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Posted By Steve Douglas,
Wednesday, May 01, 2013
Updated: Tuesday, April 30, 2013
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Residential load calculations first appeared in the Canadian Electrical Code Part I (CE Code) in the second edition dated 1930. In the 1930 edition, the load calculation rules were in Section 6 Conductors. The calculations were quite different from the present day calculations. For residential installations, the calculations were based on the number of branch circuits being installed instead of the dwelling floor area. In the fourth edition, dated 1939, a demand factor table for lighting load based on the floor area was added. In the sixth edition dated 1953, the Conductor Section including the load calculation requirements were moved to Section 4 Conductors. This calculation format remained until the seventh edition, dated 1958. A new section called Circuit Loading and Demand Factors including the single dwelling load calculations was added as Section 8 to the eighth edition of the CE Code dated 1962.
Photo 1. Single dwelling units as defined in the CE Code Part I
In the 1958 edition, calculations were similar to the calculations of today with the main difference in the calculations being the basic load. The basic load covering lighting and convenience outlets was 3,500 W for a residence with a living area up to 500 ft² (46.5 m²). For a residence with a living area over 500 ft² and up to 1500 ft² (139.4 m²), the basic load was 4,500 W with an additional 1000 W for each 1000 ft² (93m²) or portion thereof. The 1958 basic load requirements stayed the same until the twelfth edition dated 1975 when the basic load was increased to the requirements of the current code.
Rule 8-200 of the 2012 CE Code covers load calculations used to determine the minimum feeder or service size for single dwelling units. To start off, we should establish what a dwelling unit is. Section 0 defines a single dwelling unit as "a dwelling unit consisting of a detached house, one unit of row housing, or one unit of a semi-detached, duplex, triplex, or quadruplex house,” and a dwelling unitas "one or more rooms for the use of one or more persons as a housekeeping unit with cooking, eating, living, and sleeping facilities.” Subrule (1) of Rule 8-200 is divided into two items (a) and (b). Item (a) details the specific criteria for calculations and Item (b) mandates the absolute minimum allowable ampacity of the service or feeder size – based on the floor area. 100 A is required by the Code where the floor area of the single dwelling, exclusive of basement floor area, is 80 m² (861 ft²) or more, and 60 A is the minimum permitted service/feeder ampacity – where the floor area of the single dwelling, exclusive of basement floor area, is less than 80 m². Item (a) is further divided into seven items (i) to (vii).
Items (i) and (ii) detail a basic load for the dwelling unit. This basic load includes 120 V 15- and 20-amp convenience outlets, lighting loads and motor loads rated up to 1500 W. The basic load for the dwelling unit is 5000 W for the first 90 m² (968 ft²) of living area plus an additional 1000 W for each 90 m² or portion thereof in excess of 90 m². The living area is determined as 100% of the ground floor, 100% of any area used for living purposes on the upper floor, plus 75% of the basement area of the dwelling unit.
The next step in Item (iii) is to add the electric space-heating and air-conditioning loads. Where it is known that the installed electric space-heating and air-conditioning loads will not be used simultaneously, the larger of the electric space-heating load or the air-conditioning load is added to the base load. For electric space-heating systems consisting of electric thermal storage heating, duct heater, or an electric furnace, the connected heating load is calculated at 100% of the equipment ratings. Where the electric heating installation is provided with automatic thermostatic control devices in each room or heated area, the electric space-heating load is 100% of the first 10 kW of connected heating load plus the balance of the connected heating load at a demand factor of 75%.
Photo 2. A 11 kW electric range used in the article example
The next loads to add are any electric ranges.Item (iv) allows 6000 W for a single range to be added to the basic load provided the range does not have a rating in excess of 12 kW. In the event the electric range is rated more than 12kW, 40% of any amount exceeding 12 kW will need to be added as well.
Now we add water heaters. Item (v) indicates any electric tankless water heaters or electric water heaters for steamers, swimming pools, hot tubs, or spas are added to the basic load at 100% of equipment ratings.
Item (vi) is new for the 2012 CE Code and requires that any electric vehicle charging equipment loads also be added to the basic load at 100% of equipment ratings.
The final step in Item (vii) is to add any additional loads at 25% of the rating of each load with a rating in excess of 1500 W if an electric range has been provided for, or 100% of the rating of each load with a rating in excess of 1500 W up to a total of 6000 W plus 25% of the load in excess of 6000 W if an electric range has not been provided for.
Photo 3. The nameplate of a 1500 W microwave oven
Photo 4. A 1500 W microwave oven used in the article example
As an example we will look at a 269 m² (2900 square foot) residence with the following loads:
- lighting load
- 4 small appliance branch circuits
- laundry circuit 1500 W
- natural gas heating
- air conditioner 6000 VA
- electric range 11,000 W
- hot tub 8000 W (2 hp motor)
- Level II electric vehicle charger 7200 W
- electric dryer 5000 W
- garbage disposal 800 W
- microwave 1500 W
- dishwasher 1200 W
- electric water heater 4500 W
The calculated load for the 2900 ft² (269 m²) single dwelling in this example is 173.1 amps.
The basic load is calculated based on the floor area of the single dwelling. The load for the first 90 m² is 5000 W, leaving 179 m² of floor area. The next 90 m² has a load of 1000 W, and an additional 1000 W for the remaining 89 m². The total basic load of 7000 W includes the lighting, convenience receptacles, small appliance branch circuits, laundry circuit, garbage disposal, and the dishwasher.
The heating of the single dwelling is a gas furnace and no electric heat is installed, leaving the 6000 W air-conditioning load added with a demand factor of 100%.
The electric range for this single dwelling is less than 12 kW providing a load for the calculation of 6000 W.
The 8000 W hot tub and the 7200 W electric vehicle charging equipment are now added with a demand factor of 100%.
Any additional loads with a rating in excess of 1500 W are now added with a demand factor of 25%. In this example, the additional loads over 1500 W are the 5000 W dryer and the 4500 W storage type water heater; 25% of the 9500 W gives us 2375 W to be added to the calculation.
Table 1. Summary of the single dwelling service load calculation
Now that we know the calculated load, we can determine the minimum service and conductor size. The ampacity of the load is 173.1 amps (41550 / 240 = 173.1). In most installations, the continuous load on a service is limited to the continuous load rating of the equipment being used. Subrule (3) of Rule 8-104 considers all loads continuous unless it can be shown that in normal operation the load will not persist for a total of more than one hour in any two-hour period for loads not exceeding 225 amp, or a total of more than three hours in any six-hour period for loads in excess of 225 amp. In the case of single dwelling units Subrule (2) of Rule 8-200 allows these loads to be considered as a non-continuous load for application of Rule 8-104. However, although Section 86 considers the EV charging equipment to be a continuous load, when this load is calculated for the purpose of defining the ampacity of a service in a single dwelling, such EV charging equipment load is not considered as continuous load, similarly to all other loads under this Rule. This means 100% of the calculated load for a single dwelling can be used to determine the service equipment ampere rating. For our 269 m² example a standard rating of 175 amp overcurrent device could be selected in the service box as the calculated load is 173.1 amp. Typically 175 amp rating of the overcurrent device will necessitate installation of the 200 A rated service box, as 175 A rating for the service fused disconnect or the service circuit breaker for residential installations is not available. In most cases a 200-amp service would be installed with a 175-A or 200-A trip setting or rating. Let’s consider that the trip setting of the service overcurrent device was selected at 200 A.
The size of the service conductors are now established using Rules 4-004, 4-006, and 14-104. As all distribution equipment presently available has a temperature limitation of 75⁰C, Rule 4-006 requires the allowable ampacity to be based on the 75⁰C column of either Table 2 or 4 for conductors installed in a raceway. The smallest 75⁰C conductor allowable ampacity from Tables 2 and 4 for the calculated load of 173.1 amp are 2/0 copper with an allowable ampacity of 175 amp, or 4/0 aluminum with an allowable ampacity of 180 amp.
Photo 5. The nameplate for a 1200 W dishwasher used in the article example
The next step is to verify the conductor selected meets the requirements of Rule 14-104. Rule 14-104 requires the overcurrent device to have a setting not higher than the allowable ampacity of the conductors being protected. Where the conductor allowable ampacity does not correspond with the overcurrent protection commercially available, Table 13 provides details on acceptable limits for over protection settings. Based on the fact that a 200-amp service will be installed with a main 200-amp trip setting for the breaker, Table 13 limits the conductor allowable ampacity to be not less than 176 amps. In the case of the copper conductors, the 2/0 copper conductor with the allowable ampacity of 175 amp is undersized. At this point some code users will try to apply "the 5% rule” in Subrule (1) of Rule 8-106 to the 175 allowable ampacity of Table 2. Subrule (1) of Rule 8-106 allows loads calculated in accordance with Section 8 to be within 5% of the allowable ampacity of the conductors selected. This means the 5% allowance can be applied to a calculated load. Applying the "5% rule to a conductor allowable ampacity table is a misapplication of Subrule (1).
In summary, the minimum conductor size allowed for the 200-amp service where the conductors are installed in a raceway for the example in this article is either 3/0 copper or 4/0 aluminum. If the main breaker of this service was reduced to 175 amp the minimum copper conductor size could be reduced either 2/0, and the aluminum conductor would remain at 4/0.
Read more by Steve Douglas
Posted By Steve Foran,
Wednesday, May 01, 2013
Updated: Tuesday, April 30, 2013
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In the early 90s, utilities were in the midst of massive change as downsizing, right-sizing—or whatever you called it — swept the continent. Driven by technology, fewer people were needed to get the same work done and from this emerged an industry called Process Re-engineering.
Our utility was engulfed in change. Our new business processes resulted in many changes in responsibilities for many people, but one affected our department very significantly. At the time we were responsible for technical training associated with revenue metering.
The proposed change would reduce both travel and the number of people needed to deliver metering services to residential customers by combining two separate job functions together into a single job. I cannot recall the exact numbers, but for illustrative purposes it was projected that we could combine the work of 15 meter installers and 95 meter readers into, say, 100 multi-disciplined metering workers, resulting in a net reduction of 10 people.
The challenge was that the technical competence required in the newly created position was higher than that of the 95 meter readers.
To safely perform their duties, meter installers must understand the meter nameplate, know how to identify the proper device for a service and be competent to work around energized equipment. Quite simply, the meter readers were not competent to do this work.
A comprehensive training program was developed and delivered. It covered many aspects of the residential service which included both theoretical and practical components where employees had to demonstrate their competency.
From the training, participants learned about the risks associated with metering and energized equipment. Most importantly, they obtained the knowledge and skills needed to safely manage the risks.
Of the many risks at the electrical service entrance, there is one that stands out above all others. This risk came as a surprise to every single participant in our training. In fact, none of the meter readers were aware that this risk even existed.
Most meter readers thought the greatest risk was electrical shock. Contact with 120 V is a risk; however, a far greater risk is the fault level available at the service entrance in the event of a ground fault. The potential physical harm to people and property as a result of a short circuit in a meter base can be catastrophic.
For our system, we calculated the maximum possible fault level at a 200 A 120/240 Volt service (close to a large substation, short service run, large distribution transformer, etc.). Here’s what we found: the power delivered in the event of a short circuit (even though only momentarily) is comparable to the power delivered by a typical jet engine that you see on the wings of a large airplane.
In our training, we explained this to our participants and asked them, "Would you stick a screw driver into a jet engine while it’s running? What kind of precautions would you take around a jet engine?”
A fault at a meter base has the ability to instantaneously produce the same power delivered by a jet engine. But unlike the jet engine, which makes all kinds of noise and produces so much wind that you wouldn’t dare get too close, a meter base just sits there — you can’t even tell if it is energized by looking at it.
Trainees told us that their biggest take-away was their newfound appreciation of something which they were previously unaware.
As for me, I learned that we must be open to looking at situations in new ways so we can see what was once invisible. Secondly, use appreciation (appreciation of the risk, work methods, design, etc.) to replace feelings of fear and lack of understanding.
The new service model was safely implemented and I hear from colleagues who still work at the utility that they continue to re-engineer their metering and customer services processes.
Read more by Steve Foran
Posted By Christel Hunter,
Wednesday, May 01, 2013
Updated: Tuesday, April 30, 2013
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Load calculations in the National Electrical Code have evolved over many decades. It was in the 1933 NEC that load calculation requirements began to resemble a format that the modern code user would find familiar. Since then, many things have changed, but the primary requirement remains the same — service equipment and conductors must be sized to handle the expected load.
Article 220 of theNational Electrical Code lays out the primary requirements for performing load calculations that are necessary for determining the size of a residential service. The calculations are based on the expected loads present in a dwelling unit, along with appropriate demand factors that are used to account for the diversity of electrical use by occupants. There are two methods available, standard and optional calculations. Optional calculations require fewer steps and generally result in smaller conductors, but the dwelling unit must meet more restrictive requirements. We will only be considering one-family dwelling units in this article, including single family residences, apartments, etc.
Be aware that some authorities having jurisdiction adopt the International Residential Code for One- and Two-Family Dwellings (IRC) and use the method for calculating the service size using the requirements found in Chapter 36. The IRC calculations are based on the National Electrical Code, but are not identical. Always check with your local jurisdiction to find out what method(s) are acceptable.
Photo 1. Electric range
The standard method for calculating service sizing is found in Part III of Article 220. Of course, we can’t find all the requirements in this Part, so we will also be looking at additional requirements in Articles 210, 220, 230, 250 and 310. An example load calculation using the standard method is shown in Table 1.
The first thing we need to determine is the lighting load. Table 220.12 requires that for dwelling units, we multiply the floor area (based on the outside dimensions of the dwelling unit) times 3 volt-amperes/square foot. Section 220.14(J) states that the following loads are also included in the general lighting load calculations:
- all general-use receptacle of 20-ampere rating or less, including the receptacles connected to the bathroom branch circuit required in 210.11(C)(3),
- the outdoor outlets in 210.52(E),
- the receptacle outlets in basements, garages, and accessory buildings in 210.52(G), and
- the lighting outlets required in 210.70(A), which includes habitable rooms, a variety of additional locations, and storage or equipment spaces.
Table 220.42 gives us demand factors for lighting loads. Most homes will take the first 3000 VA at 100% and the remainder at 35%. (If you are calculating a multifamily dwelling service, you might use the third demand factor category, where anything over 120,000 VA is taken at 25%.)
Small Appliance and Laundry Loads
Section 210.11(C)(1) requires a minimum of two small appliance branch circuits. Section 220.52(A) tells us that we must use a minimum load of 1500 VA for each of these circuits, but also allows the small appliance branch circuits to be included with the general lighting load when applying the demand factors in Table 220.12. Section 220.52(B) requires that 1500 VA be added for the required laundry circuit in 210.11(C)(2). This circuit can also be added to the general lighting load and demand factors may be applied.
Photo 2. Washer/dryer
Electric Dryers and Cooking Appliances
Section 220.14(B) refers us to requirements for electric dryers in 220.54 and electric cooking appliances in 220.55. Electric clothes dryers are calculated at either the minimum of 5000 watts or the nameplate rating, whichever is larger. The demand factors in Table 220.54 may be helpful if there are more than four dryers, but this is unlikely in a one-family dwelling unit, so we will not use this table for the examples in this article. Electric ranges and other electric cooking appliances (rated in excess of 1.75 kW) shall be permitted to be calculated in accordance with Table 220.55, which takes up an entire page and has five notes. There are also informational notes directing the code user to Annex D for examples. It is worthwhile to review this table and read all the notes and examples to become familiar with the various options.
If there are four or more fixed appliances in the residence, 220.53 permits all of these loads to be totaled and then a demand factor of 75% applied. Fixed-appliance loads include items such as a water heater, garbage disposal, dishwasher, microwave, etc.
Photo 3. Garbage disposal
Largest motor load
Section 220.14(C) tells us that motor loads shall be calculated in accordance with the requirements in 430.22, 430.24 and 440.6. For the service calculation, this means that we must determine the largest motor load and add 25% of its value to the total calculation. Common motor loads in residential applications include air conditioning, water pumps, disposals, blowers, etc. Often, the largest motor load in a home is the air conditioner. Even if the air conditioning is dropped from the total load calculation in favor of electric heating (see below), you may still be required to use the AC motor load for this calculation. Check with your jurisdiction to see what the policy is locally. Many jurisdictions publish residential load calculation worksheets to help with determining the size of the service.
When two loads are not likely to be energized at the same time, 220.60 allows us to use only the largest load for the calculation of the service. This is typically applied to dwelling units with both electric heating and air conditioning, since they are not expected to run at the same time.
Specific appliances or loads
There are certain loads that may be found in residences that are not included in the previous list. Section 220.14(A) requires that an outlet for a specific load or appliance not covered elsewhere must be calculated based on the ampere rating of the load served. Some examples might include a spa, RV hookup, etc. These must be included in the load calculation at their full value.
The optional method is much simpler than the standard calculation, but is restricted in 220.82 to "... a dwelling unit having the total connected load served by a single 120/240-volt or 208Y/120-volt set of 3-wire service or feeder conductors with an ampacity of 100 or greater.” Most one-family dwelling units meet this requirement, so the optional method is used frequently. An example calculation using the optional method is shown in Table 2.
For the purposes of the optional method, everything except heating and air conditioning is considered to be a general load. For this method, the general calculated load shall be not less than 100 percent of the first 10 kVA plus 40 percent of the remainder of all loads other than heating and air conditioning.
Lighting and general-use receptacles are again based on the outside dimensions of the dwelling unit multiplied by 3 volt-amperes/square foot. The small-appliance branch circuits and laundry branch circuit are each included at 1500 VA.
The next step is to determine the nameplate rating of each of the following items:
- all appliances fastened in place, permanently connected, or located to be on a specific circuit
- ranges, wall-mounted ovens, counter-mounted cooking units
- clothes dryers that are not connected to the laundry branch circuit
- water heaters
For all permanently connected motors not included in the previous list, the nameplate or kVA rating must be included in the calculation.
Heating and Air Conditioning
The largest heating and air-conditioning load must be chosen from six options:
- 100 percent of the nameplate rating of the air conditioning and cooling
- 100 percent of the nameplate rating of the heat pump when it is used with no supplemental electric heating
- 100 percent of the nameplate rating of the heat pump compressor and 65 percent of the supplemental electric heating for central electric space-heating systems (If the heat pump compressor is prevented from operating at the same time as the supplementary heat, it does not need to be added to the supplementary heat for the total central space heating load.)
- 65 percent of the nameplate rating(s) of electric space heating if less than four separately controlled units
- 40 percent of the nameplate ratings of electric space heating if four or more separately controlled units
- 100 percent of the nameplate ratings of electric thermal storage and other heating systems where the usual load is expected to be continuous at the full nameplate value.
Comparing Standard and Optional Calculations
To see how the two methods compare, let’s take a look at a 2900 square foot residence with the following loads:
- lighting load
- 4 small appliance branch circuits
- laundry circuit 1500 W
- natural gas heating
- air conditioner 6000 VA
- electric range 11,000 W
- hot tub 8000 W (2 hp motor)
- Level II electric vehicle charger 7200 W
- electric dryer 5000 W
- garbage disposal 800 W
- microwave 1500 W
- dishwasher 1200 W
- electric water heater 4500 W
The standard calculation method is shown in Table 1 and the optional calculation method is shown in Table 2. Using the standard calculation, our total load is 47,520 VA. Dividing that by 240 volts gives us 198 amps. Using the next standard service rating requires that we use a 200-amp service. Since we have a 120/240-volt single-phase dwelling service, we are allowed to use NEC Table 310.15(B)(7) and use either 2/0 AWG copper or 4/0 AWG aluminum service conductors.
Using the optional calculation, our total calculated load is 34,160 VA. Dividing that by 240 volts gives us 142 amps. Using the next standard service rating requires that we use a 150-amp service. Once again, we are allowed to use NEC Table 310.15(B)(7), which requires either 1 AWG copper or 2/0 AWG aluminum conductors.
For this example, it is clear that the optional calculation permits a smaller service. From a practical perspective, due to equipment availability, it is likely that a 200-amp service will be installed rather than a 150-amp service.
Neutrals are permitted to be smaller than the phase conductors in most residential service installations. Section 220.61 requires that the neutral load be determined by calculating the maximum unbalanced load between the neutral and any one ungrounded conductor. The values used for calculating the neutral size when using the standard or optional methods will often be different, as shown in Tables 3 and 4.
Section 230.42 states that the grounded conductor for a service shall not be smaller than the minimum size as determined in accordance with 250.24(C). If we have a single raceway (as is most common for service conductors), 250.24(C)(1) tells us that the conductor cannot be smaller than specified in NEC Table 250.66, but is not required to be larger than the ungrounded conductors.
For our standard service calculation, our minimum ungrounded conductor size was a 2/0 AWG copper or a 4/0 AWG aluminum. Using NEC Table 250.66 would require a neutral no smaller than a 4 AWG copper or a 2 AWG aluminum. In Table 3, we found that our calculated neutral load is 28,035 VA. Dividing that by 240 volts gives us 117 amps, which will require either a 2 AWG copper or 1/0 AWG aluminum from Table 310.15(B)(7). These sizes are larger than the required minimum, so we choose one of these conductors.
For our optional service calculation, our minimum ungrounded conductor size was a 1 AWG kcmil copper or 2/0 AWG aluminum. Using NECTable 250.66 would require a neutral no smaller than 6 AWG copper or a 4 AWG aluminum. In Table 4, we found that our calculated neutral load is 30,320 VA. Dividing that by 240 volts gives us 126 amps, which will require either a 1 AWG copper or 2/0 AWG aluminum from NEC Table 310.15(B)(7). Since these sizes are larger than the required minimum, we would choose one of these conductor sizes.
Note that for this example in our optional method calculation, the neutral conductor is the same size as our phase conductors. However, if a 200-amp service is installed based on the standard calculation, the neutral is significantly smaller due to the calculation method. Table 5 shows a summary of the ungrounded and neutral conductor sizes for our example using both the standard and optional calculation methods.
To accurately calculate the service size for residential installations, the designer and installer must be familiar with many requirements in the National Electrical Code. The requirements are not necessarily straightforward, and it is recommended that additional resources be reviewed. Available resources include the examples in Informative Annex D of the NEC, the IAEI publication One- & Two-Family Dwelling Electrical Systems, and other published examples.
310.15(B)(7) – Changes for the 2014 NEC
residential services, the service conductors and main power feeders are allowed
to be sized based on Table 310.15(B)(7) instead of Table 310.15(B)(16), which
permits a smaller size conductor to be used in many cases. This allowance has
been in the NEC since the 1950s in recognition of the fact that only a
small portion of the electrical loads in homes are typically used at the same
time, so the load on the service conductors at any one time is generally much
smaller than the total calculated load.
The language in
Section 310.15(B)(7) and the associated table have been a subject of great
debate in code-making panel 6 (CMP-6) over the last few cycles. CMP-6 has
considered each of the proposals and comments received over the last few years
and come up with new wording to address the concerns and suggestions submitted.
CMP-6 has agreed to
delete the existing wording and table and replace them with the following
For one-family dwellings and the individual dwelling units of two-family
and multifamily dwellings, service and feeder conductors supplied by a single
phase, 120/240-volt system shall be permitted be sized in accordance with
310.15(B)(7)(a) through (d).
(a) For a service rated 100 through 400 amperes, the service conductors
supplying the entire load associated with a one-family dwelling or the service
conductors supplying the entire load associated with an individual dwelling
unit in a two-family or multifamily dwelling shall be permitted to have an
ampacity not less than 83% of the service rating.
(b) For a feeder rated 100 through 400 amperes, the feeder conductors
supplying the entire load associated with a one-family dwelling or the feeder
conductors supplying the entire load associated with an individual dwelling
unit in a two-family or multifamily dwelling shall be permitted to have an
ampacity not less than 83% of the feeder rating.
(c) In no case shall a feeder for an individual dwelling unit be required
to have an ampacity greater than that of its 310.15(B)(7)(a) or (b) conductors.
(d) Grounded conductors shall be permitted to be sized smaller than the
ungrounded conductors provided the requirements of 220.61 and 230.42 for
service conductors or the requirements of 215.2 and 220.61 for feeder
conductors are met.
Note No. 1: It is possible that the conductor ampacity will require other
correction or adjustment factors applicable to the conductor installation.
Informational Note No. 2: See example DXXX in Annex D.
In effect, the same
size conductors that are allowed in the 2011 NEC will still be allowed
in the 2014 NEC, assuming that temperature correction factors or
adjustment factors are not required for the installation. The changes to the
code language were necessary to take into account certain limitations inherent
in the language in previous code cycles. Because Table 310.15(B)(7) is based on
service or feeder ratings and not the temperature rating of conductors, there
is no clear way to apply adjustment or correction factors for installations at
higher temperatures or if there are more than three current-carrying conductors
in a conduit.
It should be noted that
the conductor sizing will still be based on the service or feeder rating, not
the calculated load. For example, if you have a calculated load of 184 amps and
are required to install a 200-amp service, the conductors would be required to
have an ampacity of 166 amps or more:
200 amps times 83 percent equals 166 amps. So, for a 200-amp service,
you would still be allowed to choose a 4/0 AWG aluminum or 2/0 AWG copper, but
you would choose it from the 75 degree C column in Table 310.15(B)(16).
Read more by Christel Hunter
Posted By Randy Hunter,
Friday, March 01, 2013
Updated: Wednesday, February 13, 2013
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In Part One of Article 240, we left off with a basic review of circuit protection. Now that we have an understanding of how it operates, let’s get back to Part II of Article 240 which starts at 240.21 Location in Circuit. Here we find a very simple statement which requires the overcurrent protection to be located at the point where ungrounded conductors receive their supply. As simple as this is, we naturally have exceptions which will allow taps to be made under certain conditions. For this section, a tap is a conductor which is connected to a system; however, it is not sized to handle the ampacity of the upstream overcurrent protective device. We will cover the limited allowable conditions. But first, the last sentence of 240.21 needs to be emphasized; it explains that under the tap allowances in 240.21(A) through (H), you may not have an additional tap made to any of these allowed taps, or as I used to tell inspectors, "You can’t tap a tap.”
Feeder Taps Rules
In 240.21(A) Branch-Circuit Conductors, we find a method of performing taps for branch circuits, but note that you are referred back to Article 210.19 and 210.20 for the exact applications. In subsection (B) we start to deal with the Feeder Taps, which are further broken down to five conditions. I’m not going into details on each of these sections, just a brief overview of the different rules; so again, I will challenge you to open your code book and review the specific conditions for each of these allowances. First, we have a tap rule not over 10 feet in length. Please note that when we have a length in this section it is the actual length of the conductor from connection point to connection point. The 10-foot rule has four conditions which must be met. Note that if these conductors leave the enclosure from which they are tapped, the conductors must have the minimum ampacity of 1/10th of the protective device from which the tap is being made.
The second tap rule is for a 25-foot long tap, and in my experience this is the most common tap rule that I have seen applied. Here the intent is to have the conductors extend from the tap location to another piece of equipment. There are three simple rules: the ampacity of the conductors is at least 1/3 of the protective device ahead of the tap; the conductors terminate in a single overcurrent device that limits the load they can carry; and the conductors are protected in an approved raceway or other means. As I mentioned, this is the most common tap used in my experience. It is commonly used in a case where the equipment has a load that is not close to its rating, but there is no more room for sub-feed devices. We had a situation years ago in a commercial laundry where a 1200-amp switchboard fed several pieces of equipment, and they needed to install some additional washers. The actual load on the 1200-amp board was only about 700 amps, but there was no remaining space for additional overcurrent units. The solution was to use the 25-foot tap rule and install a panelboard with a main breaker. The trick was that there was a door right next to the 1200-amp unit, so by the time we made the tap, ran the conduit, and set the new panel on the other side of the door, the 25-foot rule barely made it.
The third tap rule deals with transformer taps which include the primary and secondary conductors in the 25-foot allowance. The fourth rule provides an allowance for taps over 25 feet when applied in high bay manufacturing buildings where the walls are 35 feet tall, which would make overhead taps impossible under the other rules when the distance from the tap to a piece of equipment is over 25 feet. Again there are specific conditions to follow for each rule, so please review them. The last feeder tap rule is for outside taps, while the taps covered so far are assumed to be inside facilities. However, if the taps are located outside of a building or structure, the tap can be unlimited in length. Please read the details in 240.21(B)(5).
Photo 2. In this condition, if the feeders to this transformer were taps then according to 240.21(B)(3) the length of the transformer feeders and the secondaries together shall not be over 25 feet in length.
Rules for Protection
In 240.21(C) through (H), rules for protection of transformer secondaries, service conductors, busway taps, motor circuits, generators and battery conductors are covered. In keeping with the combination inspector direction of these articles, I have just detailed the most common cases. I’ve never tried to memorize any of the tap rules, as there are simply too many details and conditions, so please remember to utilize the code book and check each installation you are inspecting.
Locations and Accessibility
Next we will jump to 240.24 Location in or on Premises. We will hit the highlights here, the first being that the overcurrent devices have to be readily accessible and shall be installed so that the center of the grip of the operating handle is not more than 6 feet 7 inches in its highest position from the floor or working platform. This is an enforcement issue and is often caught in the field.
In (B) we discover that each occupant shall have ready access to the overcurrent devices protecting the conductors supplying that occupancy, and this applies to both residential and commercial. We have a couple of conditions which may modify this. One is a facility that has continuous electrical building maintenance and supervision, and this may apply to various facilities such as guest rooms or similar locations. The overcurrent devices shall be located where they will not be subject to physical damage. This may be a judgment call, but one that comes to mind is storage facilities which set their electrical equipment on the outside of the storage buildings where they are subject to damage by various vehicles. Here we usually use some type of bollard to provide the protection needed.
The last three conditions for locations requires the locations not to be near ignitible materials, not to be located in bathrooms of dwelling units, dormitories, guest rooms or guest suites, and the last condition to avoid is over steps or stairs. The thought here is that we want to provide a stable and level working area for anyone who may have to service or operate the overcurrent devices.
Photo 3. Here is an example of providing physical protection to this equipment by the use of bollard posts.
Plug Fuses, Fuseholders and Adapters
In Part V of Article 240 we have requirements for Plug Fuses, Fuseholders, and Adapters. We will cover this since many of the combination inspectors working in parts of the country that have older facilities may still find in-use plug fuses, more commonly referred to as screw-in fuses. First is a limitation that these will only be used on circuits not exceeding 125 volts between conductors or on a system having a neutral point where the line-to-neutral voltage doesn’t exceed 150 volts. This really applies only to branch circuits. Each fuse and, if used, adapter will be marked with its ampere rating. Further, if 15 amps or lower, they will be identified by a hexagonal configuration of the window or other prominent part to allow them to be identified from other fuses of higher ratings.
Next, we need to visit some screw-in fuse basics. First, they shall have no exposed energized parts after the fuses have been installed, and second, the screw shell they screw into shall be connected to the load side of the circuit. With this, let’s consider this further. If the screw shell was connected to the line side of a circuit and you screwed in a fuse, if your fingers contacted the outer portion of the fuse you would get shocked. So this is a critical requirement to make sure the screw shell is not energized until the fuse has been completely installed.
Photo 4. Edison-base fuses are shown in the bottom four examples; please note the hexagonal configuration of the 15 amp units. The top fuses are examples of Type S fuses with the appropriate adapter shown in the middle; note the spring tang which prevents the adapter from being unscrewed once installed.
Edison-base fuses are covered in 240.51; however, these are only to be used for replacements in existing installations where there is no evidence of overfusing or tampering. This is because Edison-base fuses are totally interchangeable, so you can replace a 15-amp fuse with a 20-amp fuse or a 30-amp fuse. That interchangeability leads to applications where misapplications may occur due to availability issues or ignorance of proper circuit protection.
So to solve this issue, Type S Fuses were invented to limit the size of the fuse that can be installed through unique physical dimensions to the screw portion of the device. This is covered in 240.53 and they have three ranges 0–15, 16–20 and then 21–30 amps. This makes it so they are not interchangeable; and if you have a circuit which is using 20-amp conductors, you can’t install a 30-amp fuse. Now to follow up with this additional safety provision, they made adaptors for Type S Fuses so they could be installed in standard Edison-base fuse holders. These adapters have a spring tang on them so that once an adapter has been installed it can’t be removed, thus providing the same limitations and additional safety of the Type S fuses in older installations. These adapters are covered in Article 240.54.
Photo 5. These are renewable fuses — on top is the interior of a 200-amp fuse and below is a 20-amp fuse, as mentioned in 240.60(D).
Cartridge Fuses and Fuseholders
In Part VI we cover Cartridge Fuses and Fuseholders. In 240.60 we have general rules for cartridge fuses which cover the voltage limitations and the need for unique sizing to make it difficult to install a fuse into a fuse holder which is designed for a lower current. Also, the marking requirements are covered, which should help both the contractors and inspectors to verify we have the proper fuse for the application.
Renewable fuses are cartridge fuses which disassemble and allow one to replace the internal fuse element. As I travel around, I often ask if anyone has seen these and only those who have been in the field for some time have usually heard of them and even fewer have seen them. The last item to cover in the fuse area is that fuses that are rated 600 volts nominal or less shall be permitted to be used for voltages at or below their ratings.
Coming down to the end of overcurrent devices, we step into the circuit breakers in Part VII. Circuit breakers come in many sizes and options, and they have the ability to be reset once they are tripped if they have been installed in the proper application. Breakers shall be capable of being opened manually, and they shall clearly indicate whether they are in the open (off) or closed (on) position. If the handles operate in the vertical, then the "up” position of the handle shall be the "on” position. This is sometimes a challenge when doing service work, because some breakers have various orientations which can be used that may not provide the "on” position to be up. Breakers also have a requirement that they are not subject to tampering, or unauthorized alteration of the factory calibration, unless intended for adjustment.
Marking of breakers must show the amperage rating in a manner that will be durable and visible after installation. In some cases the marking may be under a trim or cover; however, generally the amperage is listed on the operating handle. Units made for 100 amps or less shall have this marking on the handle or escutcheon areas. The interrupting rating of breakers shall be marked on the breaker. It might be referred to as AIC (amps interrupting capacity) or IR (interrupting rating). If it is not marked, then it is assumed the breaker is only listed to be used in an installation which has an available fault current of 5000 amperes or less. This is a reminder to make sure the overcurrent devices used are listed for the available fault current at that point of the distribution system.
If a breaker is to be used as a switch, then it has to listed for this usage and the marking will either be a SWD (switched) or HID (high-intensity discharge). These applications usually happen in locations such as warehouses or small retail shops. So if the only method of turning on the lights is the breaker, then it would make sense that you have to enforce the switch rating requirement. If they are utilizing high-intensity discharge lighting, the breakers have to be marked with the HID rating.
Applications in 240.85, reminds us that we have to use breakers that are compatible with the system voltages in each installation. Please see this section of the code regarding the use of straight rated breakers and slash rated breakers. Most of us deal with solidly grounded systems, which require slash rated (120/240V or 480Y/277V) breakers.
Breaker Series Ratings
Breaker Series Ratings is found in 240.86, which allows breakers to be installed under some very special conditions. Series rating is a way to install breakers which will rely on the upstream device to protect it in the case of a high-fault current. This allows breakers that are not rated for the available fault current to be utilized only in two conditions.
One is under engineering supervision in existing installations, which at times happens when work is being done to older systems. Here the engineer who is qualified may apply a series rating if the system is properly documented and field marked to identify it is a series system.
The second is under the condition of 240.86(B), Tested Combinations, that allows series rating if the combination of devices have been tested and certified to operate together to provide the protection of the system from the available fault current. This is often done to lower the cost of the equipment package. This requires that the system be identified as a series system and the documentation is maintained on-site for the inspector and future electricians who may have to service the installation. This is a very complicated item because when it is used and any additions or modifications are made, the original requirements of the series rating still apply and must be followed.
In my jurisdiction, we had a strip mall constructed using a series rated system using brand X gear. As each individual suite had their tenant improvements done, each contractor performing the work for the new tenant had to match the exact requirements and brand of the series system. One contractor who installed brand Y sub-panels learned the hard way and had to change out two panels to keep the system installed within its series rating. If a system has a motor load connected on the load side of the higher rated device and on the line side of the lower rated device, or if the motor load is more than 1 percent of the rating of the lower rated device then a series system is not allowed to be used. So if you have a series rated system, review the code and insure the conditions are met.
The last subject to cover in this article on overcurrent protection is fairly new to the code. In 240.87, Noninstantaneous Trip, a circuit breaker may be used that doesn’t have an instantaneous trip feature or has that feature turned off. The instantaneous portion of an overcurrent device gives the device the ability to react as fast as possible to a short-circuit condition within the system. The instantaneous portion of the overcurrent devices also reduces the incident energy at a point of fault within the system.
You might ask why would we do this; in cases where breakers are used to selectively coordinate a system, the upstream device may have the instantaneous set higher or turned off so the downstream device has the time needed to operate. So as the upstream device waits for the downstream unit to do its job, the current will not be limited and will continue to flow for an extended period of time increasing the incident energy at the fault location. Under these conditions we must provide one of three equivalent means, (1) zone-selective interlocking, (2) differential relaying, or (3) energy-reducing maintenance switching with local status indicator. The purpose of these three options is to reduce the stress on the equipment but most of all for the protection of personnel who may be working on the system.
These requirements are unique to breakers and do not apply to fused systems. From my experience when we dealt with systems like this, we asked for third party verification that the electrical system was installed properly and that any adjustments to breakers were done properly and coordinated to function together. After the third party report was done, the engineer of record would review it and then submit it to the AHJ for inclusion in the approved set of plans for the project.
This concludes Article 240. I hope you have had your code book open as we continued, due to the fact that I cover only the issues which I feel you need to have a basic knowledge of as a combination inspector. It is always good for you to review the portions I skipped; this will help you when a situation may occur in your area and you will recall that you heard or read something about that, and you’ll be able to find it that much easier in the code book.
Read more by Randy Hunter
Posted By Steve Douglas,
Friday, March 01, 2013
Updated: Wednesday, February 13, 2013
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The fourth edition of the SPE-1000 Model Code for the Field Evaluation of Electrical Equipment (SPE-1000) is scheduled to be published by June of this year. The SPE-1000 was first published in 1994 providing requirements used by inspection bodies accredited by the Standards Council of Canada when field evaluating unapproved electrical equipment. Used in conjunction with the requirements of the Canadian Electrical Code (CE Code), Part I, the SPE-1000 document provides construction requirements for the equipment being evaluated along with testing criteria, and minimum marking for equipment nameplates, warning and caution notices. The SPE-1000 does not cover the field evaluation of equipment for use in hazardous locations, medical electrical equipment, equipment connected to line voltage in excess of 46 kV, and individual stand-alone components such as conductors, cable, wiring devices, switches, relays, and timers.
Photo 1. Testing of a photovoltaic module
The changes in the fourth edition focused on the addition of specific requirements for industrial control equipment, high-voltage equipment, solar photovoltaic modules, wind turbines, inverters, and additional requirements for instrument transformers, energy usage metering devices, and associated equipment. During these additions, the SPE-1000 Working Group identified the need for general requirements for transformers, motors, switches, receptacles, enclosures for outdoor use, conductor ampacities, and supplementary protectors. New clauses have been added for the allowable ampacity for wires and cables. These new clauses reference allowable ampacities in the CE Code Part I for conductors external to control panels and to a new table that matches Table 18 from CSA Standard C22.2 No 14 Industrial Control Equipment for conductors within control panels. In addition the temperature limitations of CE Code Part I Rule 4-006 have been added to align the SPE-1000 with the CE Code Part I and existing equipment testing in CE Code Part II standards. For supplementary protectors a new definition has been added to the definition section of the SPE-1000 in Clause 2.2, and a new clause has been added detailing the limited acceptability for supplementary protectors.
New clauses have been added for industrial control equipment. These clauses include details for wire bending space including new tables from C22.2 No 14, disconnection means for control panels, protective devices, control transformer protection, minimum allowable conductor size, permanent connection provisions, terminal size requirements, enclosure thermal insulation limitations, observation windows, intrinsically safe equipment requirements, and additional marking requirements for control panels.
Photo 2. 27.6kV Switchgear modified to metering specification of the supply authority then field evaluated to the requirements in the SPE-1000
The high-voltage equipment requirements added to the SPE-1000 are limited to certified deadfront indoor enclosed and an outdoor enclosed assembly of switchgear and components that have been modified, and then only where adequate testing and review is performed or where documentation is provided to demonstrate that all type testing and mechanical review have been conducted as detailed in CSA Standard C22.2 No. 31 or CSA TIL D-25, as applicable, on a similar design of equipment. These clauses for high voltage equipment are not intended to approve a new design of high-voltage equipment where all relevant sections of CSA C22.2 No. 31 or another recognized document (ORD) are deemed necessary. The requirements in the clauses for high voltage equipment include construction, grounding and bonding, conductor spacing, circuit breakers and switching devices, force-cooled equipment, pad-mounted switchgear, and requirements switchgear intended to be used for service entrance. Testing for high voltage equipment includes all tests from CSA Standard C22.2 No 31 Switchgear, and additional requirements for short-circuit withstand rating, and dielectric strength testing.
New clauses have been added for solar photovoltaic modules. These clauses provide separate direction regarding additional testing and markings required for modules that have been either certified to UL 1703 or tested in accordance with IEC 61730-1 and 61730-2. Modules without UL 1703 certification or appropriate IEC 61730-2 testing cannot be field evaluated using the SPE-1000.
Wind turbines clauses have also been added to the SPE-1000. These clauses include details for generators and motors, generator overcurrent protection, wind turbine components, overcurrent protection for all components of the wind turbine, emergency blade pitch control power supplies, capacitors, low and extra low voltage cables, high voltage cables, grounding and bonding, lightning protection, disconnection means, and safety controls. In addition, clear direction is given for the main nameplate, and warning and caution markings.
Specific requirements for inverters/converter have also been added to the SPE-1000 with separate details for inverter components, and utility interconnected inverters. These clauses tie closely with the requirements and testing in CSA Standard C22.2 No 107.1 General Use Power Supplies.
Photo 3. Modified distribution panel with CTs installed for non-utility metering; ready for a field evaluation inspection
Additional requirements for instrument transformers, energy usage metering devices, and associated equipment have been added to the SPE-1000. These clauses were added as a direct result of the addition of Subrule (4) for CE Code Part I Rule 12-3032 allowing enclosures for overcurrent devices, controllers, and externally operated switches to be used as a raceway for wiring associated with instrument transformers and energy usage metering devices. These new clauses address the wiring space, wire bending space, and additional marking requirements when current transformers CTs are added to a panelboard.
In addition to the changes detailed in this article, tables have been added for
- Wire bending space
- Wiring space
- Full-load motor-running currents in amperes corresponding to ac horsepower ratings
- Full-load motor-running currents in amperes corresponding to dc horsepower ratings
- Allowable ampacities of insulated copper conductors inside industrial control enclosures
- Maximum acceptable rating of primary overcurrent device for control transformers
- Maximum acceptable rating of secondary overcurrent device for control transformers
- Minimum spacings for high-voltage power circuits
- Impulse and corona-extinction test voltages for high-voltage switchgear assemblies
- Dielectric strength test voltages for high-voltage switchgear assemblies, and
- Voltage and frequency operation limits
Two new figures have also been added, one for an articulated finger probe, and one for the typical arrangement of intrinsic safety barriers.
One more major change is in the process and scheduling of future editions of the SPE-1000. Future changes for the SPE-1000 will use a similar approach as the CE Code Part I in that separate proposals will be addressed when submitted then sent for ballot. The existing process held proposals until a sufficient number were received, and then a working group would work on all the proposals at once. The new process will address proposals sooner in an effort to encourage more proposals. The cycle for the SPE-1000 will also change to a three year cycle with new editions being published following the publication of the CE Code Part I.
Read more by Steve Douglas
Posted By Barry O’Connell,
Friday, March 01, 2013
Updated: Wednesday, March 13, 2013
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In September 2012, both UL and ULC withdrew certification for Electrical Circuit Protective Systems (FHIT and FHITC) that employed fire resistive cables. This included UL Classified Fire Resistive Cable (FHJR), UL Listed cable with "-CI” suffix (Circuit Integrity), and ULC Listed Fire Resistant Cable (FHJRC). Certification was retained for systems that used protective materials like intumescent wraps, tapes, composite mats, etc.
Fire resistive cables are used for emergency circuits in many applications, including high-rise buildings and places of assembly. Emergency circuits include feeders for fire pumps, elevators, smoke control equipment, fire alarm systems and other similar circuits. These circuits are required by the National Electrical Code and the Canadian National Building Code to have a 2-hour fire rating. This added level of survivability is intended to allow sufficient time for building occupants to exit a building during an emergency and to provide uninterrupted power for fire fighting equipment and emergency communication systems.
There are two types of fire resistive power cable systems: polymer insulated cables that require conduit protection and armored cables that do not. Armored cable types include both mineral-insulated and metal-clad cable. The events that led to the certification withdrawal were based on systems employing polymer insulated cables, not armored cable.
In 2011, UL was informed of an issue with using polymer insulated fire-resistive cables in conduit systems coated with zinc. UL confirmed that a problem existed and issued a notice stating that fire-resistive cables should be used only with components free of zinc. UL expanded their research and conducted extensive additional testing that showed an unacceptable level of variability with all non-armored polymer insulated cables.
These findings led to the conservative decision to withdraw all certifications, including armored cable systems, even though there was no indication that similar issues existed with either metal-clad or mineral-insulated cables. Shortly thereafter, however, UL offered an interim test program to manufacturers of fire-resistive cable for possible re-certification of existing products.
After UL/ULC withdrew all fire resistive cable certifications, a joint meeting of the UL Standard Technical Panel on Fire Resistive Cables and the ULC Standards Committee on Fire Tests was arranged. The meeting took place on October 24, 2012, in Ottawa, Canada, where the committee reviewed available information and agreed to form task groups to evaluate and update the fire resistance test standards for cable systems. This process will include additional testing and review, and a revised standard is likely to take at least two years to complete.
Because mineral-insulated cable construction is completely different from the cable-in-conduit technology under investigation, UL/ULC worked with Pentair Thermal Management to reinstate Pyrotenax mineral-insulated cable as a 2-hour fire-rated cable system. The process included reviewing MI cable designs in detail, detailed technical explanations of the critical design factors related to mineral-insulated cable’s fire resistance, and extensive fire tests, in accordance with the interim test program performed at the UL facility in November and December of 2012.
On December 21, 2012, UL and ULC re-established certification of fire-resistive cables used in electrical circuit integrity systems. The first system to be included is Pyrotenax Mineral Insulated Cable, and it has been assigned a new identification: System No. 1850. Information can be found at www.ul.com in the Online Certifications Directory.
Read more by Barry O'Connell
Posted By Jim Dollard,
Wednesday, February 13, 2013
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There were 120 proposals submitted to raise the 600-volt threshold in the NEC to 1000-volts in the 2014 NEC cycle. These proposals were submitted by the High Voltage Task Group (HVTG), which was appointed by the NEC Correlating Committee. The HVTG was charged to review all NEC requirements and/or the lack of requirements for circuits and systems operating at over 600-volts.
FPN reference to NESC deleted
The origin of this task group began at the end of the 2008 NEC cycle when a fine print note (FPN) [now Informational Notes] referencing the National Electrical Safety Code (NESC) was deleted from 90.2(A)(2). The substantiation provided in the proposal to delete the FPN stated that "conductors on the load side of the service point are under the purview of the NEC, and the FPN sending NEC users to the NESC creates serious confusion for designers, installers and the authority having jurisdiction (AHJ) working on premises wiring at voltage levels over 600-volts”. The proposal was supported by comments that pointed out conflicts that place the AHJ in a very difficult position and the FPN was deleted in the 2008 NEC. The NEC Correlating Committee then appointed the High Voltage Task Group to address issues with installations over 600 volts”.
Article 399 created
In the 2011 NEC, a proposal submitted by the HVTG created a new Article 399, Outdoor Overhead Conductors over 600 Volts. This new article was developed to provide the AHJ with NEC requirements to address the outdoor installations referenced in 90.2(A)(2) without a broad reference to another standard. It is interesting to note that most of the requirements in Article 399 are performance based. The requirements for conductor support and structures outline the installation requirements to consider without being prescriptive. In each case there must be documentation of an engineered design submitted by a licensed professional engineer engaged primarily in the design of such systems. This new article permits an engineer to design the installation in accordance with NESC requirements provided the design is documented and available to the AHJ.
Photo 1. XHHW cable labeled 600 volts
Raising the voltage threshold
The work of the High Voltage Task Group continued into the 2014 NEC cycle with a primary focus on raising the voltage threshold in the NEC from 600 to 1000 volts. This is not the first coordinated attempt to raise the voltage threshold. In the 1990 NEC revision cycle, a Correlating Committee task group tried unsuccessfully to raise the voltage threshold. Finding substantiation on how the NEC settled at 600 volts is difficult. The threshold was raised from 550 to 600 volts in the 1920 NEC. In 1990 it was difficult to substantiate a need to raise the voltage threshold. Today, emerging technologies are operating at just over the 600-volt threshold. We need product standards and installation requirements to facilitate their safe installation. The electrical industry is changing rapidly and codes/standards must keep pace; otherwise, we will be forced to use the International Electrotechnical Commission (IEC*) products and installation standards other than the NEC.
Is going from 600 to 1000 volts the right number? Small wind electric systems often operate at 690 volts AC but solar photovoltaic (PV) systems are currently being installed at dc voltages over 600 volts up to and including 1000 volts, 1200 volts, 1500 volts, and 2000 volts dc. These dc systems are expanding and have become a more integral part of many structures. Small wind electric systems and solar photovoltaic (PV) systems are employed regularly in and on all types of structures from dwellings units to large retail and high rise construction.
Photo 2. Cube fuses labeled 600 VAC or less
The first direction that the Higher Voltage Task Group took was to simply suggest revisions in Chapter 6 of the NEC for Special Equipment. It was quickly understood that changes throughout the NEC were required. Chapter 6 requirements simply modify and/or supplement the rules in Chapters 1 through 4. A review of the UL White Book reveals that UL has many products that are utilized in these systems rated at and above 600 volts, including but not limited to, 1000-V dc PV switches, 1500-Vdc PV fuses, and 2000-V PV wiring. The HVTG realized that the NEC must recognize those products through installation requirements or we will continue to have problems installing and inspecting systems for PV and small wind. The HVTG proposals to raise the voltage threshold recognize emerging technologies that are in many cases operating at just over 600 volts. Everyone needs to play a role in this transition. The present NEC requirements would literally require that a PV system operating at 1000 volts dc utilize a disconnecting means rated at 5 kV. The manufacturers, research and testing laboratories, and the NEC must work together to develop installation requirements and product standards to support these emerging technologies.
Moving the NEC threshold from 600 volts to 1000 volts will not, by itself, allow the immediate installation of systems at 1000 volts. Equipment must first be tested and found acceptable for use at the higher voltage(s). The testing and listing of equipment will not, by itself, allow for the installation of 1000-volt systems. The NEC must include prescriptive requirements to permit the installation of systems that are over 600 but less than or equal to 1000 volts. It will take both tested/listed equipment and changes in our installation code, the NEC, to meet the needs of these emerging technologies that society demands.
Eighty-two percent of the proposals submitted to raise the voltage threshold were accepted in some form. Where a code-making panel felt there was a safety issue or where manufacturers did not want to pursue having their products evaluated at 1000 volts, the Higher Voltage Task Group agreed to reject.
Moving the NEC to 1000 volts is just the beginning. This is just the first step of many to recognize emerging technology with prescriptive requirements to ensure that these systems and products can be safely installed and inspected in accordance with the NEC.
*The IEC is a nonprofit, nongovernmental international standards organization that prepares and publishes international standards for electrical and electronic technologies.
Read more by Jim Dollard
Posted By Randy Hunter,
Friday, January 18, 2013
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Overcurrent protection is a subject on which we could write volumes; however, our objective here is to cover the basics in order to provide the information needed for the combination inspector. This is actually a fun portion of training, as we usually take apart devices and explore how they operate. Check out the included photos that illustrate some of the details that we usually look at in training classes, and don’t be hesitant about disassembling equipment (that you don’t plan to install later!) to see what is inside.
To make sure we understand our topic, we need to start with the scope of this article. It provides the general requirements for overcurrent protection and overcurrent protective devices not more than 600 volts, nominal. There are two parts to Article 240 that we will not address: Part VIII dealing with supervised industrial installations and Part IX dealing with over 600 volts. Combination inspectors are generally not involved with these installations, so we will leave those topics to articles and books specifically concerned with those subjects.
Photo 1. Overcurrent protection comes in many types, sizes and shapes
As with most NEC articles, we need to start with some unique definitions. Article 240 has only three definitions, and they are located in 240.2. First we have a definition of current-limiting overcurrent protective device, which is a device that when interrupting currents in its current-limiting range will reduce the current flowing downstream to a level much less than if there were just a solid conductor having comparable impedance. Current-limiting devices are very instrumental in reducing incident energy (arc flash, arc blast) in our electrical systems, making them safer for personnel and providing protection of equipment.
The second definition deals with a term that is also used in other parts of the code. Quite often we have a question as to what exactly is an "industrial installation”? In 240.2, we have a definition of supervised industrial installation with a list of conditions which must be met to fall under this definition. Note that this definition is specifically limited to use in Part VIII of Article 240; this means that these limitations do not apply to any other code provision where supervised industrial installation (or similar term) is used. You can’t use this definition when applying 392.10(B), for example, which allows certain cable tray wiring methods in industrial establishments. Since we are not covering Part VIII in depth, I simply point this out so that you know that this definition is very limited in application.
Photo 2. Here is a very good example of a neutral main bonding jumper that was never properly connected after the completion of the ground fault testing. This caused several problems within this facility, including voltage fluctuations and equipment failures.
The last definition is that of a tap conductor as used in this article. This definition is needed as we have various tap rules within Article 240 with very specific rules. Simply put, a tap conductor is a conductor other than a service conductor that has overcurrent protection ahead of it which is oversized compared to the normal requirements found in Article 240.
Basic minimum overcurrent protection
So, let’s get down to the most basic of rules for overcurrent protection. The go-to article here is 240.4. Other than flexible cords, flexible cables and fixture wire, we refer to the ampacities of the conductors as specified in Article 310.15, unless covered in specific applications as described in 240.4(A) through (G). In these subparagraphs, we cover basic minimum overcurrent protection device sizing according to conductor sizes and properties. These are very basic rules that will at times get overlooked when using ampacity tables, but you absolutely need to try to commit these to memory. Also, many code test questions will ask about sizing a certain type and size conductor, and you’ll get sidetracked into doing a calculation and don’t want to forget that you have limitations in 240.4. Please review these and remember some basic ones such as 14 AWG copper must have 15 amp protection, 12 AWG copper is limited to 20 amps and 10 AWG copper at 30 amps, just to name a few.
There are two very important rules in 240.4 that we have to examine more closely, as they deal with protection for 800 amps and lower and then over 800 amps. These general rules state that if you are sizing protection for 800 amps or lower, you are allowed to round up to the next largest device, as long as you don’t violate the specific conductors mentioned in 240.4(D). If you have a system over 800 amps, then the ampacity of the conductors has to equal or exceed the rating of the overcurrent device.
Photo 3. This is a photo showing the interior of a 600-amp fuse after the sand filler has been removed. This fuse opened during an overload condition. You can see how five of the six alloy contact points melted and released; when the last one interrupted the flow of current, it arced and caused the burned sixth contact. Please note in the left photo you can see the short-circuit elements which are still intact; these are the webs which melt out during a high fault current condition.
The next question that usually comes up here is: When allowed to round up to the next size device, what is the next size? The next size according to what is available from the manufacturer? No, you round up to the next standard size device according to 240.6. Please notice that in 240.5 you will find the specific overcurrent sizes for flexible cord, cables and fixture wire; again, the rules are very specific and in part mention exact size overcurrent protection according to the wire size. Please review, but know that generally speaking, we don’t see these wire types that often in combination inspections.
Standard ampere ratings
Let’s go back to Standard Ampere Ratings in 240.6. This is one section of your code book you will need to reference almost as often as conductor ampacities, so please remember it. Here you find what the code considers "standard sizes” when references are made to sizing overcurrent devices. I always make a big point of making sure everyone uses the sizes listed in this article when working on code questions or test questions. Another note here, remember when we had the rule that you round up when working at 800 amps and below? Well, you will notice the size differences below 800 amps are much closer, so when we round up it’s not much of a shift; however, when you get over 800 amps you will notice there are not as many options and the sizes start to jump in pretty large increments. This helps to explain the reasoning behind this code requirement, since rounding up in such large steps could lead to a large disparity between conductor ampacity and overcurrent protection levels.
Also in 240.6 are two paragraphs, (B) and (C), which work together to address adjustable trip circuit breakers. If you have an adjustable breaker which has the adjustment exposed with ready access, then the rating of that breaker will be the maximum possible setting. However, if these controls have a restricted access feature meeting the requirements as set forth in (C), then the rating of this device will be at the set value. The one gray area here is that some of the breakers have a field-fitted rating plug. It was my opinion as an inspector that if this plug type device could not be removed without the removal of sealable covers or special tooling of some sort, we applied 240.6(B). One such case had a plug which could easily be removed using a pair of needle nose pliers, so we opted for the conservative approach and applied (C).
Photo 4. This photo shows the inside of a breaker. You can see some of the mechanical parts that are required for proper operation.
Fuses or circuit breakers in parallel
Questions appearing on some tests ask if it is permissible to use fuses and circuit breakers in parallel. This is addressed in 240.8. The correct answer is: only where they are a part of a factory assembly and listed as a unit. I have seen this from time to time, but never outside a factory listed unit.
Electrical system coordination
Continuing on, 240.12 starts us on the path for an interesting concept that often isn’t considered in design. The subject is electrical system coordination, and it simply states that to minimize hazard(s) to personnel and equipment where an orderly shutdown is required, a system of coordination based on two conditions shall be permitted. The first condition is coordinated short-circuit protection and the second is overload indication based a monitoring system or devices. Notice that this language says "shall be permitted.” This means that is allowed, but not required. This language is generally applied to situations where it is more hazardous to shut down the electrical source than it is to shut down the process. This is not the same as selective coordination, which is required for many emergency systems and will be covered when we finally get to Chapter 7.
Ground-fault protection of equipment
In 240.13 we find requirements for Ground-Fault Protection of Equipment. Let’s first consider the difference between ground-fault circuit interruption (GFCI) and ground-fault protection. The easiest way to explain this is that GFCI protection is for people and the threshold levels are extremely low (around 5 ma), whereas the protection of equipment is meant to minimize the damage to equipment in the event of a fault condition to ground. This applies to only a very specific power configuration and that is a solid wye-connected system, where the voltage to ground is more than 150 volts and the phase-to-phase voltage does not exceed 600 volts. Commonly this will be our 277/480 volts wye-connected three-phase systems that are 1000 amperes or more in size. Ground-fault protection offers a level of protection in the event of a ground fault at a much lower level of current than what the breaker is equipped to handle under normal operation. I’ll tell you a personal experience related to this. We had a bank in our jurisdiction where the service was rated at 1000 amps, 277/480 and so the inspector made a note that this installation would require ground-fault protection. The engineer stated it didn’t need it because he had specified an 800-amp main breaker. A little gray area I guess, but we decided to stick with the rating of the manufacturer’s label which stated the service equipment was 1000 amps. The end result was that the factory sent out new equipment labels and had a field inspection done by a listing agency to change the equipment to an 800-amp service officially.
Ground-fault protection needs a little deeper look, first to understand how it works and then to understand some of the complexities to look for as you are doing your inspections. First, these devices have a sensing ability to verify that the amount of current being called for is balanced and all accounted for between the other phases or the grounded conductor. In the event that we have current going to ground or not following the normal paths, then the ground-fault protection will open the device (this could be a breaker or a bolt switch equipped with the ground-fault protection). When these devices are sent out, they are set at factory minimums. If the levels are not adjusted at the time of installation, this minimum setting may cause nuisance tripping. The setting should be evaluated and specified by the engineer of record, and then set in the field to match the engineer’s design.
Once I got a service call for a large grocery store that had lost power. The main at this store had ground-fault protection, and the original contractor didn’t set up the device as requested by the engineer of record. So, it was still set at the minimum value. On the evening I got the call, the manager had asked a box boy to paint the hallway going upstairs to the break room, and the young man had saturated his roller to the point it caused some large drips to start running down the wall. The paint flowed into a 277-volt switch box and shorted out the switch. Well before the individual circuit breaker which fed the lighting circuit could interrupt the fault, the ground-fault device saw the fault to ground, did its job, and shut down the entire store. Because the device was not properly set by the installer, the entire facility lost power.
Photo 5. This collage of photos shows a before-and-after for breakers. The top row shows a breaker which has not been in operation. The left and middle photos show both sides of the contacts, and the right photo shows the arc chutes. The bottom row shows an example of the same parts of another breaker which has been subjected to a high fault current condition and had to open, causing damage to its components.
Total separation of grounds and neutrals
Now one of the critical items we must look at during inspection is the total separation of grounds and neutrals downstream of the ground-fault device. This must be done all the way throughout the system down to each branch-circuit device and the equipment connected to the system. At the main service we have to pay special attention to have the grounds connected only to the grounding bar, and neutrals connected only to the neutral bar. Now this is different from our normal method, say in a residential main panel, where we can just mix the grounds and neutrals as we see fit. In these larger systems, there is a neutral bonding jumper (which could be a conductor but is generally a piece of busing) that comes from the factory and is not connected to the ground bar. As an inspector, you have to verify that the grounds terminate on the ground bar and the neutrals, to the neutral bar. If these are not done properly, the system will have issues.
Locally we always required third party verification and testing of the ground-fault system before we would approve it to be energized. This was our insurance that the unit had the grounds and neutrals separated throughout the entire facility and that the system wasn’t left at factory minimums. Once the system has been checked, then the neutral bonding jumper is connected between the neutral and the ground bar. So after we got this report, we would make sure the bonding jumper link was connected between the neutral and ground bar and then allow the contractor to have the system energized. This was our solution for enforcement of ground-fault protection of equipment.
In the next issue we will pick up with Part II of Article 240, but this is a good time to cover a related issue. While teaching classes on the code, I always tried to take the mystery out of the electrical system as much as possible. So I am going to spend a little time explaining how overcurrent and short-circuit protection works. In class this always led to a lot of show and tell, taking apart devices and physically seeing their operation. I will try to explain this here and supplement it with some good photos.
Basics of circuit protection
We need to explore some of the basics of circuit protection. First we’ll start with the most common circuit breaker in the industry today, that being an inverse time circuit breaker. These come in all sizes and ratings, from 15 amps and up. They are rated by amperage, voltage and interrupting rating. The first two items we should already be familiar with; however, the last is often overlooked. The interrupting rating is the amount of fault current the device is able to safely handle without catastrophic failure. Insuring the available fault current is less than the rating of the device is one of the inspection items we need to look for. Breakers are mechanical equipment; similar to any other mechanical device, they require a lot of pieces to work together with the proper timing to do their job. A car is a good comparison, as it is a mechanical device that has many pieces that have to operate in a certain sequence for proper operation. Also similar to cars, the need for exercise and maintenance for breakers should be considered. Inside a breaker we have two distinct methods which cause it to open, one being an overload which is normally up to about 6 times the handle rating, and the other being the short-circuit portion which reacts to shorts causing a high level of fault current to flow. The overload is normally handled by a bi-metallic element which when exposed to excessive current starts to heat up and it then deflects to contact the trip bar and release the trip mechanism. Breakers handle short circuits with a magnetic sensor which reacts to high current flow and opens a breaker as fast as it can. Remember these are mechanical, and as such they take a certain amount of time to react and then to complete the operation of shutting down the circuit. The photos illustrate the number of components inside a breaker and also show a close up of the contacts, the arc shields which control and manage the arcing when operating in high-fault conditions, and a bi-metallic strip.
The other most common method of circuit protection is fuses, which in many circles is considered old style due to the fact they’ve been used to protect electrical systems practically since the beginning. However, they still have a distinct use in today’s systems and provide some very unique methods of protection due partially to their simpler operation. The most commonly used fuse for construction is a dual element time-delay fuse. These have two distinct portions within each fuse; the first portion is a thermally reactive element which handles an overload situation. This element uses a melting alloy which has been specifically created for each size fuse. When it is exposed to an overload condition it will melt out and release, allowing the fuse to open. The short-circuit section of a fuse consists of a web-style design which is designed to react to high-current flows and very rapidly melt out; as these melt and break away, the amount of metal mass left is diminished which limits the amount of current which can continue to pass through the fuse. Therefore, they are considered current-limiting by design. In order to control this arcing within the fuse, it is filled with sand. When the sand comes into contact with the extreme heat and arcing of the webs, it turns into glass to quench the arcing event and extinguish it.
I know the operation of a fuse sounds basic compared to a device full of mechanical components which have to work in unison, but that’s just how simple they are. Once a fuse opens, you replace them with a new fuse which restores the system back to its original level of protection. When a breaker has been subjected to fault current near its operating limit, it should be taken out of service and tested before using again. There are companies that test breakers to insure they operate in the proper time and current levels required and then re-certify them. If the breaker is not tested and re-certified, it may not protect the system during a future fault.
In the next issue we will continue with Article 240. Continue to review the code as these articles only cover the highlights you need to know.
Read more by Randy Hunter
Posted By Joseph Wages, Jr.,
Tuesday, January 01, 2013
Updated: Friday, January 18, 2013
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Think you might know what lurks behind the meter cover? Chances are you will think twice after reading this article!
Electrical Metering Devices
Meter enclosures are part of every electrical system. But how often do you look inside the enclosure after it has been installed and energized? A utility provider provides electricity to a customer in order to make a profit. Typically, this is accomplished by metering the electrical system at the point of connection. Electronic receiver/transmitter (ERT) meters are becoming more of the norm in today’s electrical metering systems. They provide many benefits to the electric utility provider and to the customer. But, this could allow for many unforeseen problems as well.
Photo 1. A 400-ampere, single-phase meter enclosure located at the front entrance of a church and preschool. Men, women, and children visiting this location walk by this meter enclosure on a daily basis. What’s located behind the cover could prove deadly! And chances are it will not be discovered until there is a problem.
The Good Ole Days and the Standard Electrical Meter
For many years the measurement of electricity went relatively unchanged. These metering devices worked adequately for the utility provider to accomplish the goal of registering electrical usage so that a utility bill could be generated. The customer received the monthly statement and would pay his or her bill.
In the event the customer fell in arrears and did not pay the monthly bills, a customer service representative would visit the location and disconnect the power. To accomplish this, the meter seal would be cut, the electric meter removed and booted off, and then reinserted into the meter base. The cover is then reinstalled and re-sealed until a time in which the customer pays the outstanding bill and any reconnection charges. In the event of tampering, the meter would either be removed and a blank inserted or the service conductors cut and disconnected at the weatherhead or utility pole.
Photo 2. Within the meter enclosure unknown to daily passersby are several potential dangers as can be seen, such as corrosion, effects of overheating to busbars and conductors, insulation failure as well as conductor damage.
When the service is ready to be reconnected, the customer service representative then returns to the property, cuts the meter seal, removes the meter, removes the boots and reinserts the meter. The cover is then installed and another seal applied. This returns electricity to the customer and usage is recorded for the next monthly billing cycle.
During this entire process there were several opportunities for the utility representative to notice and report any problems developing within the meter enclosure. Upon discovery, action could be taken to alleviate potential problems before they happened. Technology has changed the way that utilities gather billing information and even the disconnection of delinquent accounts. This has in turn made it even more imperative that electrical work within the meter enclosure be installed in a code-compliant fashion.
A statement must be mentioned concerning safety regarding this issue. Unbelievably, some customers take it upon themselves to reconnect their electricity without the approval of the serving utility. This has resulted in additional fees being accessed by utility and the electrical meter being removed or the conductors disconnected at the weatherhead or utility pole. This illegal activity can result in electrical accidents up to and including death. Qualified personnel are necessary to reconnect these services to assure the safety of the electrical system. Never attempt to reconnect your electrical service! Contact your friendly utility provider for help with this situation.
Photo 3. The interior of this meter enclosure depicts damage due to overheating and corrosion. Remember, none of this damage is visible from the outside of the meter enclosure.
The ERT Meter
An electronic receiver/transmitter meter (ERT meter) is used in a network meter reading environment. It can be retrofitted into existing meter enclosures and is available in single- phase and three-phase models. The meter uses electronic modules to communicate power consumption and power quality to the utility provider. The meter also allows two-way communications from the utility provider to the customer. This allows the utility provider the opportunity to be aware of outages that occur and to respond much more quickly.
The use of ERT meters saves the utility provider from physically visiting the meter location on a monthly basis. The ERT meters have a low-powered radio device that permits them to be read from a distance. This allows meter readings to be collected electronically with a mobile data collector (usually a laptop computer) or with a handheld receiver. Technicians are able to download the readings for multiple meters at one time rather than walking from house to house to look at each individual meter.
In some cases, the utility can also disconnect and reconnect the customer remotely. This can be for nonpayment of their monthly bill or to head off high demand issues on the utility system. This can be handy during high usage periods where the provider needs to disconnect loads within structures to prevent brownouts or blackouts from affecting the system. Many household devices are being produced with communication features that communicate with the electronic meters. Utilities can disconnect AC units briefly to prevent issues on the system from occurring. Most generally the customer is unaware that this has even taken place. Some consumers have expressed concerns regarding this issue as it applies to privacy. Many customers enjoy the features of ERT meters. This technology allows the customer to monitor their electrical usage. This has also allowed the customer to change some of their usage patterns in order to save money on their electrical bills by using electricity in the off-peak periods.
As you can see, the use of this technology removes the "hands on, eyes in the field” that may have visited the enclosure and discovered a problem. This can result in minor situations arising that develop into major issues. These issues can be lessened by proper equipment installation and inspection.
Photo 4. Conductor damage due to insulation failure that is not detectable from the exterior of the meter enclosure.
Preventing These Problems Begins with You
The first line of defense in addressing this issue starts with the electrical contractor. The contractor must make sure that all material used for the installation is listed and labeled as per requirements found in NEC 110.3(B). Proper application of NEC requirements will help ensure a safe and compliant installation. NEC Article 110 covers the requirements for electrical installations. Requirements found within this article include working clearances, interrupting rating, mechanical execution of work, mounting and cooling of equipment, illumination, electrical connections, arc-flash, and field marking. There are other requirements that are useful and required throughout the NEC as well.
Care must be taken to follow the manufacturer’s installation instructions. Special care must be taken to use anti-oxidant compounds as required by the NEC and the manufacturer. These requirements are found in NEC 110.14. Conductors should be stripped and prepared properly so as to not damage the conductor. Specialized tools are necessary to assure that the specific torque requirements are followed for the lugs and conductors. Informative Annex I includes recommended tightening torque values to be used in the absence of the manufacturer’s recommended torque values. These values are taken from UL Standard 486A-B.
The next line of defense lies with the electrical inspector. The inspector needs to assure the customer and utility provider that the electrical contractor has followed the guidelines for properly installing the metering equipment. A good understanding of the NEC and any additional electrical requirements required by the utility provider are necessary. Some of these requirements have been previously discussed. Additionally, the inspector needs to make sure that proper grounding and bonding has been accomplished. Grounding and bonding requirements can be found in NEC Article 250. Also, remember that the insulated fitting required at NEC 300.4(G) is required due to conductor size, not conduit type. Bushings are required for various conduit types throughout the NEC such as at 344.46 for rigid metal conduit. There have been many instances where the electrical installation has been turned down by an inspector due to a missing conduit bushing or insulated fitting.
Photo 5. For years a standard 2s meter was adequate for the needs of the utility company. Today’s technological advances have spurred changes within the utility industry in order to compete and reduce operation costs. Electronic radio transmission (ERT) meters may be able to control some electrical devices within the structure through the electrical utility to prevent system problems. These include the refrigerator, the air conditioner or some other high usage item. Privacy issues have been expressed by some people.
Arc Flash and Available Fault Current, a Deadly Combination
New to the 2011 NEC are 110.16 and 110.24 which deal with arc-flash and available fault current markings and requirements. Section 110.16 states that the marking shall be located so as to be clearly visible to qualified persons before examination, adjustment, servicing, or maintenance of the equipment. This includes meter socket enclosures as well as switchboards, panelboards, industrial control panels and other motor control centers. These requirements do not apply to dwelling units.
Service equipment must be marked with the maximum available fault current per NEC 110.24. This Code requirement further states that the field marking shall be legible and include the date that the fault-current calculation was performed. It must also be sufficiently durable to withstand the environment in which it has been installed. Modifications require that this calculation be verified and recalculated as necessary to ensure the service ratings are sufficient for the maximum available fault current at the line terminals of the equipment. An exception exists for industrial installations where conditions of maintenance and supervision ensure that only qualified persons service the equipment. Again, as previously stated these requirements do not apply to dwelling units.
These requirements help to ensure that whoever works on this equipment in the future is aware of the potential available fault current. This also brings up an interesting question. Who is responsible to adjust the modification markings to the existing equipment as per the requirements found within 110.24? Suppose the providing utility changes out the transformer to the building. Suppose the impedance is different from the existing transformer to the newly installed transformer. Who makes the changes to the marking at the service equipment? Is the building owner aware that these changes have been made and what effect it has on the available fault current to his equipment? Does the utility provider even know that this requirement is found within the NEC? How does this information get upgraded on the electrical equipment?
Photo 6. This electrical service location has a meter blank installed and the meter retired or taken out of service. This could be due to non-payment for services, tampering or because the electrical equipment is no longer in service.
Most utilities work under the guidelines of the National Electrical Safety Code (NESC). During a power outage at 2:00 A.M. in the pouring rain and lightning, who will make these adjustments in the field? Is there communication between the utility provider and the customer concerning these changes? What happens when the utility changes the substation feeder to this area of town from one substation to another? There are different characteristics present in both substations that will affect the calculations towards what is marked on the service equipment. These are just a few questions and situations that could arise and affect the accuracy of the field markings for these installations.
Interestingly, during the 2011 NEC Report on Proposals (ROP) and Report on Comments (ROC) meetings these situations were vigorously debated and discussed. The inclusion of the date of when the calculation was conducted was agreed upon and included so that the future electrical contractor would be aware of when the calculation was conducted. Under no circumstances should the electrical contractor rely on a marking on the equipment to determine the level of personal protective equipment (PPE) required. Changes to the system may have taken place after the date the calculation was performed, changing the available fault current at the terminals of the equipment.
Technology Always Has its Ups and Downs
In conclusion, a properly installed and inspected electrical service should provide years of service to the customer. If the customer increases the electrical load by adding new electrical appliances, the service size may need to be re-evaluated. There are many existing older homes and commercial locations with electrical services that were acceptable at the time they were built. Over the years with the addition of new electrical appliances, these services may no longer be adequate for their situation. An electrical contractor should review these services and determine if modifications are needed.
Believe it or not, there are still several locations within utility territories that have only 120-volt services. Usually these are only 60-amp services. This service was all that was necessary to provide electricity to the few devices available at that time. Technology has brought us many new items to add comfort and convenience to our daily life. Many homeowners are shocked when they purchase a new air conditioner or electric dryer to find out that they will need to modify their electrical service to utilize the equipment. Many tend to be elderly and on a fixed income.
Field markings are crucial to the safety of the equipment, the electrician and the electrical inspector. All attempts should be made by the utility and the customer to maintain the accuracy of these markings. Doing so may mean the difference between life and death!
And remember to consult the utility provider and to secure the required permits from the authority having jurisdiction (AHJ) before beginning the electrical upgrade. The utility provider may need to re-evaluate transformer sizes and make adjustments to their system due to your planned modification. What worked for the utility years ago may need modification today.
Read more by Joseph Wages, Jr.
Posted By Steve Douglas,
Tuesday, January 01, 2013
Updated: Friday, January 18, 2013
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The new standard for cablebus C22.2 No 273 is scheduled for publication by September of this year. This new standard will be the first standard for cablebus in North America. The committee includes the six major cablebus manufacturers in North America, two switchgear manufacturers, CSA, and an IAEI representative.
Cablebus is an assembly
Cablebus is an assembly of insulated conductors with fittings and conductor terminations in a completely enclosed, ventilated, or non-ventilated protective metal housing. In most cases, cablebus will be approved by either certification or field evaluation and is typically assembled at the point of installation from the components furnished by the cablebus manufacturer. Accompanying the cablebus, the manufacturer will provide installation instructions and drawings for the specific installation to facilitate:
a) system design;
c) fire stop rating (where applicable);
d) weatherproof entrance fittings (where appli-cable);
e) bonding, conductor and shield terminations (where applicable);
f) grounding of shields (where applicable) and installation;
g) inclusion of electrical detail of the conductor configuration, together with enclosure dimensions;
h) specification of maximum allowable span support; and
i) vertical installations.
To assist the electrical contractor and electrical inspector the main nameplate will include:
a) The manufacturer’s name, trademark, or other descriptive marking by which the organization responsible for the product can be identified;
b) The electrical ratings:
– rated nominal voltage, (Vrms or Vdc)
– frequency in Hz
– allowable ampacity (Amps), based on ambient temperature* of XX*C, and based on a maximum operating temperature of XX*C- short circuit current rating
– number of phases (poles for dc);
– 3-wire or 4-wire; and
–Maximum continuous current rating _XX_A, when connected to a 100% continuous rated overcurrent device
– Maximum continuous current rating _XX_A, when connected to a 80% continuous rated overcurrent device
*Note: the temperature is the maximum ambient temperature that the equipment was designed to operate in.
c) The month and year of manufacture, at least, shall be marked on the cablebus system in a location accessible without the use of tools.
d) The number of conductors and size per phase.
e) As a minimum, the allowable ampacity (amps) based on a maximum operating temperature of 75°C shall be included on the nameplate.
f) Type of material, such as stainless steel (including the type), aluminum, etc., and, if carbon steel, Type 1 (hot-dip galvanized), Type 2 (mill galvanized), or Type 3 (electrodeposited zinc), as applicable. If the manufacturer’s catalogue number marked on the product would readily lead the user to the required information published by the manufacturer, this marking is not mandatory;
g) a warning label that reads, "WARNING! DO NOT USE AS A WALKWAY, LADDER, OR SUPPORT FOR PERSONNEL; and
h) the design drawing number for the specific installation.
Maximum continuous current rating
The maximum continuous current rating will assist in the application of CE Code Rules 12-2260 and 8-104 and help provide consistency with respect to conductor loading. In addition to these nameplate markings, cablebus will be one of two classes corresponding with the Items (a) and (b) in CE Code Rule 12-2252. CE Code Part I Rule 12-2252 states:
12-2252 Use of cablebus (see Appendix B)
Cablebus shall be permitted for use where
(a) protection from contact with conductors is provided by design and construction of the enclosure; or
(b) installation is intended in areas
(i) accessible only to authorized persons;
(ii) isolated by elevation or by barriers; and
(iii) where qualified electrical maintenance personnel service the installation.
Class A cablebus is designed with protection from conductors contact provided by the design and construction of the enclosure. Class B cablebus is intended to be installed in areas accessible to authorized persons, isolated by elevation or by barriers, and where qualified electrical maintenance personnel service the installation.
Read more by Steve Douglas