed service disconnect

Another consideration is the size of the service-entrance conductors, the new tap conductors, and the size of the terminals on available switchgear rated at 30 or 60 amps. The added conductors between the existing service-entrance conductors and the new service disconnect will be subjected to available fault currents and will have no protection except that provided by the fuse on the primary of the utility transformer. Making them as large as possible, with an upper limit of the size of the existing service-entrance conductors would seem prudent, but small disconnects will not accept very large conductors.

For these reasons, I suggest that Section 230.79(D) be used as the requirement for the smallest service disconnect for PV inverter supply-side taps. Section 230.79(D) requires that the disconnect have a minimum rating of 60 amps. This would apply to a service-entrance rated circuit breaker or fused disconnect.

Section 230.42 generally requires that the service-entrance conductors be sized at 125% of the continuous loads (all currents in a PV system are worst-case continuous). The actual rating should be based on 125% of the rated output current for the utility-interactive PV inverter as required by 690.8. The service tap conductors must have a 60-amp minimum rating from 230.79(D). Temperature and conduit fill factors must be applied.

For a small PV system, say a 2500-watt, 240-volt inverter requiring a 15-amp circuit and overcurrent protection, these requirements would appear to require a minimum 60-amp rated disconnect, with 15-amp fuses; fuse adapters would be required. Fifteen-amp conductors could be used between the inverter and the 15-amp fuses in the disconnect.

Photo 5. Meter-main combo—do not tap.

Section 230.42(B) requires that the conductors between the service tap and the disconnect be rated not less than the rating of the disconnect; in this case, 60 amps.

How we would deal with the 60-amp disconnect, 15-amp over-current requirements using circuit breakers is not as straightforward. A circuit breaker rated at 60-amps would serve as a disconnect, and it could be connected in series with a 15-amp circuit breaker to meet the inverter overcurrent device requirements. In this case, the requirements of 690.64(B)(2) should be applied for the series connection. See "Perspectives on PV” in the November-December issue of the IAEI News for details.

Section 110.9 of the NEC requires that the interrupt capability of the equipment be equal to the available fault current. The interrupt rating of the new disconnect/overcurrent device should at least equal the interrupt rating of the existing service equipment. The utility service should be investigated to ensure that the available fault currents have not been increased above the rating of the existing equipment. Fused disconnects with RK-5 fuses are available with interrupt ratings up to 200,000 amps.

Section 230.43 allows a number of different service-entrance wiring systems. However, considering that the tap conductors are unprotected from faults, it is suggested that the conductors be as short as possible with the new PV service/disconnect mounted adjacent to the tap point. Making these tap conductors as large as the service-entrance conductors, while not a Code requirement, would also add a degree of safety. Of course, the added disconnect must be able to accept the larger conductors. Conductors installed in rigid metal conduit would provide the highest level of fault protection.

All equipment must be properly grounded per Article 250 requirements. See 250.24(B) for bonding requirements. As a service disconnect, neutral-to-ground bonding would generally be required at the new disconnect, and a grounding electrode conductor should also be added.

The actual location of the tap will depend on the configuration and location of the existing service-entrance equipment. The following connection locations have been used on various systems throughout the country.

On the smaller residential and commercial systems, there is sometimes room in the main load center to tap the service conductors just before they are connected to the existing service disconnect. In other installations, the meter socket has lugs that are listed for two conductors per lug. Of course, adding a new pull box between the meter socket and the service disconnect is always an option. Combined meter/service disconnects/load centers frequently have significant amounts of interior space where the tap appears to be possible between the meter socket and the service disconnect. However, tapping this internal conductor or bus bar in a listed device such as a meter-main combination would violate the listing and should not be done. See photo 5.

Where the service-entrance conductors are accessible, a new meter base (socket) could be added ahead of the combination device. A tap box would then be added between the new socket and the combination device. The meter would then be moved from the combo device to the new socket, jumper bars added to the old socket and the old socket covered.

In the larger commercial installations, the main service-entrance equipment will frequently have bus bars that have provisions for tap conductors. The tap can only be made by the organization supplying the service equipment and that is usually a UL 508 panel shop. They can tap the equipment and maintain the listing on the equipment.

In all cases, safe working practices dictate that the utility service be de-energized before any tap connections are made. Additional service-entrance disconnect requirements in Article 230 and other articles of the NEC will apply to this connection.

Summary

Supply-side service entrance taps are useful for larger PV systems where the conditions of the load-side tap cannot be met. These supply-side taps normally require that the power be removed from the service to ensure a safe installation.

The next "Perspectives on PV” will address the new micro inverters and ac PV modules.

Sharp-eyed inspectors will note in the last issue that the 45 -amp breaker used for the PV system will be too large for the 200- amp panel. The panel must be 225 amps or the main breaker reduced to at least 195 amps if loads allow.

For Additional Information

If this article has raised questions, do not hesitate to contact the author by phone or e-mail. E-mail: jwiles@nmsu.edu Phone: 575-646-6105

A color copy of the latest version (1.9) of the 150-page, Photovoltaic Power Systems and the 2005 National Electrical Code: Suggested Practices, written by the author, may be downloaded from this web site: http://www.nmsu.edu/~tdi/Photovoltaics/Codes-Stds/Codes-Stds.html

The Southwest Technology Development Institute web site maintains a PV Systems Inspector/Installer Checklist and all copies of the previous "Perspectives on PV” articles for easy downloading. Copies of "Code Corner” written by the author and published in Home Power Magazine over the last 10 years are also available on this web site: http://www.nmsu.edu/~tdi/Photovoltaics/Codes-Stds/Codes-Stds.html

The author makes 6–8 hour presentations on "PV Systems and the NEC” to groups of 60 or more inspectors, electricians, electrical contractors, and PV professionals for a very nominal cost on an as-requested basis. A schedule of future presentations can be found on the IEE/SWTDI web site.


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Tags:  Featured  January-February 2010  Perspectives on PV 

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Making the AC Utility Connection

Posted By John Wiles, Sunday, November 01, 2009
Updated: Friday, January 18, 2013

Connecting the utility-interactive inverter to the utility grid properly is critical to the safe, long-term, and reliable operation of the entire system. The ac output circuit requirements and the circuits that carry the inverter current in the premises wiring are somewhat complex. However, meetingCoderequirements can and should be accomplished to ensure a safe and durable system.


The circuit sizing and overcurrent device on the ac output of the utility-interactive inverter was covered in the September-October issue ofIAEI News. Even though power and current flow from the inverter to the utility, it should be noted that the utility-end of this circuit is where the currents originate that can harm the conductors when faults occur. Any overcurrent protection should be located at the utility end of the inverter ac output circuit and not at the inverter end of this circuit.

Photo 1

Although the inverter may require an external disconnect, if that disconnect function is achieved, as it commonly is, by a circuit breaker, then the conductor ampacity calculations may be more complicated as noted below. It is good practice to install the inverter near the backfed load center so that the backfed breaker commonly used to interconnect the inverter with the utility can also be used as the ac inverter disconnect required by 690.15. This places the overcurrent device at the utility-supply end of the circuit and groups the ac disconnect for the inverter with the dc disconnect.

Load-side connection

There are two types of connections allowed by the Code for interfacing the output of the utility-interactive inverter to the utility power. They are made on either the supply side or the load side of the main service disconnect of a building or structure (690.64). The load side of the main service disconnect is the most common connection used for the residential system and smaller commercial system under about 10 kW. NEC 690.64(B) [moving to 705.12(D) in 2008 and 2011] covers the requirements and it is heavy reading at best.

There were changes in 690.64 between the 2005 NEC and the 2008 NEC and the IAEI News issue for July-August 2008 discussed them.

Inspectors need to know this material and how to apply it because many PV installers are not familiar with the details of the requirements.

Photo 2. Multiple inverters require special connections

Code-making panels since 1984 have maintained that 690.64(B)(2) will be rigorously applied to any circuits supplied from multiple sources where protected by overcurrent protective devices (OCPD) from each source. Such sources would include the output of PV inverter(s) and the utility supply.

This Code section requires that the ratings of all OCPD supplying power to a conductor or busbar be added together. The sum of the ratings of those breakers must be less than or equal (in other words: may not exceed) 120% of the rating of the busbar or the ampacity of the conductor. In equation form:

PV OCPD + Main OCPD <= 120% R, where R is the ampacity of conductor or rating of busbar.

120% factor depends on breaker location

The 120% factor came about in previous code cycles because it was determined that the demand factors on residential and small commercial systems would be such that it was unlikely that the conductor or panel would ever be loaded to 100% of rating. Even if the sources could supply 120% of the rating of the busbar or conductor, loads connected to that same busbar or conductor not exceeding the busbar rating would not pose an overload problem. In order to use this 120% factor, any backfed breaker carrying PV currents must be located at the opposite end of the busbar from the main breaker or main lugs supplying current from the utility (photo 1). The same location requirement would apply to any location of the supply overcurrent devices on any conductor. If the PV inverter OCPD cannot be located as required, then the 120% in the above requirement drops back to 100% and the installation under the load side connection becomes more difficult.

Photo 3. Two inverters with PV ac combining panel

The Article 240 tap rules do not apply to these inverter connections since the tap rules were developed only for circuits with one source. The OCPD for the inverter output circuit should be located, as mentioned above, at the point nearest where the utility currents could feed the circuit in the event of a fault.

Examples

1. A dwelling has a 125-amp rated service panel (bus bar rating) with a 100-amp main breaker at the top. How large can the backfed PV breaker be that must be located at the bottom of the panel?

PV OCPD + Main OCPD <= 120% of Panel rating

120% of panel rating = 1.2 x 125 = 150 amps

PV + 100 <= 150, therefore the PV OCPD can be up to 50 amps

2. Suppose it was 100-amp panel with a 100-amp main breaker. What PV breaker could be added?

PV + 100 <= 1.2 x 100 = 120

The maximum PV backfed circuit breaker would be rated at 20 amps.

3. A 200-amp main panel with a 200-amp main breaker would allow up to 40 amps of PV breaker, which could be any combination of breakers that added up to 40 amps on either line 1 or line 2 of the 120/240V panel.
PV + 200 <= 1.2 x 200 = 240

PV <= 240-200= 40 amps

4. Working the problem from the inverter end, we start with the continuous rated inverter output current. This is usually the rated power divided by the nominal line voltage, unless the inverter specifications list a higher continuous output current (sometimes given at a low, line voltage).

A 3500-watt, 240-volt inverter has a rated ac output current of 3500/240 = 14.58 amps.

Photo 4. Utility-required disconnect, fused

The output circuit must be sized as 125% of 14.58 = 18.2 amps (690.8). The next larger overcurrent device would be a 20-amp OCPD and this would be consistent with the use of 12 AWG conductors if there were not any very large deratings applied for conditions of use. This system could be connected to a 200-amp panel or a 100-amp panel providing that the backfed 20-amp breaker could be located at the bottom of the panel.

There is sometimes a tendency to use that 30-amp breaker and those 10 AWG conductors that happen to be on the truck. While this would pose no problems for conductor ampacity or protection, the inverter specifications may limit the maximum size of the output OCPD and larger values may not be used [110.3(B)].

No bottom breaker position?

From the above equations, it can be seen that if the backfed PV OCPD cannot be located at the bottom of the panel or at the end of the circuit, it is not possible to install the backfed breaker without changing something. That 120% allowance drops to only 100%.

Any panel that has a main breaker rated the same as the panel rating in the above equations would not allow any OCPD to be added. The 100%-of-the-panel-rating factor (instead of 120%) would equal the rating of the main breaker and the equation would force the PV breaker rating to be zero.

In a few cases, an NEC Chapter 2 load analysis might reveal that the service for the dwelling needed to be only 150 amps, but a 200-amp panel was installed with a 200-amp main breaker just to provide extra circuit positions. In this case, it might be possible to substitute a 150-amp main breaker for the 200-amp breaker, and even without the bottom position being open, 50 amps of PV breaker could be installed.

Systems with multiple inverters

Many residential and small commercial systems use more than one inverter (photo 2). If the local utility requires an accessible, visible-blade, lockable disconnect on the output of the PV inverters, then more than one inverter could not be connected directly to the main panel (photo 3). The two or more inverters would have to have their outputs combined in a PV ac inverter combining subpanel (PV ac subpanel) before being routed through the utility disconnect and then to the main panel (photo 4). The disconnect is not normally fused, but some are, depending on the system configuration. The PV ac subpanel rating, the rating of the disconnect, and the ampacity of the conductor to the main panel are also controlled by 690.64(B) requirements.

Here is another example

The dwelling has a 200-amp main service panel with a 200-amp main breaker and there is an empty breaker position (2-poles) at the bottom of the panel. The utility requires an external disconnect switch and it is desired to install a PV system that has a 3500-watt and a 4500-watt inverter. A PV ac panel will be used to combine the outputs of the two inverters and the output of that PV ac panel will be routed through the utility disconnect and then to a single backfed breaker in the main service panel.

The ratings of the output circuits of each inverter are:
3500/240 = 14.58 amps, 1.25 x 14.58 = 18.2 amps; use a 20-amp breaker and 12 AWG conductors.

4500/240 = 18.75 amps, 1.25 x 18.75 = 23.43 amps; use a 25-amp breaker and 10 AWG conductors.

The 20 and 25-amp breakers are mounted in the bottom of a PV ac panel, and a main-lug only panel will be installed. Normally, no loads will be connected to this subpanel. It will be dedicated to the PV system.

The next step is to calculate the backfed breaker that must be placed in the main service panel to handle the combined output of both inverters from the PV ac subpanel and to protect the conductor carrying those combined outputs under fault conditions from high utility currents.

The combined currents from both inverters are:

14.58 + 18.75 x 1.25= 41.6
and the overcurrent device should be 45 amps.

The ratings of OCPD supplying the conductor from the PV ac subpanel to the 45-amp breaker, the utility disconnect switch, and supplying that PV ac panel are now defined as 45, 20, and 25 amps.

The panel rating and the ampacity of the conductor are controlled by 690.64(B)(2) and it would be incorrect to guess that the answer might be 45 amps as it would be in a normal load subpanel.

45 + 20 + 25 <= 120% R,
where R is the panel rating or the ampacity of the conductors.

90 <= 1.2 R, R >= 90/1.2 = 75 amps.

With this number, we would round up to a 100-amp panel, and a 100-amp disconnect would be used. The conductor size for this ampacity would be 4 AWG since the breakers would typically have 75°C terminal temperature limits.

If the reader really wants to see why we must use 75 amps and not 45 amps for sizing the panel and the conductors, send the author e-mail for a white paper on the subject.

Summary

The load-side connection for the utility-interactive PV inverter is not the easiest subject to understand, but the correct application of these requirements will yield a safer, more durable system. When the requirements of load-side connections become complex and expensive, a supply-side connection is used, and we will examine those requirements in the next Perspectives on PV.

For Additional Information

If this article has raised questions, do not hesitate to contact the author by phone or e-mail. E-mail: jwiles@nmsu.edu Phone: 575-646-6105

A color copy of the latest version (1.9) of the 150-page, Photovoltaic Power Systems and the 2005 National Electrical Code: Suggested Practices, written by the author, may be downloaded from this web site: http://www.nmsu.edu/~tdi/Photovoltaics/Codes-Stds/Codes-Stds.html

The Southwest Technology Development Institute web site maintains a PV Systems Inspector/Installer Checklist and all copies of the previous "Perspectives on PV” articles for easy downloading. Copies of "Code Corner” written by the author and published in Home Power Magazine over the last 10 years are also available on this web site: http://www.nmsu.edu/~tdi/Photovoltaics/Codes-Stds/Codes-Stds.html

The author makes 6–8 hour presentations on "PV Systems and the NEC” to groups of 60 or more inspectors, electricians, electrical contractors, and PV professionals for a very nominal cost on an as-requested basis. A schedule of future presentations can be found on the IEE/SWTDI web site.

This work was supported by the United States Department of Energy under Contract DE-FC 36-05-G015149


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Connecting the Inverter

Posted By John Wiles, Tuesday, September 01, 2009
Updated: Friday, January 18, 2013

Connecting the utility-interactive inverter properly is critical to the safe, long-term and reliable operation of the entire system. Proper grounding of the inverter will minimize the possibility of electrical shocks and damage from surge currents. Understanding and applying the requirements of NEC 690.47 to the inverter grounding connections is somewhat complex but ensures that the user will be safe and that the inverter and other equipment will suffer minimum damage under surge conditions.


Equipment Grounding Conductors

In a typical residential or small commercial PV system (less than about 20 kW), the inverter serves as a central focal point for grounding connections. The dc equipment grounding conductor from the PV array and the dc disconnect are connected to the inverter. The ac inverter output circuit equipment grounding conductor leading to the point of connection with the utility is connected to the inverter. Under the 2005 NEC, the dc equipment grounding conductors may be the only connection the module frames have to earth. UL Standard 1741, quoted in part below, requires equipment grounding terminals for both the ac and dc circuits.
 

Photo 1. Three grounding terminals on bus bar

18.1.8 Equipment grounding leads or equipment grounding terminals shall be provided for each input and each output circuit.

Grounding Electrode Terminal

Nearly all utility-interactive inverters installed today (2009) employ transformers, are connected to grounded PV arrays, and have an internal ground-fault indication/detection (GFID) system. This GFID system includes the internal bonding jumper between the dc grounded conductor and the grounding system. The presence of this dc bonding jumper requires, according to UL Standard 1741, that the inverter have a dc grounding electrode terminal. Here is what UL Standard 1741 requires (in part) for the dc grounding electrode terminal.
 


Photo 2. Grounding bus bar

18.2.1 Equipment intended to be installed as service entrance equipment or equipment containing the main dc or ac bonding connection shall be provided with a grounding electrode terminal.

Photo 3. Three grounding terminals

These grounding connection requirements will require that each inverter have a minimum of three terminals available for making the proper connections. All three terminals may be on a common bus bar or mounted separately in the inverter. They will normally all be connected (bonded) together electrically in the inverter and they will be connected to the inverter chassis. See photos 1, 2 and 3.

To ensure proper grounding of the entire PV system, it is necessary to connect all three of these terminals properly. Unfortunately, some manufacturers and their certification/listing agencies are letting inverters get on the market that do not have all three of these terminals. Because other countries do not ground PV systems like our Code requires, some inverters get certified/listed without a dc grounding electrode terminal. The Europeans use the term protective earth (PE) terminal instead of equipment grounding terminal. Others have only one equipment grounding terminal, not the required two and do not even have a grounding electrode conductor terminal. See photo 4.

Photo 4. Only one equipment grounding terminal (PE) and no grounding electrode conductor terminal

Some inverters have an external grounding electrode terminal and the equipment grounding conductors are permanent leads coming out of the inverter. See photos 5 and 6.

When the installer or inspector finds one of these inverters with missing grounding terminals, the manufacturer and the listing agency should be contacted. It is possible, in some cases, to splice the ac and dc equipment grounding conductors together and connect them to a single equipment grounding terminal. However, the grounding electrode conductor must be connected directly to the proper terminal and should not be spliced.

Connecting the Inverter to Ground (Earth)

The Code had significant changes between the 2005 and 2008 editions in Section 690.47(C) that addresses the dc grounding electrode connection. As far as the author can determine, either the requirements of this section in NEC-2005 or the permissive requirements in NEC-2008 may be applied to connect the grounding electrode conductor when installing a system in jurisdictions using either Code. A proposal has been submitted for NEC-2011 that includes all three methods and will have improved clarity. That proposal is repeated and may help in understanding what the requirements are for 690.47(C) in NEC-2008. Note that paragraphs (1) and (2) align with 690.47(C)(1) and 690.47(C)(2) inNEC-2005 and paragraph (3) aligns with 690.47(C) inNEC-2008. Reviewing this proposal may assist the reader in understanding the existing 690.47(C) in the 2005 and 2008 Codes.

690.47(C) Systems with Alternating- and Direct-Current Grounding Requirements. PV systems having direct current (dc) circuits and alternating current (ac) circuits with no direct connection between the dc grounded conductor and ac grounded conductor shall have a dc grounding system. The dc grounding system shall be bonded to the ac grounding system by one of the methods listed in (1), (2), or (3).

This section shall not apply to ac PV modules.

When using the methods of (2) or (3), a visual inspection shall be made to ensure that the existing ac grounding electrode system meets the applicable requirements of Article 250, Part III.

FPN No. 1: ANSI/Underwriters Laboratory Standard 1741 for PV inverters and charge controllers requires that any inverter or charge controller that has a bonding jumper between the grounded dc conductor and the grounding system connection point have that point marked as a grounding electrode conductor (GEC) connection point. In PV inverters, the terminals for the dc equipment grounding conductors and the terminals for ac equipment grounding conductors are generally connected to or electrically in common with a grounding busbar that has a marked dc GEC terminal.

FPN No.2: For utility-interactive systems, the existing premises grounding system serves as the ac grounding system.

(1) Separate DC Grounding Electrode System Bonded to the AC Grounding Electrode System. A separate dc grounding electrode or system shall be installed, and it shall be bonded directly to the ac grounding electrode system. The size of any bonding jumper(s) between ac and dc systems shall be based on the larger size of the existing ac grounding electrode conductor or the size of the dc grounding electrode conductor specified by 250.166. The dc grounding electrode system conductor(s) or the bonding jumpers to the ac grounding electrode system shall not be used as a substitute for any required ac equipment grounding conductors.

Exception: Where the existing ac grounding electrode is not readily accessible, the bonding conductor shall be permitted to be connected to the ac grounding electrode conductor as close as possible to the ac grounding electrode with an irreversible splice.

(2) Common DC and AC Grounding Electrode. A dc grounding electrode conductor of the size specified by 250.166 shall be run from the marked direct-current grounding electrode connection point to the ac grounding electrode. This dc grounding electrode conductor shall not be used as a substitute for any required ac equipment grounding conductors.

Exception: Where the existing ac grounding electrode is not readily accessible, the dc grounding electrode conductor shall be permitted to be connected to the ac grounding electrode conductor as close as possible to the ac grounding electrode with an irreversible splice.

(3) Combined DC Grounding Electrode Conductor and AC Equipment Grounding Conductor. An unspliced, or irreversibly spliced, combined grounding conductor shall be run from the marked dc grounding electrode conductor connection point along with the ac circuit conductors to the grounding bus bar in the associated ac equipment. This combined grounding conductor shall be the larger of the size specified by 250.122 or 250.166 and shall be installed in accordance with 250.64(E).

Photo 5. External grounding electrode terminal

While any of the three methods of making connections to the inverter grounding electrode terminal may be used, there are advantages and disadvantages to each.

Method 1, in the above proposal, (similar to 690.47(C)(1) in NEC-2005) has the advantage of routing surges picked up by the array more directly to earth than methods 2 and 3. However, since the bonding conductor between the new dc grounding electrode must be bonded to the existing premises ac grounding electrode, there is the size, routing and cost of that conductor to consider.

Method 2 (similar to 690.47(C)(2) in NEC-2005) uses fewer components than the other two methods and also routes surges to earth without getting near the ac service equipment.

Photo 6. Equipment grounding conductors as leads attached to the inverter in conduit

Method 3 (similar 690.47(C) in NEC-2008) combines the inverter ac equipment grounding conductor with the dc grounding electrode terminal and thereby uses less copper. However, the requirement to bond the conductor at the entrance and exit of each metallic conduit and enclosure may become difficult with conductor sizes greater than about 6 AWG, especially since the conductor must remain unspliced or irreversibly spliced. Also, any surges picked up by the array will be routed directly to the service equipment and may be more likely to enter the premises wiring system than when grounding electrode conductors are routed more directly to ground.

Summary

Proper grounding connections at the inverter are critical to a safe and properly operating PV system. These connections may be the only connections that the entire system has to earth. All connections must be made and that may prove difficult if manufacturers have not included the proper number of terminals.

In the next Perspectives on PV, we will cover the ac output circuits of the utility-interactive inverter.

For Additional Information

If this article has raised questions, do not hesitate to contact the author by phone or e-mail. E-mail: mailto: jwiles@nmsu.edu Phone: 575-646-6105

A color copy of the latest version (1.9) of the 150-page, PhotovoltaicPower Systems and the 2005 National Electrical Code: Suggested Practices, written by the author, may be downloaded from this web site: http://www.nmsu.edu/~tdi/Photovoltaics/Codes-Stds/Codes-Stds.html

The Southwest Technology Development Institute web site maintains a PV Systems Inspector/Installer Checklist and all copies of the previous "Perspectives on PV” articles for easy downloading. Copies of "Code Corner” written by the author and published in Home Power Magazine over the last 10 years are also available on this web site: http://www.nmsu.edu/~tdi/Photovoltaics/Codes-Stds/Codes-Stds.html

The author makes 6–8 hour presentations on "PV Systems and the NEC” to groups of 60 or more inspectors, electricians, electrical contractors, and PV professionals for a very nominal cost on an as-requested basis. A schedule of future presentations can be found on the IEE/SWTDI web site.


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Tags:  Featured  Perspectives on PV  September-October 2009 

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The Inverter

Posted By John Wiles, Wednesday, July 01, 2009
Updated: Friday, January 18, 2013

In our top-to-bottom perspective of a PV system, we have arrived at the inverter. The utility-interactive inverter is a key element in the PV system that helps to ensure safe and automatic operation of the system.


Photo 1. Inverters

Peak Power Tracking

A PV array is a current source of energy and the output power depends on the load that the inverter places on the array. No loading (zero current) would operate the array at the open-circuit voltage point (Voc) and the heaviest loading (a short-circuit, not achievable) would operate the array at the short-circuit current (Isc) point. Neither of these operating points would produce any power output from the array. However, for every condition of sunlight intensity (irradiance) and array temperature, there is a load that will extract the maximum power from the array that the array can produce under those conditions. The utility-interactive inverter will find that maximum or peak power point and track that point as the sunlight and temperature vary throughout the day.

Automatic Operation

Today’s utility-interactive (U-I) inverter is designed, manufactured, tested and certified/listed to operate automatically in the PV system. It seamlessly converts dc power from the PV array into ac power that is fed into the utility supplied premises wiring system. The output of the inverter is functionally connected in parallel with the premises wiring and the utility service.

One of the most important aspects of the inverter is the anti-islanding circuit. The anti-islanding circuit is designed to keep the utility electrical system (both premises wiring and utility feeder) safe in the event that the utility is being serviced or is disconnected at some point in the transmission system, distribution system or premises wiring system.

Unlike the engine-driven generator, which can feed power into a blacked out/disconnected local utility feeder system, the anti-islanding system prevents the inverter from energizing the "dead”power system.

This circuit prevents the inverter from delivering ac power if the utility voltage and frequency are not present, or if they are not within narrowly defined limits. This circuit monitors the voltage and frequency at the output terminals of the inverter. If the voltage varies more than plus ten percent or minus twelve percent from the nominal output voltage the inverter is designed for (120, 240, 208, 277, or 480 volts), the inverter cannot send power to the output terminals. Nor, is there any voltage on these terminals when the inverter shuts down. In a similar manner, if the frequency varies from 60 Hz more than 60.5 Hz or less than 59.3 Hz, the inverter also cannot send power to the ac output. If the utility power is suddenly not present at the output terminals for any reason (inverter ac output disconnect opened, service disconnect opened, meter removed from the socket, utility maintenance, or utility blackout), the inverter senses this and immediately ceases to send power to the output terminals.

The anti-islanding circuit in the inverter continues to monitor the ac output terminals and when the voltage and frequency from the utility return to specifications for a period of five minutes, the inverter is again able to send PV power to the ac output. When the inverter is not processing dc PV power into ac output power, it essentially disconnects from the PV array by moving the load on the PV system to a point where there is no power. Usually this is the Vocpoint for the PV array.

Circuit Sizing

DC: The dc input circuit to the inverter is sized based on the dc short-circuit current in those conductors. Normally the PV array is rated in watts at standard test conditions (STC) of 1000 watts per square meter (W/m2) of irradiance and a cell temperature of 25 degrees Celsius (°C). In most cases, the array will operate, on average, at a lower power output due to the normal and expected power lost due to module heating. For this reason, inverter manufacturers typically suggest sizing the PV array (STC dc watts) at ten to twenty percent greater than the inverter ac power output rating. It does no short-term harm to connect an even larger PV array to the inverter since the inverter must limit its output to the rated value no matter how much array power is applied. If this over-sized array is used, the inverter will spend more operating time each day at rated power output than it would with a normally sized array. The penalty for designing a system in this manner will be increased module cost for the larger array, some lost power on sunny, cool days, and possibly some slight reduction in the inverter life due to longer high temperature operating temperatures.


Photo 2. Cold weather increases Voc

AC: The ac output circuit of an inverter must be sized at 125 percent of the rated output current of the inverter (690.8). Some inverter manufacturers specify the rated current or a range of values (due to varying line voltages from nominal). If this specification is not given, then the rated power may be divided by the nominal line voltage to determine a rated current. For example, a 2500-watt inverter operating at a nominal voltage of 240 volts would have a rated current of

2500 watts/240 volts = 10.4 amps

These inverters are not capable of providing any sustained (more than a second) surge currents, so the rated output current is all that can be delivered. When faced with a short-circuit, the rated output current is all that can be delivered, but more than likely, the reduced line voltage due to the fault will cause the inverter to shut down.

Dedicated Circuit

NEC 690.64(B)(1) requires that the inverter output be connected to the utility power source at a dedicated disconnect and overcurrect protective device (OCPD). In most systems this is a backfed breaker in a load center/panelboard [690.64(B)]. Inverters may not have their outputs connected directly to another inverter or directly to an ac utility-supplied circuit without first being connected to the dedicated disconnect/OCPD. The utility-interactive micro inverters and the ac PV module are an exception to this rule since they are tested and listed to have multiple inverters connected in parallel on a single circuit with only one OCPD/disconnect device for the entire set of inverters.

The OCPD must be sized at a minimum of 125% of the rated inverter output current (or total of the output rated output current from multiple micro inverters or ac PV modules) and it must protect the circuit conductor from overcurrents from the utility side of the connection. It is usuallynota good idea to install a larger OCPD than the minimum required value (allowing a round up to the next standard value is OK and needed) because the inverter may (as part of the listing/instructions) be using the OCPD to protect internal circuits.

Is It a Branch Circuit?Out #@$%^ Typo!Consider the typical residential branch circuit.

1. It is protected by an OCPD at the source of power (the utility) that can damage it (emphasis added).

2. If the breaker protecting the branch circuit is opened, it becomes completely "dead” (deenergized).

3. If the branch circuit has a solid ground fault or a line-to-line fault, the OCPD will open and protect the conductor.

4. The branch circuit may be wired with Type NM cable in residential applications.

Now consider the circuit between the utility-interactive inverter and the dedicated disconnect/OCPD (usually a breaker).

1. This circuit is protected by an OCPD at the source of power (the utility) that can damage it. Since the circuit is sized at 125 percent of the rated output current of the inverter and the inverter current is limited to the rating, the inverter is not a source of power that can damage the conductor.

2. If the breaker protecting this circuit is opened, it becomes completely "dead” (deenergized).

3. If this circuit has a solid ground fault or a line-to-line fault, the OCPD will open and protect the conductors. And the inverter will shut down.

4. It would appear in every practical sense that this utility-interactive inverter ac output circuit is just like a branch circuit and it, too, may be wired with Type NM cable in residential applications.

So, these ac output circuits from the utility-interactive inverters can be wired like any other branch circuit in a residence. Of course, the inverters are surface-mounted devices and there may be the possibility of exposed Type NM cables being subject to physical damage. If they are, then conduit or other wiring method would be required.

There are no flush mounted inverters on the market yet, but I expect they will appear with internal fans and vents to get rid of the heat they generate.

The typo

Inspectors. There is a typo in 690.31(E) of the2005 NECand2008 NEC. I am the guilty party that let it slip through—on two code cycles. The first sentence starts out:


Photo 3. Cold weather increases module voltages and sunlight helps too.

"Where direct-current source or output circuitsofa utility interactive inverter from a building-integrated or other photovoltaic system ….”

The word "of” should be "to” and will be corrected in the2011 NEC.The requirement for metallic raceways applies only to the sunlight-energized dc PV source or PV output conductor.

GFCIs and AFCIs

The ac output of a utility-interactive inverter should not be connected to a GFCI or AFCI breaker as these devices are not designed to be backfed and will be damaged if backfed. These devices have terminals marked line and load and have not been identified/tested/listed for back feeding.

Summary

A detailed understanding of PV equipment and how power flows in a PV system should enable better, more thorough inspections of these systems. Better inspections will result in better, safer PV installations. We will continue with more information on the utility-interactive inverter ac output in the next "Perspectives on PV” in our top-to-bottom tour of the PV system.

For Additional Information

If this article has raised questions, do not hesitate to contact the author by phone or e-mail. E-mail: jwiles@nmsu.edu Phone: 575-646-6105

A color copy of the latest version (1.9) of the 150-page,Photovoltaic Power Systems and the 2005 National Electrical Code: Suggested Practices, written by the author, may be downloaded from this web site: http://www.nmsu.edu/~tdi/Photovoltaics/Codes-Stds/Codes-Stds.html

The Southwest Technology Development Institute web site maintains a PV Systems Inspector/Installer Checklist and all copies of the previous "Perspectives on PV” articles for easy downloading. Copies of "Code Corner” written by the author and published inHome Power Magazineover the last 10 years are also available on this web site: http://www.nmsu.edu/~tdi/Photovoltaics/Codes-Stds/Codes-Stds.html

The author makes 6–8 hour presentations on "PV Systems and theNEC” to groups of 60 or more inspectors, electricians, electrical contractors, and PV professionals for a very nominal cost on an as-requested basis. A schedule of future presentations can be found on the IEE/SWTDI web site.

This work was supported by the United States Department of Energy under Contract DE-FC 36-05-G015149


About John Wiles: John Wiles works at the Southwest Technology Development Institute (SWTDI) at New Mexico State University. SWTDI has a contract with the US Department of Energy to provide engineering support to the PV industry and to provide that industry, electrical contractors, electricians, and electrical inspectors with a focal point for code issues related to PV systems. He serves as the secretary of the PV Industry Forum that will be submitting 30+ proposals for Article 690 in the 2008 NEC. He provides draft comments to NFPA for Article 690 in the NEC Handbook. As an old solar pioneer, he lives in a stand-alone PV-power home in suburbia with his wife, two dogs, and a cat - permitted and inspected, of course.

Tags:  Featured  July-August 2009  Perspectives on PV 

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Approaching the Inverter

Posted By John Wiles, Friday, May 01, 2009
Updated: Friday, January 18, 2013

/01/In our top-to-bottom perspective of a photovoltaic (PV) system, we are still on the dc circuits from the PV array and are approaching the inverter. There are always a few details that get overlooked in designing, installing and inspecting these systems.


Photo 1. Enphase 175 Watt Micro inverter

The Conductors

We have noted previously that single-conductor, exposed cables (type USE-2 or the new PV cable/PV wire) will be used for the module interconnecting cables. Both of these cable types will generally be available only in basic black. And as 200.6(A)(2) notes, this black cable, even when smaller than 4 AWG, may be marked white as a grounded conductor at the time of installation.

Normally the exposed single-conductor cables are transitioned to a conduit wiring method when the circuits leave the PV array. Conductors in conduit, while they could be USE-2/RHW-2 (for flame and smoke retardant) or PV wire, are typically THHN/THWN-2 because they are less costly and the -2 rating is needed for the outdoor, wet environment and the high temperatures of conduit in sunlight [310.15(B)(2)]. Unfortunately, 14–10 AWG conductors with THHN/THWN-2 insulation are not widely available due to low demand. (I’ll get e-mails telling me differently if I am misinformed). Of course THHN/THWN is available, but it doesn’t have a wet, 90°C rating. I expect that demand will increase for the small-conductor THHN/THWN-2 cables as inspectors start applying 310.15(B)(2) to roof top HVAC installations. Due to the limited availability of 14–10 AWG THHN/THWN-2, I tend to support the use of USE-2/RHW-2 in the conduits with a white marking although theCodedoes not clearly state that it can be used with markings in that location. XHHW-2 would also be a suitable alternative.


Photo 2. Xantrex 100kW Inverter

Although most PV arrays installed in the recent past have had the dc negative conductor grounded (and colored white), new arrays may have the dc positive conductor grounded and colored white. Of course, there are no designated color codes for the ungrounded conductor, but common sense would indicate that on a negatively grounded array with the negative conductor colored white, the positive, ungrounded conductor would be most clearly marked and understandable if it were colored red. However, many installations use a black positive conductor and that is still acceptable under theCode. In the positively grounded systems where the positive grounded conductor is colored white, the ungrounded negative conductor would be most clearly understood if it were black.

Now, and in increasing numbers in the next few years, the use of transformerless inverters will dictate the use of ungrounded PV arrays (690.35) and then we can go to a "red is positive” and "black is negative” color coding since there will be no grounded conductor.

Oh yes. We should not ignore the newest bipolar PV arrays and bipolar inverters. In these systems, we will have red, positive conductors, black, negative conductors, and white, grounded conductors.


Photo 3. SATCON 1 MegWatt Inverter

As before, the grounded conductor in a PV dc disconnect should never be switched, although bolted, isolated, terminal-block connections are acceptable.

Wiring Methods, Continued

All circuits in a PV system, as in other electrical systems, must be wired using a Chapter 3 or 690.31 method that is suitable for the application and the environment. However, there are frequently questions about the circuits between the dc PV disconnect and the inverter. As far as theNECis concerned, if these circuits are in protected environments, they could be wired with type NM cable. Of course, local codes may dictate other requirements such as the need for using raceways inside commercial structures for all electrical wiring.

The Inverter

Utility-interactive inverters range in size from 175 watts (photo 1) to 1 megawatt and come in all shapes, sizes and colors (photos 2 and 3). New models are being introduced monthly. These inverters will be listed by UL, CSA, ETL, and TUV Rheinland, all of whom are designated as nationally recognized testing laboratories (NRTL) by OSHA for testing and listing PV modules, inverters, combiners, and charge controllers using standards published by UL.


Photo 4. Solectria 13kW Inverter with external disconnects

Some inverters have only a single set of dc input terminals. With these designs, an external dc PV disconnect must be installed. Even if the inverter has more than one set of input terminals for parallel separate strings (source circuits) of modules, external dc PV disconnects must be used on each input. (See photo 4).


Photo 5. Inverter with internal disconnects as part of the inverter

Other inverters have internal dc disconnects or disconnect housings that attach to the main inverter section containing the electronics package. The method used to mount the internal disconnects, the ease and accessibility of the disconnects, and the manner in which they are separated from the inverter proper vary from brand to brand and from product to product. The installer and the AHJ must reach a mutual conclusion on the suitability of these disconnects for meeting the various disconnect requirements in theCode.

Since the inverters are listed with the disconnects, it can be presumed that the disconnects are properly rated for the dc load break operation. If the inverter is installed in a location that meets the 690.14 requirements for the main PV dc disconnect, then it would appear that the internal disconnect would meet this requirement.


Photo 6. Inverter with internal disconnects that can be separated from inverter

Meeting the requirement for maintenance disconnects (690.15) will require additional considerations. If the inverter were to require factory service, can the energized PV source or output circuits be disconnected from the inverter safely when there is no external disconnect? If a disconnect housing is attached to the inverter and that housing does not have to be removed to service the inverter, then some degree of safety is assured. However, if the energized conductors must be disconnected from internal switches and pulled through small conduit knockouts, the situation must be examined carefully. Will qualified people who know how to disable the array be doing the removal? Or will the unqualified person try to pull energized conductors through the knockouts? (See photos 5 and 6).

DC Input Fusing

Some models of both small (<10kW) and large (>100 kW) inverters have dc input fuses mounted inside the inverter or inside a combiner/disconnect device attached to the inverter. The smaller fuses (30 amps or less) are usually mounted in "finger-safe” fuse holders that allow the fuse to be safely replaced in an un-energized state.


Photo 7. PV combiner with disconnect at output

However, when the fuse rating goes over 30 amps with values as high as 400 amps or more, these fuses are mounted in exposed fuse holders or bolted directly to a dc busbar. One side of each fuse is tied together with the dc input of the inverter. The other side of each fuse is hardwired to the output of a PV dc combiner and these combiners will be scattered throughout the PV array—sometimes over acres of real estate. Although the inverter can be turned off and the dc input capacitors allowed to discharge (up to five minutes), each fuse is still energized from its own input and the combined inputs of all of the other fuses through the common bus bar. The only way to safely service these fuses is to go through the entire PV array, find all of the combiners, and open or pull each and every source circuit fuse (those less-than-30 amp "finger safe” fuse holders). An optional disconnect at the output of every combiner speeds this process and makes servicing the combiner fuses safer, but all disconnects must be located and opened. (See photo 7).


Photo 8. Disconnects for each input installed near the inverter

When these fuses are present in the input of the larger inverters, the safest way to provide for service is to install a dc disconnect near the inverter on each dc input to a fuse. (See photo 8). These collocated disconnects can be easily opened, and with the inverter turned off, the fuses can be safely removed in a de-energized state.Removedis a term used to describe prying out 400-amp fuses with a screwdriver because you have broken two plastic fuse pullers trying to remove them from those very tight fuse holders.

Summary

A detailed understanding of PV equipment and how power flows in a PV system should enable better, more thorough inspections of these systems. Better inspections will result in better, safer PV installations. We will continue with more information on the utility-interactive inverter in the next "Perspectives on PV” in our top-to-bottom tour of the PV system.

For Additional Information

If this article has raised questions, do not hesitate to contact the author by phone or e-mail. E-mail: jwiles@nmsu.edu Phone: 575-646-6105

A color copy of the latest version (1.9) of the 150-page,Photovoltaic Power Systems and the 2005 National Electrical Code: Suggested Practices, written by the author, may be downloaded from this web site: http://www.nmsu.edu/~tdi/Photovoltaics/Codes-Stds/Codes-Stds.html

The Southwest Technology Development Institute web site maintains a PV Systems Inspector/Installer Checklist and all copies of the previous "Perspectives on PV” articles for easy downloading. Copies of "Code Corner” written by the author and published inHome Power Magazineover the last 10 years are also available on this web site: http://www.nmsu.edu/~tdi/Photovoltaics/Codes-Stds/Codes-Stds.html

The author makes 6–8 hour presentations on "PV Systems and theNEC” to groups of 60 or more inspectors, electricians, electrical contractors, and PV professionals for a very nominal cost on an as-requested basis. A schedule of future presentations can be found on the IEE/SWTDI web site.


Read more by John Wiles

Tags:  Featured  May-June 2009  Perspectives on PV 

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Still on the Roof

Posted By John Wiles, Sunday, March 01, 2009
Updated: Monday, January 21, 2013

In our top-to-bottom perspective of a PV system, we need to look at one more component usually located on the roof. This is the PV source-circuit combiner and it will be followed in the dc circuit by the PV dc disconnecting means.

The PV Combiner

The PV source-circuit combiner is found on larger residential systems and on most large commercial systems. PV systems that have a dc rating above about 6 kW may have sufficiently large numbers of modules that more than two strings of modules are required to get the desired array power. Since module voltages range widely and module power ratings can vary from 40 watts to 300 watts, there are no hard and fast rules relating the need for a dc combiner to a specific number of modules in an array.

Photo 1. PV source-circuit combiner with fuses

Photo 1. PV source-circuit combiner with fuses

Multiple Strings May Need Combiner

The rated output voltage of the PV modules and the inverter dc input characteristics determine how many modules may be connected in a series string. See "PV Math” in the January-February 2009IAEI News.The power rating of each module and the number of modules in a string determine the power rating of a string. The desired array power rating and the power rating of the inverter determine how many strings can be connected in parallel. Normally two strings can be connected in parallel without requiring a combiner containing overcurrent devices [See 690.9(A) Ex]. If more than two strings are needed, then overcurrent protection on each string may be required and these overcurrent devices are placed in a PV source-circuit combiner. See "Questions from the AHJ—To Fuse or Not to Fuse” in the May-June 2008 IAEI News for the details.


Photo 2. PV source-circuit combiner with circuit breakers' poor workmanship

A combiner may use either fuses (typically on high-voltage, utility-interactive systems) or circuit breakers (used on systems operating at a nominal 48 volts or below). [See photos 1, 2, and 3].

It should be noted that the combiners shown in photos 1 and 3 have exposed circuit terminals and busbars near the overcurrent devices. They are not dead front when opened, and voltages on the exposed terminals and busbars may approach 600 volts on many systems in cold weather. Although not dead front, these combiners meet the "intent” of the NEC where a tool is required to get access to energized surfaces (terminals and busbars). In these cases, the combiners have screw-on covers and the tool required to open the combiner is a screwdriver. NEC-2008 requires that combiners be listed and UL has determined that the listing must be to UL Standard 1741 (the PV inverter standard) [690.4(D)]. Although listing is required, UL 1741 has not yet been modified to specifically require that combiners be dead front. Some of the newer units are dead front, and eventually that requirement will be in the standard. See photo 4 of a dead front unit that has terminal covers made of clear plastic.

Photo 3. PV source-circuit combiner on large system

Photo 3. PV source-circuit combiner on large system

Wiring from the PV Array to the PV Disconnect

Although the conductors between the modules and the return circuit from one end of a string to the other are permitted to be single conductor cables in free air, as soon as these circuits leave the array location they must transition to a standard chapter 3 wiring method. That wiring method must be suitable for the hot, wet environment found on roofs, and sunlight resistance is also a must. Electrical metallic tubing (EMT) is frequently used. The ampacity of the conductors is based on the short-circuit current being carried in that circuit and must be corrected for the conditions of use. In many cases, terminal temperature limitations on combiners or fused disconnects may dictate further ampacity corrections. See the "The Nature of the PV Module: Limited Currents Have Benefits and Drawbacks,” September-October 2007 IAEI News for more details.

An equipment grounding conductor should be run with the circuit conductors in the conduit. Where the PV source circuits are unfused, the size under the 2005 NEC was based on 125 percent of the rated short-circuit current (Isc) for the circuit. In the 2008 NEC, Isc is used directly in Table 250.122 to select an equipment-grounding conductor. The reduction in size of the dc equipment-grounding conductor is due to the 2008 NEC requirement that nearly all PV systems have ground-fault detectors that will limit ground-fault currents in the equipment grounding conductors. On systems with PV source or output circuit fuses, the normal procedure of using the fuse value in Table 250.122 is used (690.45).

Photo 4. Dead front PV source-circuit combiner (photo credit AMTec Solar)

Photo 4. Dead front PV source-circuit combiner (photo credit AMTec Solar)

In many systems, the equipment grounding conductors may be as small as 14 AWG between the PV modules. However, in areas where winds, snow, ice and other environmental factors are significant, a larger equipment grounding conductor should be considered to provide additional mechanical protection (690.46).

The dc PV Disconnect

The dc PV disconnecting means (PV disconnect) should be installed in a readily accessible location, either inside or outside the building at the point of first penetration of the conductors (690.14) (photo 5). Since Section 690.31(E) allows the PV source or output conductors to penetrate the building surface on the roof (if they are routed in a metal raceway inside the building), it appears that the PV disconnect can be mounted inside the building in any readily accessible location. However, this NEC allowance may not be the safest option or even very clearly defined in the national Code.

Photo 5. PV dc disconnecting means

Photo 5. PV dc disconnecting means

This parallel wording of 690.14(C)(1) to the requirements for the ac service disconnecting means 230.70(A)(1) may need further examination since in the world of ac utility-power, removal of the ac revenue meter can effectively disable the ac power in a structure where the ac service disconnect is inside a locked structure. With a dc PV disconnect inside a locked structure, the readily accessible definition may not be appropriate. Many jurisdictions require that the PV disconnect be located within sight of the ac service disconnect or meter on residences. This is usually on the outside of the building. On commercial buildings, the PV system may be some distance from the ac service disconnect, and a directory may be used to show the location of all disconnects, both ac and dc (705.10).

The disconnect should break all ungrounded conductors, but should not open a grounded conductor. Grounded conductors in PV systems may be either the negative or positive source-circuit conductors and should have white insulation, or where larger than 6 AWG, be marked with a white marking. The type of module used determines which circuit conductor should be grounded and the inverter must be compatible with that polarity of grounded conductor. The dc bonding jumper in a utility-interactive PV system is commonly inside the inverter and is a part of the ground-fault detection/interruption systems required by 690.5.

If the grounded source-circuit conductor is opened by the switch in the disconnect, the marked grounded conductor becomes ungrounded and may be energized with respect to ground up to the open-circuit voltage of the system. This represents an unsafe condition for people servicing the PV array and for that reason the Code prohibits the use of disconnects, breakers or fuses in grounded PV dc conductors unless they are part of an automatic ground-fault detection/interruption system (690.13).

Photo 6. Disconnect Labels

Photo 6. Disconnect Labels

Photo 6 shows the front of a PV dc disconnect with the labels required by 690.17 and 690. 53. The 690.17 warning is required because the load terminals of this disconnect are connected to the inverter dc input which may be energized for up to five minutes after the disconnect has been opened. The filter and energy storage capacitors in the inverter will be discharged after this time. The 690.53 label with the dc voltage and current ratings will allow the AHJ to determine if the correct cables have been installed.

Power flow in a PV system is from the PV array through the dc PV disconnect, the inverter, the ac disconnect and finally to the grid. This power flow sometimes confuses installers on how to properly connect the dc and ac disconnects. Note the upper line-side terminals on the disconnect shown in photo 7 are covered by an insulated cover. Also note the switchblades, the fuse holder terminals (if any), and the load-side lower terminals are exposed and easily touched. A general safety rule is that the most dangerous circuits should be connected to the protected line-side terminals. If this is done, it is less likely that energized terminal connections will be accidentally touched when the door of the disconnect is open. In the PV dc disconnect, the PV source or output circuits should always be connected to the line-side terminals. The dc input to the inverter is connected to the load-side terminals and the 690.17 warning label is required as shown in photo 6.

Photo 7. Line and load connections are important

Photo 7. Line and load connections are important

For the ac PV disconnect the circuit connected to the utility source should be the line side of the disconnect with the inverter ac output connected to the load side. No warning label is required because, when the disconnect is opened, the inverter ceases to produce power within a fraction of a second and the exposed load side terminals pose no shock hazard.

Summary

Attention to theCoderequirements in 690 and other articles plus an understanding of PV equipment and how power flows in a PV system should enable these systems to be installed and operated in a safe manner. The utility-interactive inverter is next on our top-to-bottom tour of the PV system.


For Additional Information

If this article has raised questions, do not hesitate to contact the author by phone or e-mail. E-mail: jwiles@nmsu.eduPhone: 575-646-6105

A color copy of the latest version (1.9) of the 150-page, Photovoltaic Power Systems and the 2005 National Electrical Code: Suggested Practices, written by the author, may be downloaded from this web site: http://www.nmsu.edu/~tdi/Photovoltaics/Codes-Stds/Codes-Stds.html

The Southwest Technology Development Institute web site maintains a PV Systems Inspector/Installer Checklist and all copies of the previous "Perspectives on PV” articles for easy downloading. Copies of "Code Corner” written by the author and published inHome Power Magazineover the last 10 years are also available on this web site: http://www.nmsu.edu/~tdi/Photovoltaics/Codes-Stds/Codes-Stds.html

The author makes 6–8 hour presentations on "PV Systems and the NEC” to groups of 60 or more inspectors, electricians, electrical contractors, and PV professionals for a very nominal cost on an as-requested basis. A schedule of future presentations can be found on the IEE/SWTDI web site.

This work was supported by the United States Department of Energy under Contract DE-FC 36-05-G015149


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Tags:  Featured  March-April 2009  Perspectives on PV 

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