Posted By Jim Hayes,
Monday, September 03, 2012
Updated: Thursday, September 20, 2012
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We seem to live in a world where technology changes weekly; we’re inundated with so much new hardware like smartphones and tablets and broad concepts like "Smart Grid” that it’s difficult to keep up-to-date. Everybody seems to think "fiber optics” is a new technology, too, although some of us have been involved with the technology for over thirty years!
Without these last three decades of fiber optics, we’d be missing a lot of communications we depend on — like the Internet, mobile smartphones or cheap worldwide phone calls. Computer networks would be slower and more expensive. Cities would not be able to ensure security with video surveillance or to manage traffic with smart traffic lights. If it involves data and/or communications, it probably involves fiber.
Within the same thirty years, think how many other technologies have been introduced, peaked and become obsolete: VCRs, CDs, desktop PCs, and more. Yet fiber optics has slowly matured, becoming the preferred medium for most communications.
Along the way, all those who install cabling have embraced fiber technology. I started training NECA electrical contractors how to install fiber optics over twenty years ago. Today the majority of these contractors install fiber optics for the same customers they have always served. Installers of data cabling, security systems, building management systems or just about any cabling inside buildings or out have been trained in fiber optic installation.
Thankfully, we rarely hear "fiber optics is expensive, hard to install and fragile because it’s made of glass” any more. On the other hand, fiber has become so common, we sometimes don’t hear about some of the more important uses of the technology and how it facilitates many other technologies.
Photo 1. Telecommunications. Photo courtesy of John Watson
Let’s start with data centers. All that Internet data that creates billions of web pages or enables "cloud” computing has to be stored somewhere and made easily accessible — over fiber optics, of course. What we call a "data center” is really a "data warehouse” with shelves of servers that find and distribute the data and disk storage from many terabytes of data. A typical data center will have thousands of links connecting servers to storage and consume megawatts of power.
Speed and power are the keys to data center networks (sometimes called SANs or storage area networks). 1 gigabyte Ethernet is being phased out for the 10 gigabyte version, and 40 and 100 gigabytes are becoming available. Copper cabling was OK for gigabyte links, but its development has been five years behind fiber at 10G and it’s not seriously being considered for 40 to 100G. Besides the bandwidth advantage, fiber consumes about ⅕ as much power per link as copper at 10G, and power consumption is a big issue for data centers.
Cellular Phone Networks
You are probably already addicted to smartphones or tablets and depend on cellular phone networks for these mobile devices. Cellular phones have always connected into the worldwide phone network that is based on fiber for long distance and metropolitan links. The growth in mobile data use is staggering. In its first 3½ years on the market, the iPhone alone caused an 8000% growth (that’s 80 times) in AT&T’s mobile network traffic. To meet traffic demand, cellular towers have been converting copper or digital radio to fiber connections to allow more radio bandwidth for user connections.
I’m sure you’ve noticed the proliferation of antennas on cellular towers to meet this demand for cellular bandwidth. Each of those antennas requires a connection to the base, typically with large coax cables running up the tower. These are being replaced with fiber, too, using composite cables that carry copper conductors as well as fiber, providing power and connections to all the antennas with a much smaller, less costly cable.
Photo 2. Traffic signal. Photo courtesy of John Watson
Besides widespread use of those smart mobile devices, we’ve increased our consumption of data in the home too, with Internet video being the fastest growing source of traffic. Streaming video, led by Netflix, now accounts for the bulk of Internet traffic. Video consumes vast amounts of bandwidth; just think about streaming a movie over your Internet connection that would fill a 4–5 gagabyte DVD!
To keep up with consumer demand, fiber is being brought closer and closer or even all the way to the home. CATV was an early adopter of fiber since it increased system reliability as well as allowing the first broadband Internet access using cable modems. Using the latest cable modem technology, DOCSIS 3, CATV can deliver Internet access up to 100 megabits/s or more.
Telecommunications companies (Telcos) are still trying to catch up with CATV for Internet access. Most telcos continue to use their old copper wires to connect the home but are installing fiber closer and closer to the home. Copper is limited in bandwidth by physics; longer links of copper telephone wire have considerably less bandwidth than shorter ones. While technology called DSL (digital subscriber line) can provide 10–40 megabits per second connections, it requires "clean” copper wires. Many homes in the U.S. have older copper that has deteriorated over the years, making DSL problematic. One telco guy I know says we’ve already mined all the copper we can for DSL — it’s time to convert to fiber.
And it’s fiber to the home (FTTH) that is the long-term solution for home connections. With fiber, you have virtually infinite bandwidth. Fiber costs more but has two potential paybacks: (1) the potential to sell consumers more services and (2) lower maintenance costs. In addition, new technology like passive optical networks (PON) that split one fiber link to serve up to 32 users can save lots of money, making fiber only slightly more expensive than DSL.
In 2005, a Telcordia survey said that FTTH could pay for itself in lower maintenance costs compared to the high maintenance required by older copper wires. Verizon took this report to heart, creating FiOS, the biggest FTTH system in the U.S., and saving billions of dollars in maintenance costs by replacing their aging copper cable plant with fiber. Converting customers from just POTS (plain old telephone service) to HDTV and Internet users added billions more in revenue too.
Photo 3. Fiber has been used by utilities for years. Photo courtesy of John Watson
Today, there are about 800 FTTH projects being built in the U.S. Google made waves in the industry by announcing they would build a gigabyte FTTH network in a U.S. city as a demonstration project. Kansas City (KS+MO) was chosen for this project that is now being installed. While Google was talking about gigabyte FTTH, Verizon actually demonstrated it in their FiOS network in Massachusetts. But the city of Chattanooga, Tennessee, outdid everybody, installing a citywide gigabyte FTTH network using the local electrical utility network.
Chattanooga’s gigabyte FTTH network truly illustrates the reasons for cities to install FTTH. The local telco and CATV companies did not do the work, but the city electrical utility had been looking at reading meters online. It was an easy — I said easy, not cheap — expansion to offer broadband Internet. But the city’s commitment to providing broadband helped convince Volkswagen to bring their U.S. manufacturing plant to Chattanooga, creating 3000 jobs initially and 2000 more in the future.
Other regions and cities are also promoting FTTH for its ability to attract high-tech jobs. A consortium of college towns has founded "Gig-U,” an organization dedicated to bringing gigabyte FTTH to their cities to attract tech companies. A startup has even received $200 million to provide those networks.
FTTH networks must have fiber equipment at every home (or subscriber, if in an apartment or condo building) to interface to the customer’s phone, TV and computer. These optical-to-electrical conversion boxes require power and connection to the customer’s devices, either through current cabling or cables installed by the system operator. Power generally is by AC power cubes although more complex uninterruptible power supplies are sometimes used to allow customer connections during power outages.
Cities are installing municipal networks for many other uses than FTTH. Municipal networks are expanding rapidly because they encompass many types of services, not just broadband Internet.
Cities and suburbs are installing communications systems to connect administrative, public safety and other city offices. They are installing surveillance cameras for security and traffic monitoring. Traffic signals are being monitored and controlled to improve traffic flow to save energy. Schools are being connected to provide high speed Internet to students and school administrations.
Wireless (WiFi) systems are being installed for both private municipal use and public access. Utilities are expanding their communications systems to increase efficiency and to read meters remotely. Many cities are also realizing that they can lease "dark fibers” to phone or CATV companies as well as other companies that desire high-speed connectivity.
Photo 4. Utilities with long distance companies to install fiber along high voltage lines. Photo courtesy of John Watson
While consumer demand for broadband drives much of the growth in fiber optics, many other applications are growing as well. The buzz about "Smart Grid” refers to the work being done to increase the efficiency and security of the power grid. Just imagine the scope of the problem. In the U.S., there are over 2000 utilities generating power and hundreds of thousands of photovoltaic solar systems pumping power into the U.S. power grid.
Keeping this network under control is a massive job. Last year here in Southern California, we had a power outage because a tech at one small generating station flipped the wrong switch and put 5 million users in the dark for the evening. Making a power grid more efficient requires communications and that’s done on fiber.
Utilities and Optical Power Ground Wires
Fiber has been used by utilities for decades. Utilities were among the first to recognize the advantages of fiber as a non-conductive communications cable that could not only connect generating or distribution facilities but even monitor high-voltage lines using fiber optic sensors. Utilities partnered with long distance companies to install fiber along high-voltage distribution lines, with the communications companies selling telecom services and the utilities getting free links for their use.
Together they pioneered an extremely useful cable design, a high-voltage transmission line with fiber in the center for communications. It’s called optical power ground wire (OPGW) and forms the communications backbone in many areas of the country. Since fiber is totally immune to electromagnetic interference, power lines can carry massive amounts of communications signals at much lower costs than when fiber cables must be installed independently.
Fiber is a major component of smart grid technology. One college we know is developing a smart grid program with a major utility, and fiber represents about one-third of the course. And as utilities add more fiber to manage their systems, they get closer to the home, eventually reaching what one person called FTTM — fiber to the meter — and they can offer gigabyte connections to every home.
Fiber has also made big inroads into industrial environments. Over the last few decades, most machines have become automated with computerized controls that send data back to computers that analyze and manage the factory floor. Auto plants use fiber for welding robots, critical assembly and inspection tools, control of other machinery and the assembly lines. Aluminum smelters are another place where fiber is popular because they use extremely high electrical currents in processing the aluminum that interferes with other communications media. Video cameras monitor the activity of workers, machines and materials. HVAC has become computerized in part to improve the factory environment but also to reduce energy costs.
Photo 5. In the desert in the southwest, solar generation stations are being built
Most factories have in common large physical plants and a harsh environment.
Industrial fiber optic cable plants usually require more protection than in commercial buildings. Cables are often run in metal conduit and terminations are made in sealed enclosures. Cables are sometimes run along the roof and dropped to floor locations, so extra support for the vertical runs can be an issue.
Business and Government Uses
Perhaps the most complex fiber optic cable plants are in the biggest buildings like airports, convention centers and other government facilities. These buildings are likely to include networks for phones and computers, indoor antennas for mobile WiFi and cellular devices, security cameras and entry systems, building management systems and entrance facilities with connections to many outside communications systems.
These large buildings have been used for some interesting experiments in cabling and networks. Here in the U.S., some contractors have been using FTTH PONs [fiber to the home, passive optical networks] scaled for use in buildings. A PON network has many advantages including easier installation with only one fiber per user, higher bandwidth capacity and lower cost. The cost is even lower than a copper network because of the network architecture and market volume; with millions of units having been built and installed in FTTH networks. Perhaps the two biggest advantages of a PON network in a building are easier network management and security. PON networks are scaled to handle millions of users so network management in a smaller building network is easy. And since it splits fibers to connect users using the same signal, it’s necessary to encrypt every user, increasing security.
A network architecture we saw recently in a foreign airport is making inroads into computer networks too. Called fiber to the office (FTTO), it uses a standard Ethernet architecture over fiber with small, inexpensive switches placed near the user’s desk and shared by four computers or VoIP phones. Besides replacing four big copper cables with one small fiber cable, it also eliminates the need for expensive telecom rooms on every floor of a building with their requirements for electrical power, AC and data-quality grounding.
Alternative Energy Systems and Fiber
Fiber has also become an enabler for energy. One of the earliest uses for fiber was in mines where it was used for its long distance capability and inherent safety since it does not involve electrical current in its transmission. Fiber was also used for energy exploration, allowing the connection of many seismic sensors over a large area on land and underwater remote vehicles to reach the deepest parts of the ocean (and discover the Titanic too). Pipelines rely on optical fiber running along their lengths for communications and monitoring for problems.
Perhaps the most interesting use of fiber in energy is in two growing fields, wind and solar power. Wind power requires careful monitoring of each tower to make it efficient as the wind changes and to connect it to the grid. Each tower is connected to fiber as part of the generation network.
Solar power is even more interesting. In the desert in the Southwest, several gigawatts of solar generation stations are being built. Unlike the typical small system that uses photovoltaic panels, these large systems combine hundreds or thousands of mirrors to focus extremely high power light to create steam that runs a generator. It’s really just like a nuclear power plant but the reactor is 93 million miles away — and much safer!
Some systems heat water or antifreeze to generate steam, but a new system uses plain old salt (NaCl) in the heat exchanger. The higher heat capacity of salt allows storing heat underground so steam can be generated during night hours, a big advantage over photovoltaic systems.
These solar systems require moving the mirrors continuously to keep the light focused properly. Every one of the mirrors spread out over hundreds or thousands of acres requires control and feedback, and that is provided by optical fiber. And of course the generators and heat exchangers require equally good monitoring and control, all on fiber.
How Fiber Impacts You
For electrical inspectors, most of what you need to know about fiber optics is in the NEC or on the Fiber Optic Association website (www.thefoa.org). We even have created with NECA an ANSI standard on fiber optic installation that describes what installing fiber optics "in a neat and workmanlike fashion” means. FOA provides the ANSI/NECA/FOA-301 Standard free to the industry. Contact FOA for your digital copy. But the proliferation of fiber uses in every aspect of today’s technical world means it’s harder to keep track of where the fiber is and what the fiber is doing.
Here at the Fiber Optic Association, the professional society of fiber optics, we try to track all the uses of fiber and help the industry keep up-to-date with our newsletters and websites. Most of our focus is on the designers and installers of fiber optic cable systems, but recently we’ve had requests from electrical inspectors for more information and even requests from IAEI chapters for seminars on fiber optics.
Generally, we send callers to our website or our online training website (fiberu.org) for free technical information online. For one chapter, the FOA sent one of our Master Instructors, Arnie Harris in Philadelphia, to speak. Arnie’s talk was well-received and we’re certainly willing to entertain more requests for seminars.
Read more by Jim Hayes
Posted By Randy Hunter,
Sunday, September 02, 2012
Updated: Wednesday, September 19, 2012
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Outside branch circuits and feeders — what is so special about these circuits that we require a completely separate article when they are located outside? What makes them different from inside branch circuits and feeders? The scope of this article covers the requirements for branch circuits and feeders running on or between buildings, structure, or poles on the premises, which would also include the wiring for the supply of utilization equipment that is located on or attached to the outside of buildings, structures or poles.
Photo 1. Feeders — any conductors downstream of the main service disconnect is considered a feeder (to the last set of overcurrent devices).
Before we start this article, we need to review just a little. Some of these outside systems may look very similar to and can easily be confused with utility-owned wiring methods. In order to understand the difference between the two, we need to establish where the service point is located. Once this point is found, then anything that is both outside and downstream of the service disconnect would come under the scope of this article.
A new definition has been included to describe a substation. While substations are normally under the control of utilities, in some instances customers are supplied with power at higher than normal voltages from the utility and therefore own their own substations. This has created the need for theNECto include rules for these substations, as they are part of the system past the point of service and therefore under the scope of theNEC. Please review this new definition in 225.2. As with several other articles, in 225.3 we are referred to a table that lists other articles within the code that have additional requirements for specific situations or equipment.
Moving on to the general requirements, the first item is the requirement for conductor covering, which is required within 10′ of any building or structure other than supporting poles. There is an exception for the grounded or grounding conductor which may still be bare within 10′ of buildings, but only where specifically permitted elsewhere in the code. The ampacity of the conductors for under 600-volt systems will be according to the load as calculated in 220.10 and Part III of Article 220 and then meeting the requirements of Section 310.15. As usual in the code, not all the answers can be found in one location.
Photo 2. Here is the service point for the overhead conductors in photo 1.
By now we should be used to bouncing from article to article to complete an installation. It’s very important to be aware of how the code works and the interaction between various parts of the code. When teaching in a classroom style location, it is actually quite fun to play the game of asking where in the code do we go for this and that as we work through installation scenarios. Even those who are very good at the code will often find themselves searching the code for an exact reference, which they recall but can’t remember exactly where it is located. The more I deal with the code the more I find it important to locate the exact language to ensure I am using that portion of the code exactly as written and not just how I "remembered” it. This became especially critical in the position of AHJ. So the message here is not to feel like you are not good at the code if you can’t quote verse and chapter. We are really challenged because the code is a living document which changes every three years.
Conductor size and support requirements
We now step into 225.6 where we have the conductor size and support requirements. This is broken down into two categories, first Overhead Spans and then Festoon Lighting. Let’s identify where we commonly see these installations.
First, the overhead spans would be wiring going from building to building or pole to pole, usually contained within the confines of one’s property. These may be providing power to detached buildings, wells, signs, area lighting, or other exterior equipment. Depending on the voltage, over 600 volts or below, we are limited to how small the conductor is that may be used for these spans. Over 600 volts requires a minimum of 6 AWG copper, or 4 AWG aluminum, individual conductors; if using a cable assembly, the limits go down to 8 AWG copper or 6 AWG aluminum. For 600 volts or less, we are allowed to use 10 AWG copper or 8 AWG aluminum for spans up to 50 feet, and for spans over 50 feet you are required to step up one size to 8 AWG copper, or 6 AWG aluminum. There is one exception which is the use of a messenger wire, which normally utilizes some type of a steel cable or hybrid wire which has a center strand of steel; in this case, you do not have to increase conductor size when going over 50 feet.
Festoon lightingis not a commonly used term. If we check back in Article 100 we can find the definition, which is: "a string of outdoor lights that is suspended between two points.” This is the lighting method used for outdoor carnivals, flea markets, or holiday sales lots. Here we have the requirement that the conductors not be smaller than 12 AWG unless they are supported by a messenger wire, and if any spans exceed 40 feet then they shall be supported by a messenger wire. It goes further and even tells us how the messenger is to be supported, that being strain insulator devices. We are also told what the conductors or messenger wires may not be attached to; it is not permissible to attach to a fire escape, downspout or plumbing equipment. This makes sense when you consider the issues that may arise if we have a conductor get damaged and short out to the messenger wire, which may then energize a fire escape, downspout or plumbing system and present a dangerous condition. Often, these systems run over long distances and the current needed to have the overcurrent device operate in a timely function may not be optimal, so we can’t take the risk of energizing anything accidentally.
Photo 3. This shows a medical office building where the main service is in the building on the right and the emergency power system and generator are in the parking garage to the left (see inset photo).
In our part of the country (Southern Nevada), festoon lighting was something that we seldom had the opportunity to inspect during most of the year. However, during the fall it seems to come around for Halloween and the holiday season, where we have quite a few pumpkin sales locations and holiday tree lots. These often present several conditions which challenge a good inspector, as they seem to find unique ways to create power and lighting systems, often without the benefit of qualified electrical professionals. As these seasons would come around, I would remind the inspectors to review 225.6(B), to freshen their memory, and hopefully cut down the number of phone calls I would get.
Calculating loads for outdoor installations
Sections 225.7 and 225.8 deal with Lighting Equipment Installed Outdoors and Calculations of Loads 600 Volts, Nominal, or Less. Here we find some limitations depending on the voltages used and references to other code articles for load calculations. Most of this is not out of the ordinary, so we will move on to 225.10 where we have a list of wiring methods which shall be permitted for circuits not over 600 volts on the outside surfaces of buildings. My first question when reviewing this list is, what’s not there? The one wiring method that is definitely not there is nonmetallic-sheathed cable, which when we look it up in Section 334.12(B), we find it’s not allowed to be installed in a wet location. If we review the definition in Article 100, we find that anything on the exterior of a building is considered a wet location due to the fact it is subject to exposure to various types of weather. Another thing to consider is that due to condensation, various wiring methods run outside of buildings are subject to moisture inside conduits, as well as underground installations.
Moving on to 225.11 through 16 we find requirements for conductor entrance and exit from buildings, open conductor supports, spacing, supports over buildings and attachment to buildings. Most of these are simply references to other code articles where we have the language detailing these installations. Most of them refer us to Article 230, Services, which we will cover in the next article in this series.
Outdoor installations often have many similarities whether it is a branch circuit, feeder or service conductors. Some of the installation methods overlap, and the main article used is the one for the most commonly found outside circuit, which is Services, Article 230.
Where violations frequently occur
Next we have to cover a couple of areas where violations occur frequently, Masts as Supports, 225.17 and Clearance from Overhead Conductors and Cables, 225.18. Most of the violations happen in the clearances area. In 225.17 we cover masts as supports and here we find an interpretive term. The language states that where a mast is used as the final span support it shall be of "adequate” strength. First, we have to understand that the final spans are the ones at the end of a run. Those supports in the center of a run have equal tension pulling in two directions, so they basically just need vertical strength; however, the end supports have the pull in just one direction, and therefore have to be strong enough for the lateral pull in one direction. Now what is adequate when you are an inspector? We have run across this issue several times, and locally we just defaulted to the local utility standard, and what they found is that intermediate metallic conduit (IMC) or rigid galvanized steel (RGS) are of sufficient strength for their overhead service drops, so we adopted this standard for outside feeders and branch circuits, as well.
The most critical issue, in my eyes, is the clearance for overhead conductors and cables, which is dealt with in 225.18. We have four basic rules that consider two factors, (1) the voltage of the conductors and (2) the anticipated traffic under the overhead spans. The required distances for clearances are measured from finished grade, sidewalks, or any platform or projection from which the conductors might be reached. Starting out with the lowest clearance, which is 10 feet, this is the minimum clearance for conductors operating at 150 volts or less to ground and subject only to pedestrian traffic. Moving on to condition 2, which is over residential property and driveways and commercial areas not subject to truck traffic, where the conductors operate at 300 volts or less to ground, here we find the minimum clearance at 12 feet. If we have the same conditions but we have a voltage over 300 volts, we make the clearance move up an extra 3 feet to a total of 15 feet. So what have we left out? Only the most important condition, in my thoughts, and that is the area over public streets, alleys, and roads subject to truck traffic; driveways other than residential; and other land traversed by vehicles such as cultivated, grazing, forest and orchard. In these locations, we must maintain an 18-foot clearance no matter what the voltage. A new requirement for the 2011 Code is the clearance requirement over railroad tracks, which is 24.5 feet.
Photo 4. Example of festoon lighting
Continuing with clearances, we next review 225.19, which is titled Clearances from Buildings for Conductors of Not over 600 Volts, Nominal. In (A) Above Roofs, we find the general rule which states that the conductors and cables will be 8 feet above the roof surface and that height shall be maintained for a distance of 3 feet in all directions from the edge of the roof. The main thought here is that the conductors will be high enough that you won’t come into contact when working on a roof. As with many sections of the code, the general rule is followed by several exceptions; and in this case, we have four. I’ll go into detail on one of them and then challenge you to review the rest on your own. In Exception 2, where the roof has a pitch of 4 inches of elevation change in 12 inches of horizontal distance, which makes it quite steep and therefore un-walkable, the clearance may be reduced to 3 feet.
Continuing with clearances in (B) the clearance from signs, chimneys, radio and TV antennas, tanks and other non-building or non-bridge structures, the clearance shall be not less than 3 feet in any direction. This feeds right into (C) which covers horizontal clearances, which also is 3 feet.
The next clearances covered are those for the final span when it is near any windows, doors, porches, balconies, ladders, stairs, fire escapes, platforms, projections or similar surfaces from which the conductors may be reached, bunching all of these together as they require, again, a 3-foot clearance to conductors. With all of these distances and clearance requirements it would just be a natural assumption that conductors should not pass below any openings through which items are moved, and that very requirement is explicitly stated in (D)(3). The most obvious location mentioned is a barn loft through which things are loaded.
Vegetation as Support
Skipping down to 225.26 Vegetation as Support, vegetation such as trees shall not be used for support of overhead conductor spans. This is very commonly violated. For some reason people just have a difficult time telling the difference between a wooden pole and a living tree, I guess.
Raceway Seal is a new item in 225.27 added in the 2011NEC. A similar requirement was covered in a previous article, but I think it is important to mention that any underground raceway entering a building shall be sealed.
Supplied by Feeders or Branch Circuits
This brings us to Part II of Article 225, which is Buildings or Other Structures Supplied by a Feeder(s) or Branch Circuits(s). If you are familiar with Article 230 (or have been cheating and reading ahead), you will notice that just about every item in this section is also covered in 230. This further supports the idea that services and outside feeders and branch circuits have many conditions which are the same. With this in mind and to reduce repetition, I will only touch on the unique issues here and we will cover the rest of the basics in our next article.
In 225.30 Number of Supplies, we are informed that only one feeder or branch circuit may supply a building, and of course there are exceptions. Note that for the purpose of this section, a multiwire branch circuit shall be considered as a single circuit. Also, a new item was added for the 2011 Code which states that if a feeder or branch circuit originates at one of these buildings, and is to feed something back in the original building or structure, only one circuit may feed back. This may sound totally confusing, but here is an example: the main building has the normal power service, and therefore has a feeder out to the parking garage, and this feeder goes to a panelboard which feeds all the normal lighting circuits. In this garage is located the emergency generator and the emergency distribution system. It feeds all the emergency lights for the stairs and other areas of the garage, but it also feeds the emergency power for the main building. Well, the code doesn’t allow twenty branch circuits to be fed back to the main building for this application. One circuit or feeder would feed to the main building from the emergency equipment in the garage; and once back at the main building an emergency panelboard would have to be set to feed the individual circuits within that building.
Access to Overcurrent Protective Devices
The last item to cover for 225 is 225.40 Access to Overcurrent Protective Devices, which covers the situation where one may not have ready access to the feeder overcurrent device. This could be for a variety reasons; for instance, you may be a tenant in a separate building on a property under one ownership. When that tenant does not have access to the feeder overcurrent device, the code requires that branch-circuit overcurrent devices shall be installed on the load side of the feeder, that they shall be readily accessible, and — a key point here — they shall be sized smaller than the feeder overcurrent device. The idea here is if you have an issue and a device opens, generally it will open where the tenant has access to it and can restore the power when the problem is solved. This will work most of the time, unless a high-fault current issue happens, which may also take out the feeder device on the line side.
This concludes our discussion of Article 225. Again, I challenge you to open the code and review the actual code language. Time and space certainly limit the amount of coverage possible when writing these articles, so please follow along and fill in the voids. Also, now would be a good time to compare 225 and 230 to see the similarities and be prepared for the next installment when we will dive into Article 230, Services.
Read more by Randy Hunter
Posted By Steve Foran,
Sunday, September 02, 2012
Updated: Wednesday, September 19, 2012
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Can you think of an accomplishment that is a source of great pride?
My foray into selective coordination served as my introduction to the electric utility industry. A coordination study was my very first job as an engineer. I had to review and update the protection along a number of 12-kV and 25-kV rural distribution feeders that had expanded as a result of many cottage developments. The loads had increased significantly as did the length of the feeders as well as the number of branch circuits. Quite honestly, when I started, I had no idea of what I was being asked to do.
Looking back almost 25 years to this project, several things really stand out in my memory. Namely, I was eager to do an excellent job and I learned a lot in the process of doing it. I developed a very good relationship with a senior engineer named Dave who became a mentor to me. He shared his expertise unselfishly and helped me successfully complete the project. In fact, it was from him that I learned the importance of practicality in engineering work. Dave once stoically told me, "Steve, never forget that "V” equals "I” times "R”.” Having just spent five years studying electrical engineering, I thought V=I*R was an odd piece of advice, but later realized the wisdom in its simplicity when faced with very challenging troubleshooting problems. Lastly, as I drive around that cottage region today, I can still see devices covered with my fingerprints from that coordination study back in 1987. As you might imagine, this continues to serve as a great source of pride although my wife is probably sick of hearing me drone on about the protection coordination stories I have told her dozens of times.
There are two lessons that strike me about this story. The first is that my success was almost totally dependent on others. True, I was keen and anxious to do a great job and I worked hard, but without Dave, the senior engineer who also became a good friend, I would have been lost. I have found this to be a universal truth in all aspects of life. So much of our success is dependent upon the work and contribution of others. This truth is not always acknowledged because it is sometimes hard to see. But consider that even Bill Gates had to depend on someone else to design and manufacture computers to run the software that Microsoft produces.
You likely have many successes that you can easily attribute to others. But what about that success you think you achieved on your own? For this success, I challenge you to dig a bit deeper and identify who helped you and what they did. If you have the courage to do this, here is the guarantee — you will be profoundly grateful for them and their contribution.
The second point that amazes me is how the impact of that simple coordination study has lasted for such a long time. I have not worked at the utility for more than ten years and yet the result of my contribution continues to affect how the power system operates in that area. There probably have been some changes to the protection, but these changes would have been built upon the work I did in my very first engineering role. Regardless, this remains a source of pride. The critical lesson you must remember is that everything you do makes a difference. Whether it is a coordination study in a processing plant, an inspection at a renovated home, a performance appraisal for a member of your team, how you greet your customers, or the way in which you say, "Thank-you” to the server at a restaurant —everything you do makes a difference. Remember this important caveat though: you get to decide if your actions are going to make a positive difference or a negative difference.
Thinking back to that accomplishment of yours that makes you proud, I bet you can see the positive impact that it has made in the world and I bet you had a mentor, a Dave, who helped you succeed.
Now go thank your Dave and then be a Dave for someone else.
Read more by Steve Foran
Posted By Marcus Sampson,
Sunday, September 02, 2012
Updated: Wednesday, September 19, 2012
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The carnival or traveling show will only be in town for the duration of the local festival or county fair before it’s torn down, packed up and moved to the next location. While the "jump” is something the carnie crew and roustabouts do each week, this may be the only inspection of a transient enterprise you are called on to do the entire year. Making sure your neighbors, friends and family are not exposed to electrical hazards means having an understanding of the National Electrical Code rules for transient electrical distribution systems.
Carnivals are amusement parks on wheels and a look at the wiring can reveal the negative effects of frequent teardowns, arduous road trips and rushed re-assembly. Many times, the various parts of the electrical distribution are composed and mended from random parts and pieces and inevitably show the consequences of life on the road. Equipment is subject to the moisture, dirt and vibration during hundreds or even thousands of miles traveled each year. Cords are often damaged by exposure to oils, gasoline, direct sunlight, foot and vehicular traffic and temperature extremes, arriving on-site worse for the wear. Disconnects, distribution boxes and cords are unloaded at each stop in various shades of disrepair.
Carnival work is grueling and repetitive. Workers are not always welcoming to code officials, who frequently add to their already long list of duties. Often, trucks arrive barely in time to set up before their first performance and taking time for safety inspections is not a priority. With pressure from event organizers, joint owners and ride operators, time for completing the inspections, identifying violations, and making the necessary re-inspections is limited.
With the safety of the carnival workers and the public at stake, it is important to check the entire distribution system for properly sized overcurrent devices, grounding and bonding continuity and GFCI functionality.
It’s easy to think that the portable wiring for traveling shows is permitted to rest a bit below the usual standards, but all temporary wiring must comply with the provisions for permanently installed wiring found in chapters 1 through 4. While some accommodations are found in Articles 525 or 590 for the conditions, additional provisions such as GFCI protection of branch circuits and equipotential bonding of equipment also apply.
Photo 1. Inspectors check the lighting on a kiddie ride for grounding continuity.
When you first arrive on the carnival or festival site, be sure to note the location of any overhead power distribution. Portable structures, including carnival rides, games, concessions and other units cannot be located within the area beneath a 4.5 m (15 ft) horizontal distance from conductors operating at more than 600-volts. Where the overhead conductors operate at 600-volts or less, no part of a portable structure can be located within a 4.5 m (15 ft) radius of the conductors.
Look around for rides or attractions that use large volumes of water and verify that the overhead conductor clearances of Table 680.8 are also met.
There are multiple options for power sources for carnival, fairs and other outdoor venues. Power can be obtained from permanently installed distribution in the park or fairgrounds or can be supplied directly from the local electric utility. Some shows will use a "hot truck,” a trailer with a high-voltage transformer meant to be connected to a utility primary distribution system, but often, one or more large generators will be positioned at deliberate locations.
The service equipment is generally located away from the action because it is not permitted to be accessible to unqualified persons unless it is lockable. From the service, feeder conductors and cables are laid out to the weatherproof power distribution units which, in turn, supply the game trailers, rides, joints, concession stands, ticket booths, etc.
Photo 2. Conductors used for portable power distribution should be examined for physical damage.
Grounding Electrode System
Keep in mind that both portable transformers and generators will be separately derived systems. Be sure to verify that the system bonding jumper is installed and sized per NEC 250.28. Next, explore the available grounding electrode options:
- Permanently installed rods or concrete-encased electrodes may be available if the site is host to recurring events.
- A plate electrode may also be used, if it meets the requirements of 250.52(A)(7).
- Water or other underground metal piping in the area may meet the provisions of 250.52.
- If pipe or rods are used to create the grounding electrode system, two must be installed at least 1.8 m (6 ft) apart and they must be fully driven. Alternatively, 3 or more partially driven rods spaced at least 1.8 m (6 ft) apart may be an acceptable means of achieving the required low-impedance earth contact. To assure a connection to earth with no more than 25 ohms of resistance, on-site soil conditions must also be considered.
NOTE: Before an 2.44 m (8 ft), copper-plated, pointed rod is fully driven, location services should be consulted to avoid unintentional contact with underground utilities.
Check the size of the unspliced grounding electrode conductor against Table 250.66, remembering that a conductor to a rod electrode need not be larger than 6 AWG copper, and must be securely fastened in place or protected by a raceway or armor.
Feeders and Branch Circuits
Look at the generator and at all the distribution boxes to be sure that the conductor terminations are made with suitable terminals or lugs used for no other purpose, such as securing devices within the equipment or mounting the equipment. Make sure that terminals and lugs contain only one conductor, unless specifically approved for more.
Photo 3. Cord caps and other equipment can be damaged during the road trip.
Whether the electrical distribution is extended from a transformer, generator or existing power source, it must be installed in approved wiring methods. Be sure that feeder and branch-circuit cables are sized per Article 210 and provided with overcurrent protection per Table 310.15(B)(1) or with the tap rules of Section 240.21.
Although you may see open wiring in walls of units that were built prior to today’s wiring standards, open wiring is not acceptable within the walls of attractions or concession trailers. In addition, you may find some older carnival rides that have the original open conductors installed along framework and covered with layers of paint. If these do not show signs of damage or disrepair, they may still be accepted.
That said, all damaged wiring needs to be repaired with approved methods or replaced. Cord and cable sheaths must be fully inserted and secured into cord caps, plugs or cam-lock devices.
Flexible Cords and Cables
Overcurrent protection for feeders and branch circuits extended using flexible cords and cables cannot exceed the maximum allowable ampacities in Table 400.5(A) (1) and (2) which has specifications for both single and multi-conductor cable types. Note that the adjustment factors found in Table 400.5(A)(3) for cables with more than 3 current-carrying conductors are the same as those in 310.15(B)(3)(a) for more than 3 current-carrying conductors in a raceway.
You may discover both single and multi-conductor cords with no type designation. These are not permitted, as cords are required to be type recognized in Article 400. Single conductor cables such as Type PPO or Type W are permitted in sizes 2 AWG or larger, but welding cables and locomotive cables are not allowed.
Photo 4. Conductors, cords and cables that cannot be properly repaired must be replaced.
Cords must be sunlight-, oil- and water-resistant and approved for extra-hard usage, although hard-usage cords and those with a "J” in the designation are permitted to be used within a portable unit, where not subject to physical damage. For example, SJO cords may be used to supply lighting inside a tent, where the cords are routed up and secured along the support poles.
Special precautions apply when cam-lock type connectors are used. These are quick-connect single conductor splicing and terminating devices, and while it’s easy to look at them as such, they are not attachment plugs or receptacles. They are meant to be installed and used by qualified persons and are to be guarded from accidental disconnection. When inspecting single-pole separable connectors look closely at the rules of 530.22 which apply, per 525.22(D).
As always, conductors of a feeder or branch circuit, including the equipment grounding conductor shall be part of the same cable assembly or must be grouped. Look to see that the individual conductors within cords are properly identified and that white conductors are only used as grounded — not equipment grounding — conductors.
You can easily use a simple lamp or audible-type tester on de-energized multi-conductor cords to verify continuity while verifying that conductors are properly terminated on cord caps and plugs.
Look at the overall distribution to assure that all the cords and cables are generally protected from physical damage. Splices are not permitted in cords; however, the outer covering of a cord may be repaired with listed shrink tubing products.
A good site plan will route all cables away from vehicular and pedestrian traffic, but some locations will be a challenge. In an open field, where a carnival or circus may set up for a few days, temporary shallow burial of the electrical distribution cords, permitted by NEC 525.20(G) may be appropriate.
Photo 5. Distribution boxes located in areas accessible to the public must be lockable.
If these solutions are impracticable, cable protection can be provided by mats or ramps, but be sure that they do not create a greater tripping hazard than the exposed cords alone. Extremely durable, commercial-grade ramps are available, but adequate protection can be achieved by other approved methods.
Junction Boxes and Panelboards
Check the separation of grounded circuit conductors from equipment grounding conductors and look for adequate wiring space within the enclosures, as well.
When looking at the large distribution boxes and panelboards make sure they are designed to be weatherproof and are situated so that the bottom of the enclosure is no less than 6 inches above the ground.
And when inspecting switch, circuit breaker and fuse enclosures be sure that they are dead-front, so operators and the public are never exposed to live parts.
Remember that all electrical equipment must be supplied by feeders and branch circuits that contain properly sized equipment grounding (bonding) conductors and that the continuity of the equipment grounding conductor system must be verified each time that portable electrical equipment is connected.
Not only receptacle devices, but all the equipment and enclosures must have the equipment grounding conductor connected to an approved grounding terminal. The EGCs are permitted to be bare; but if they are covered or insulated, single conductor or part of a multi-conductor cable, they must be green or identified with green paint or tape.
As with permanently installed wiring systems, the equipment grounding (bonding) conductors must be installed as a complete point-to-point system, so verify that enclosures are not used for grounding continuity.
On larger sites, you may see mobile units (rides, concessions, games, tents, etc.) supplied from different power sources and positioned with less than 3.6 m (12 ft) between them. Be sure that those units are bonded together to eliminate the possibility of potential difference, as required by 525.11. The bonding conductor can be covered, insulated or bare and is not required to be installed with other circuit conductors or in a raceway. Use the rating of the largest overcurrent device supplying the units and Table 250.122 to size the bonding conductor, keeping in mind that it cannot be smaller than 6 AWG.
Photo 6. Single-pole separable connectors (cam-locks) used for portable wiring on festival sites must be installed per 525.53(K).
Section 525.21 requires all rides, tents, and concessions to have a readily accessible switch that disconnects all power to the unit. The switch must disconnect the portable structure from all ungrounded conductors and be located within sight of and within 1.8 m (6 ft) of where the operator normally is stationed. This requirement is especially important when the ride is in operation; and for games and/or rides where the operator’s console is located away from the power disconnect switch, a shunt trip disconnecting device can be permitted. Be sure to check the functionality of the shunt-trip device.
Remember, too, that some larger rides may have a feeder for motor loads and a separate feeder for lighting, and if so, the switches must be positioned side-by-side or together, as this is intended as both a life safety switch as well as a maintenance disconnecting means. Check to see that these are clearly identified because in an emergency situation, it may not be the operator who needs to throw the switch to disconnect power.
When wiring for temporary lighting is installed inside tents, game trailers, concessions and other portable structures, it must be adequately secured with wire ties or other approved means. Festoon lighting or cord sets, both inside and outside, have to be installed at least 10 feet above ground where accessible to the public. The lamps themselves must be protected from accidental breakage by a suitable fixture guard.
While open conductors are generally not permitted for permanent or portable installations, they are allowed when used for festoon lighting or when part of a listed assembly. Section 225.6(B) requires a minimum of 12 AWG conductors for festoon lighting unless there is a messenger providing additional support. Be sure that neither the conductors nor the messengers are attached to a fire escape or secured to plumbing piping or downspouts.
Because of the electrical hazards associated with the use of electricity outdoors, all of the 125-volt, single-phase, 15- and 20-ampere non-locking-type receptacles on the festival grounds that are readily accessible to the general public or used for set-up are required to have GFCI protection.
But not only the receptacles, all equipment supplied from a 125-volt, single-phase, 15- or 20-ampere branch circuit that is readily accessible to the general public is required to have GFCI protection. This would include branch circuits used for free-standing perimeter lighting or lighting strings installed on fences as well as portable signs.
Both permanently installed GFCI receptacles and listed GFCI cord sets are permitted to be used to achieve the required level of life-safety protection. It is important for operators as well as inspectors to regularly check the functionality of the GFCI devices.
Remember the exception from the GFCI requirements for locking-type receptacles that are not accessible from grade. This provision would apply to the 120-volt interconnecting cables on removable portions of rides or concessions and that are not used to supply portable hand tools or equipment.
While 525.23 does require GFCI protection for almost all of the 15- and 20-amp, 120-volt distribution located outdoors, egress lighting that may be used in a tent or other portable structure is not permitted to be connected to a circuit or receptacle that is protected by a GFCI.
At the outset, it can seem like electrical inspections of Article 525 venues are significantly more difficult than inspections of other types of projects. However, the basic Code principals are the same whether the electricity is portable or permanent: a proper grounding electrode system; suitable overcurrent protection for services, feeders and branch circuits; guarding from weather and physical harm; intentional bonding of equipment and ground-fault circuit-interrupter protection for personnel where necessary.
Children of all ages attend the local festivals, fairs, carnivals and circuses without a thought toward their exposure to electrical hazards. Completing a thorough electrical inspection before the first customer enters the gate is the key to protecting your neighbors, friends and family.
Read more by Marcus Sampson
Posted By Steve Douglas,
Sunday, September 02, 2012
Updated: Wednesday, September 19, 2012
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The Canadian Electrical Code consists of five parts. Part I covers the installation and maintenance of electrical equipment, Part II is the safety standards for electrical products, and Part III is for outside wiring. Part IV is the objective-based industrial electrical code, and Part VI, the electrical inspection code for existing residential occupancies. This article will focus on Part I amendments.
The Part I committee consists of Part I members, associate members, subcommittee chairs, subcommittee members. A list of these members, complete with affiliation, is located in the front of the Canadian Electrical Code (CE Code). The Part I committee consists of voting members and non-voting (associate members). The voting members consist of a maximum of 41 members, 16 of whom are regulatory authority representatives and the remainder, from the industry.
Appendix C of the CE Code sets out committee and subcommittee structures detailing responsibilities and expectations. Members include inspection authorities, manufacturers of electrical equipment, employers, employees, consultants, utilities, testing laboratories, underwriters, or fire marshals, primary and secondary industries, respective code-making panels of NEC and users. Presently we have 50 IAEI member positions covering each of the 43 subcommittees.
Six Steps to a Successful Code Change
Step 1. Fill out Annex B in Appendix C and send it to the standards administrator of the Canadian Electrical Code, Part I. Proposals need to include specific wording for a proposed new rule or rule change, the reasons for the request, and background information to support the change.
Step 2. The standards administrator fills out Annex A from Appendix C and sends a copy to the subcommittee chair.
Step 3. The subcommittee chair adds comments and returns the proposal, now referred to as a subject back to the standards administrator.
Step 4. The standards administrator sends the subject to the subcommittee members for their comments. Communication of the subcommittee uses the CSA Standards Development Online Workspace (SDOW)
Step 5. After the comments are received from the subcommittee members, the subject is sent back to the subcommittee chair.
The subcommittee chair then decides if the subject is ready to be sent to the Part I Committee for a ballot, or if there is a need for it to be resubmitted to the subcommittee with a reworded proposal or additional rationale. The original submitter may be consulted at this point to ensure the intent of the proposal remains as purposed.
Step 6. When the subcommittee has achieved consensus, the subject is forwarded via the standards administrator to the Part I Committee for a ballot. The Part I Committee meets yearly in June to discuss subjects that received negative ballots. Successful subjects are filed for inclusion in the next edition of the CE Code. Unsuccessful subjects may be returned to the subcommittee or closed. The most important part of the process is the original submission; the more detail and rationale provided the better the success rate.
This article is an update of an article published in March-April 2005 issue ofIAEI News.
Read more by Steve Douglas
Posted By Steve Douglas,
Sunday, September 02, 2012
Updated: Wednesday, September 19, 2012
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How may times have you said to yourself? "I don’t understand why that rule reads the way it does. It doesn’t take ________ into account. I think I could have done a better job.”
Every IAEI member has an opportunity to participate in development of the Canadian Electrical Code (CE Code). A very effective way is through the subcommittees. That is where the work is done and key decisions are made.
Participation is very easy — no time consuming meetings. All work is done by correspondence — telephone, letter, e-mail and on-line — with adequate time to respond. If you are a subcommittee member, you are in a special position to suggest changes at the subcommittee level. You have an opportunity to comment on every proposed revision. You are advised of the eventual subcommittee recommendation and CE Code Part I Committee decision.
How does one get involved? Easy — let me know the code section where you have an interest and a brief resume with emphasis on your experience with both the code and the section where you have an interest. I’ve included a form. You may wish to indicate several sections. I’ll do what I can within certain constraints:
The IAEI already has appointees on all of sections (see table 1). If you want to be on a particular section subcommittee, you may have to wait.
Appendix C Clause C 5.3.3 and C5.3.4 of the CE Code detail limitations on subcommittee composition and numbers.
Please send the application to me at the e-mail address below, or send me an e-mail and I will send you the application as a Word document.
In conclusion, the CE Code is unique in its complete national acceptance as a model installation code. One reason is CSA’s leadership in making it a national consensus standard. Another is the high quality input from hundreds of volunteers. I can’t think of better qualified people to make recommendations on improvements to the code than members of the International Association of Electrical Inspectors. I encourage you to volunteer your knowledge and experience.
IAEI CE Code, Part I Committee Representative
Tel:(416) 241-8857 ext. 237
Application for IAEI Rep on Canadian Electrical Code Part I Subcommittee
Read more by Steve Douglas
Posted By Thomas A. Domitrovich,
Sunday, September 02, 2012
Updated: Wednesday, September 19, 2012
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One of the most basic calculations for any power system, and arguably the least understood and most misrepresented, is the calculation of available fault current. The effort of calculating fault currents flexes the basics of math and engineering. This article is not going to get into the details of the calculation; instead, we’ll have a high-level discussion to provide a general understanding of how that number is obtained. The mechanics of the calculations may be a good topic for a future article. We’ll also discuss how this relates to the NEC, especially NEC-2011 Section 110.24, Available Fault Current.
Short Circuit Study –A Basic Power System Tool
A system short circuit study is just one of many performed on power systems. Studies are performed for new and existing power systems, a few of which are described below:
Short Circuit:Determine the magnitude of the prospective currents flowing throughout the power system at various time intervals after a fault occurs. The information is used to select fuses, breakers, and switchgear ratings in addition to setting protective relays.
Load Flow:A system planning tool that determines voltage, current, active, and reactive power and power factor for a power system.
Coordination:Coordination studies select or verify the clearing characteristics of devices such as fuses, circuit breakers, and relays used in the protection scheme.
Power System Stability:The ability of a power system, containing two or more synchronous machines, to continue to operate after a change occurs on the system. This is a measure of its stability.
Harmonic Analysis:A study of power system harmonics to avoid control/computer system interferences, heating of rotating machinery and overheating/failure of capacitors. Predicts distortion levels of harmonic producing loads or capacitor banks.
Motor Starting:A study to help ensure the starting of large motors and continuous running of motors in the system operate without experiencing problems including
- Failing to accelerate up to running speed
- Stalling from excessive voltage drops
- Under voltage operating during motor starting
- Voltage dips causing objectionable flicker in the lighting system
These studies help achieve the goals that the design engineer sets for the power system. The power system engineer must evaluate initial and future system performance, reliability, safety, and ability to grow with production and/or operating requirements. Studies such as these are critical in the design process as well as throughout the life of the facility.
Types of Fault Current
There are a few important parameters that help define the electric power system; available fault current is one of those critical data points used in any and all of the following manners:
Interrupting ratings: The overcurrent protective devices must be able to interrupt the maximum available fault current.
Selective coordination: Time Current Characteristic (TCC) Curves are usually overlaid with each other to determine selective coordination. The available fault current is an important data point on these curves.
Arc flash: IEEE 1584 and NFPA 70E require fault current and clearing time to calculate arc flash values.
Equipment ratings: Fault current and time help determine withstand ratings of electrical equipment. The equipment has to be able to safely deliver this faulted current until it is cleared by an overcurrent protective device.
There are four types of faults that can occur in a power system. The engineer will have to consider more than just one fault current calculation to ultimately arrive upon a maximum value at any one point in the power system. The following are types of faults that can occur:
- Three-phase grounded or ungrounded faults
- Phase-to-phase (line to line) ungrounded faults
- Phase-to-phase ground (double line to ground) faults
- Phase-to-ground (single line to ground) faults
One of the first of many assumptions to be made during a fault study is that the above mentioned faults are bolted. This removes the arcing impedance that is normally a part of the circuit and yields a maximum available fault current. The three-phase ungrounded bolted fault is usually mistakenly assumed to be the maximum fault current when in all actuality the single phase-to-ground fault often produces a greater fault current under certain circumstances;
- when associated generators have solidly grounded neutrals or low-impedance neutral impedances
- on wye-grounded side of a delta-wye grounded transformer
In general, the power systems engineer will calculate a maximum and minimum fault current for a given distribution system. The maximum fault current is calculated on the following assumptions:
- all generators are in service (connected to the system and running);
- the fault is a bolted fault (fault impedance is zero);
- the load is a maximum (your on-peak load. Motors which contribute fault current will be connected and add to the total fault value.)
A minimum fault current is also calculated applying the following assumptions:
- The number of generators connected is minimum
- The fault is not a bolted fault (Fault impedance is not zero. A value between 30 and 40 ohms is commonly used.)
- The load is a minimum (off-peak load. Motors which contribute fault current will not be connected.)
Maximum and minimum fault currents will be used in different ways. Maximum fault currents help determine the required interrupting capacities of overcurrent protective devices. Minimum fault currents are used in coordinating operations of overcurrent devices, re-closers and relays.
The One-line Diagram
One of the first steps a power systems engineer will take is to secure an accurate up-to-date one-line diagram. New and existing construction projects present challenges to the power systems engineer. Let’s first talk about existing facilities. We have all seen one-line diagrams before, but I would say that probably very few of those have been accurate and up-to-date. The first short-circuit study of my career was for an existing industrial facility. After obtaining what was professed to be the latest accurate one-line diagram, my mentor advised a walk down of the facility. I approached that task as anyone who had the latest accurate one-line diagram in his hands would — with very little enthusiasm and feeling as if this task was a waste of time.
First visit: Main switchgear. Uneventful, it was exactly what I expected. The engineer, during a walk down, records breaker ratings and trip unit settings. Manufacturer and model numbers are very important as well. After visiting the main switchgear, it was off to visit all of those big grey boxes that you would expect to find throughout the facility. I spent an abnormal amount of time trying to find a particular MCC which proved quite illusive. After pulling floor plans, I asked questions to ensure I was in the right area. I walked that area of the plant more than once and asked an electrician for help finding a few MCCs highlighted on my drawings. He reviewed the drawings and grinned as he informed me that they had removed that equipment quite a few years ago. "You’re huntin’ a ghost, son” were his exact words.
He showed me various errors on the drawings and my efforts morphed into updating them — at least enough for me to recreate the one-line diagram at my desk. Some equipment was replaced and new equipment was installed. That’s the day I learned that an accurate updated one-line diagram is not only key to a successful short-circuit study but a rare item for some existing plants.
Missing motors and other equipment could put your calculated fault current numbers high or low depending upon the differences between paper and reality. Numbers that are high will make an expensive solution that is more than what is needed to get the job done and numbers that are low could create unsafe applications with equipment that is undersized for the job at hand.
New construction, you would think, would have the accurate up-to-date one-line diagram issue in the bag. Guess again. The challenge here is the lack of accurate information until the "as-builts” are completed. Conductor lengths are estimates and the loads you show may or may not be what actually makes it into the facility. The one-line diagram will be continuously changing as the project is being constructed.
There’s no walk down for a facility that is not there and no name plates to review. The challenge is to specify and purchase equipment based on estimated lengths of conductors and equipment with generic manufacturer data. The one-line diagrams should be updated based on as-built drawings to ensure accuracy. Updating drawings can be a laborious project but one that is very important to ensure systems analysis studies are as accurate as possible. It is one of the last tasks of the project and sometimes one of those that falls off the radar screen. The engineer will be making assumptions and using rules of thumb for many different aspects of the power system but conductor lengths, transformer impedances and other electrical equipment data that can be accurately included in systems studies should be reflected as such. This requires updated and accurate one-line diagrams.
Even with accurate one lines, the power systems engineer will make assumptions in calculating fault currents. Engineering judgment is used to estimate loads and impedances. You’ll learn more of these assumptions as we take the next step in this process, the impedance diagram.
If you thought putting together an accurate one-line diagram was difficult, you haven’t created an impedance diagram. Once you have an accurate one-line, the engineer of today will enter data in software applications that perform all of the necessary calculations. But it is not as simple as it sounds; the details of impedances for each of the components must be addressed. This step is critical for your calculations. Short circuit studies are based on the following very familiar equations.
V = IR (Eq-1)
I = V/R (Eq-2)
The impedance diagram is the tool that will give you the denominator in equation Eq-2. Electrical components are comprised of a real and reactive component for their impedance. Not too many electrical components are purely resistive. So the R value above, Resistance, is broken down into a real and reactive component usually expressed as Z shown below:
Z = R + JX (Eq-3)
The X in this equation is there due to the capacitive and reactive nature of electrical components. The above equations can be re-written as follows:
V = I (R + jX) (Eq-4)
I = V / (R + jX) (Eq-5)
The impedance diagram is an effort that takes all of the electrical components and replaces them with their equivalent R + JX. A single line equivalent is created with all R+jX’s in place of the electrical components. This sounds much simpler than it is. The challenge is on a component- or equipment-by-equipment basis. Some of the major components that require research include the following:
- Conductors / Busway
- Motors (Induction / Synchronous)
- Generators (Induction / Synchronous)
As noted above, even the utility will be represented as impedance. Even if the utility only provides available fault current, the engineer will convert that number into impedance for the calculations. But not all electrical device impedances are included. For example, impedances of breakers and fuses and other overcurrent protective devices are omitted and considered to be negligible. Conductor impedance data is readily available either from IEEE documents or NFPA 70. These numbers are usually given on an Ohms/1000Ft basis and not something that changes for each roll of conductor that you receive.
Transformer impedances on the other hand may not be the same as what you see in manufacturer’s literature when it arrives on-site. The impedance numbers found on the nameplate are calculated for each transformer when it is made. The data found in manufacturers marketing literature are usually minimum values that the engineer uses as assumptions for new construction projects and yield higher fault currents than what will be expected when the actual transformer and impedance data is used. In addition, transformer nameplates will usually only include the %Z and not the X/R ratio that is required to separate the Z into real and reactive components. Large transformers may have that additional test data but smaller transformers typically do not. The engineer often assumes the resistance is negligible and uses the number on the nameplate as X.
Similar issues arise for motors and generators. IEEE documents do provide resources to help estimate X/R ratios and other values for various electrical equipment. The engineer may leverage these resources as required.
The impedance of rotating machines is not a simple value but is rather complex and variable with time. Machines will have different values of impedance that are used in calculations for various reasons. You may recognize the following:
X”d – Subtransient Reactance:Used to determine current during the first cycle after a fault occurs. This is usually the smallest reactance yielding the largest contribution of fault current.
X’d – Transient Reactance:This impedance is used for calculations around 0.1 seconds. It helps calculate the current after several cycles at 60 Hz.
Xd – Synchronous Reactance:The impedance used for calculating currents above from .5 to 2 seconds. These currents determine the current flow after a steady-state condition.
Detailed data may or may not be readily available and assumptions may be needed to represent rotating machines in the system modeling. Again, engineering judgment and IEEE documents help provide those rules of thumb necessary to get you in the ballpark with respect to short circuit numbers. These documents and other similar documents provide industry standard assumptions that help fill in the gaps of information.
In most cases, the assumptions made are conservative in that a higher than actual fault current is a result. In most cases the higher fault current is the worse condition. But this is not always the case as such calculations as arc flash require lower available fault currents to produce longer clearing times which result in higher energy values. Smaller rotating machines will have less available data from the manufacturer as standard. Large motors will have the most detail as more data is necessary for protection schemes they require. These are larger investments for a facility and hence more detail around motor and generator performance is provided to help protect this investment. Again, this data is not something that you will find in marketing or engineering generic literature. This data is specific to the motor that was manufactured.
Short Circuit Calculation / Closing
Once the impedance diagram is created, most of the hard work has been achieved. The rest of the work is a process of mathematical equations that reduce the impedance diagram into a voltage source and impedance (Thévenin Equivalent) to calculate fault current by the equation above. The important work and most laborious efforts are made when constructing an accurate one-line diagram and its equivalent impedance diagram.
Calculating fault currents is more of an art than it is a science. The values calculated are the best possible numbers available based on nameplate data, engineering assumptions and the use of sophisticated software programs. The use of available fault currents must be done so with an understanding of the number being used and what it represents. Arriving upon a maximum fault current is a journey for the engineer during which time the power system is thoroughly investigated. The result of this effort can be easily misapplied when care is not taken.
As always, keep safety at the top of your list so that you and those that work around you live to see another day.
Read more by Thomas A. Domitrovich
Safety in Our States
Posted By Leslie Stoch,
Sunday, September 02, 2012
Updated: Wednesday, September 19, 2012
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The 2012 Canadian Electrical Code introduces some real changes. Many of them will be found in Sections 4 and 12 for wire and cable installations. No doubt we are already comfortable with the increased wire and cable ampacities in listed Tables 1 to 4. But there is more. This article discusses some of the more significant changes in these two sections of the CEC.
Rule 4-006 is spanking new. It’s a big adjustment that will make everyone stop and think twice. For the first time, the CEC requires that when electrical equipment is marked with maximum conductor temperatures, conductor ampacities must be based on the corresponding columns in Tables 1 to 4. Where equipment is unmarked, the 90º C temperature ratings can apply. If for example, a circuit-breaker has a maximum 75º C marking the connected conductor ampacity will be based on the ampacities shown in the 75º C column. And don’t forget, this requirement applies at both ends of the cable.
Rule 4-024 deals with minimum neutral conductor sizes. We are well aware that computers and other non-linear loads produce harmonic currents that impose additional loads on the neutrals of 3-phase, 4-wire systems. Engineers and designers have for ever and a day taken steps to ensure that electrical system neutrals are adequately sized, and therefore not overloaded. This issue is now recognized by Rule 4-024(2)(a)(ii) by the following amendment: "there shall be no reduction in the size of the neutral for that portion of the load that consists of non-lineal loads supplied from a 3-phase, 4-wire system.” There are other ways of fixing this problem but this change serves notice that the problem needs fixing and provides minimum requirements for shared neutrals.
Earlier versions of Rule 12-510(3) have long permitted fishing non-metallic sheathed cable through concealed spaces. A change to this rule adds some new restrictions — for obvious reasons, fishing is now no longer permitted where metal cladding, joists or plates are within the walls.
A new requirement, Rule 12-510(4) specifies that when receptacles or switches come complete with approved integral outlet boxes (separate outlet boxes not required) and an internal clamp, cables must be supported within 300 mm of the wall opening and there must be a minimum 300 mm loop of cable or 150 mm of cable end left in the wall to permit replacement. This ensures that should there be a failure, there will be enough cable remaining for reconnecting a new receptacle or switch.
There is also a series of new rules for installing armoured, jacketed cables in conduit or tubing. This narrative begins with Rule 12-602(6) which confirms that such installations are permissible. It takes us to Rule 12-614(3) which provides minimum bending radii for armoured, jacketed cable in conduit or tubing as follows:
- 10.5 x cable diameter for low voltage cables;
- 18 x cable diameter for high voltage cables; or
- as specified by cable manufacturers
This lengthy tale reaches its conclusion with Rule 12-902(2) which says — to avoid damage, one of two following conditions must be met:
1) The cable length must not exceed the calculated cable pulling tension; or
2) The conduit or tubing must not have more than two 90-degree bends between draw points and a minimum cable radius of .944 mm for cables up to 1000 V and 1.524 mm for cables over 1000 V with maximum cable lengths of:
- 15 m for 3-conductor copper;
- 45 m for single-conductor copper;
- 35 m for 3-conductor aluminum; or
- 100 m for single-conductor aluminum
As with previous articles, you should always consult with the electrical inspection authority in each province or territory for a more precise interpretation of any of the above.
Read more by Leslie Stoch
Posted By Ark Tsisserev,
Saturday, September 01, 2012
Updated: Tuesday, September 18, 2012
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In the electrical industry, there is hardly a subject that appliesmore often than the issue of bonding and grounding. And yet, this matter remains one of the most discussed, argued and misinterpreted by the electrical designers, contractors and inspectors.
Photo 1. Inspectors might note that the bonding and grounding equipment shown in this photo (a clamp) has no certification monogram indicating that it is "approved” or "listed” for the application in accordance with the relevant CSA standard in Canada or with the UL standard in the USA. Thus, such equipment may be code non-compliant in some jurisdictions.
Let’s revisit this subject from the CE Code perspective and confirm a few undisputed facts.
Section 10 of the CE Code covers general requirements for the bonding of electrical equipment (including bonding of conductor enclosure), for connecting of bonded equipment to groundand for grounding of electrical systems. Provisions of Section 10 are referenced throughout all relevantsections ofthe Code, where bondingof the specific equipment (installed under rules of those sections)is necessary.
In addition to Section10, Section 36(which deals with High Voltage Installations) addresses particular requirements for bonding and grounding of electrical equipment within boundaries of a HV station and connection of the equipment and structures forming part of the station to the station ground electrode.
Section 10 also providesthe Code users with a clear and transparent object for bonding and grounding as follows:
(1) The object of bonding metal parts and metal systems together and to the grounded system conductor is to reduce the danger of electric shock or property damage by providing a low impedance path for fault current back to the source and to establish an equipotential plane such that the possibility of a potential difference between metal parts is minimized.
(2) The object of grounding the electrical system and non–current-carrying metal parts is to connect the earth to the equipotential plane, thereby minimizing any potential difference to earth.
(3) The object of using an ungrounded system or a system incorporating neutral grounding devices is to provide an alternative to a solidly grounded system, thereby limiting the magnitude of fault current and minimizing the damage resulting from a single fault.”
Section 0 of the CE Code offers the following definitions related to the subject of bonding and grounding:
"Bonding– a low impedance path obtained by permanently joining all non–current-carrying metal parts to ensure electrical continuity and having the capacity to conduct safely any current likely to be imposed on it.
Bonding conductor– a conductor that connects the non–current-carrying parts of electrical equipment, raceways, or enclosures to the service equipment or system grounding conductor.
Ground– a connection to earth obtained by a grounding electrode.
Grounded– connected effectively with the general mass of the earth through a grounding path of sufficiently low impedance and having an ampacity sufficient at all times, under the most severe conditions liable to arise in practice, to prevent any current in the grounding conductor from causing a harmful voltage to exist
(a) between the grounding conductors and neighbouring exposed conducting surfaces that are in good contact with the earth; or
(b) between the grounding conductors and neighbouring surfaces of the earth itself.
Grounding– a permanent and continuous conductive path to the earth with sufficient ampacity to carry any fault current liable to be imposed on it, and of a sufficiently low impedance to limit the voltage rise above ground and to facilitate the operation of the protective devices in the circuit.
Grounding conductor– the conductor used to connect the service equipment or system to the grounding electrode.
Grounding electrode– a buried metal water-piping system or metal object or device buried in, or driven into, the ground to which a grounding conductor is electrically and mechanically connected.”
So, after being acquainted with the Code objective for bonding and grounding and afterbeing reminded of the Code definitions on thissubject, one could say that the bonding is a deliberateinterconnection of all exposed, non–current-carrying metal parts of electrical equipment by means of a conductor that would be able to carry a fault current back to the source. When the bonding action is completed, all such non–current-carrying metal parts of electrical equipment would be kept at the same potential. It could be said that grounding is a deliberate action of connection of the electrical system or service equipment to earth, so the neutral point of the electrical system and the enclosure of the service equipment would be kept at the potential of earth. Accordingly, bonding to ground is a deliberate action of connection of all bonded non–current-carrying metal parts of electrical equipment to earth at the service equipment via a grounding conductor, so that equipotential plane between allbonded equipment would be equal tothe potential of earth.
So, this matter appears to be quite simple. Whydoes it remain a subject of inconsistency and occasional confusion? Perhaps, the main reason for confusion is lack of appreciation for different functions of bonding and grounding. Experience has demonstrated that the main issues appear to relate to some of the following questions (we’ll limit the list to ten such questions): (1) Does a neutral conductorcarry a fault current? (2) Can grounding be done on the load side of the service disconnecting means? (3) What should be the size of a grounding conductor and does a grounding conductor carry a fault current? (4) What material could be used for bonding and grounding conductors? (5) Can a neutral be used for bonding? (6) Is isolated ground required by the Code? (7) Is the bonding conductor required to run with service conductors that incorporate a neutral conductor?(8) How to ground separate systems located in a building? (9) How to deal with bonding and grounding in buildings supplied from a single service installed in a remote building? (10) Does a ground loop around an outdoor pad-mounted transformer with HV primary and a typical LV (120/208 V or 347/600 V) secondary have to be extended to the LV service equipment in the building?
Of course, the list of the questions is not necessarily limited to these ten. But based on the practice, the Code users indeed respond to these listed questionsquite differently and apply the Code provisions in a not very consistent way.
So, let’s elaborate on the posed questions by briefly discussing each of them.
1. Does a neutral conductor carry a fault current? The neutral (or grounded conductor) that runs to each service, becomes an extension ofbonding conductors, as all bonding conductors terminate in the service enclosure, and the grounded service conductor (which is connected to the grounding conductor at the service) also terminates at the service enclosure. Rule 10-204(2)(b) of the CE Code recognizes the fact that such grounded service conductor functions as a bonding conductor (i.e., it carries a fault current between the service equipment and the source). Rule 10-624(4) further acknowledges this fact by allowing the neutral/grounded service conductor to be used for bonding of the service equipment (bonding screw is in).
2. Can grounding be done on the load side of the service disconnecting means? Except as permitted by Rule 10-208 (when more than one building is supplied from a single service, see answer to question 9 below), grounding or connection to earthis not permitted on the load side of the service disconnectingmeans. After a neutral conductor is terminated in the service equipment, is connected to a grounding electrode via a grounding conductor and is connected to the service equipment enclosure (usedfor bonding of the service equipment), the neutral conductor is no longerpermitted to be connected to ground. Rule 10-208 provides such exceptions for installations, where such neutral conductor is also run to thebuildings that are supplied from the single service.
3. What should the size of a grounding conductor be? As a grounding conductor only serves as means of connection for the neutral in the service equipment and for the bonded enclosure of the service equipment to a grounding electrode (to earth), 6 AWG copper conductor is deemed to be sufficient for this purpose. Rule 10-812 is abundantly clear on this subject. A grounding conductor is not intended to carry a fault current (a fault current will be carried by a bonding conductor to the service enclosure, and then — by the grounded service conductor/neutral — directlyto the source). The fault current will not go back to the source via a grounding conductor, grounding electrode and earth, as the impedance of this path would be much higher than the impedance of the service grounded conductor/neutral.
4. What material could be used for bonding and grounding conductors? Let’s start with grounding conductors. Rule 10-802 mandates use of copper for grounding conductors.Material for bonding conductors could vary, as long as the bonding means will provide a reliable low impedance path. The CE Code requires use of copper bonding conductors in HV installations (Rule 36-308), in patient care areas under provisions of Section 24 (Rule 24-104) and in fire alarm systems (Rule 32-100). However, in general Rule 10-804 states that bonding conductors could be "of copper or other corrosion-resistant material, insulated or bare.” This rule also recognizes that a busbar, steel pipe, metal raceways and metal cable sheath or armour could be used as bonding conductors under specific conditions.
5. Can a neutral be used for bonding? Neutral conductor is a grounded circuit conductor. Except for situations when a neutral conductor from a single service runs to the services in other buildings supplied from that single service (see Rule 10-208), neutral is not permitted to be used for bonding on the load side of the service disconnecting means. The neutral conductor is intended to carry a load current in a typical 2-wire circuit, or an unbalanced current in a single-phase, 3-wire system or 3-phase,4-wire system.However, when the neutral conductor is used in a supply or consumer’s service, it is allowed to be used for bonding of a meter mounting device and the service enclosure, as this neutral/grounded service conductor functions as a bonding conductor.
6. Is isolated ground required by the Code? This subject creates lots of misconception. The CE Code does not specifically mandate use of isolated bonding conductor. However, such approach is permitted under Rule 10-906(9) as follows: "10-906(9). Notwithstanding Rule 10-808, electronic equipment rated to operate at a supply voltage not exceeding 150 volts-to-ground and that requires a separate bonding conductor shall be permitted to be bonded to ground by an insulated conductor extending directly back to the distribution panel, provided that (a) the separate bonding conductor is enclosed in the same raceway or cable containing the circuit conductors throughout the length of that cable or raceway; (b) the separate bonding conductor is sized not less than a given in Table 16 for each leg of the run, determined by the size of the overcurrent protection for the circuit conductors; and (c) the bonding requirements of Rules 10-304 and 10-400 are met.”
7. Is bonding conductor required to run with service conductors that incorporate a neutral conductor? A separate bonding conductor should not be run with the service conductors that incorporate a neutral/grounded service conductor, since the grounded service conductor/ neutralfunctions as the bonding conductor between the upstream transformer (or other source) and the service equipment. In fact, if a separate bonding conductor runs in parallel with the service neutral, such approach may conflict with provisions of Rule 12-108 for installation of conductors in parallel. Answer to question 5 above also could be used in dealing with this subject.
8. How to ground separate systems located in a building? Each new electrical system is derived from a secondary of a transformer or from another source of power supply (i.e., a generator). There are numerous situations, where in addition to the main service in the building (i.e., 347/600 V), a number of step-down transformers is used to establish various electrical systems such as 277/480 V or 120/208 V, etc.Quite often, a building is also provided with an emergency or a back-up generator. Each new solidly grounded system must be connected to a grounding electrode via a grounding conductorin accordance with Rule 10-204(1)(a). Therefore, a separate grounding electrode would have to be installed for each new solidly grounded system that (in addition to the main service) exists at the building. However, Rule 10-206(2) of the CE Code allows use of the single grounding electrode of the main serviceas an alternative to the requirement of Rule 10-204(1)(a). This grounding electrode of the main service would become a common tie point between neutrals of different solidly grounded systems that exist in a building.
9. How to deal with bonding and grounding in buildings supplied from a single service installed in a remote building? Rule 10-208 allows for two options. Under first option, the neutral conductor running from the main service to a building supplied from this main service would function as a bonding conductor. In this case, no separate bonding conductor (between the main service and the service in any building supplied by the main single service) would be required, and the service in each building supplied from the main single service would have to be grounded in accordance with Rule 10-204(1)(b).Under the second option, each new service in the building supplied from the main single service would be considered as part ofdistribution system. In this case, noconnection to ground would be made at each such building, as the only connection to ground would be allowedat the main single service. The neutral conductor whichruns to each suchbuilding supplied from the single service, would be isolated from the service enclosure (it would be used only to carry a load current in a 2-wire circuit or an unbalanced current in a multi-wire circuit), and a separate bonding conductor would have to run from the main single service to each building supplied from this single service to bond the service equipment in each such building.
10. Does a ground loop around an outdoor pad-mounted transformer with HV primary and a typical LV (120/208 V or 347/600 V) secondary have to be extended to the LV service equipment in the building? Ground loop is required by Rule 36-302(1)(c) around high voltage station only. Such ground loop would have to be installed around the outdoor pad-mounted transformer and around all relevant HV equipment comprising high voltage station. However, such ground loop does not have to be extended from the low-voltage secondary of this outdoor transformer into the low-voltage service supplied from this transformer andlocated in the building. Such typical low-voltage service is originated from a new low-voltage system, and provisions of Section 36 of the CE Code for station ground electrode would not be applicable for such low-voltage installation.
It looks like the typical questions raised in this article in relation to the issue of bonding and grounding have been adequately answered. But there is no doubt that the nature ofthis subject will continue generate questions from the Code users. And, of course, the best way to deal with these questions (aside from readingthe Code), is to discuss them with relevant electrical safety regulatory bodies.
Read more by Ark Tsisserev
Posted By Charles Palmieri,
Saturday, September 01, 2012
Updated: Tuesday, September 18, 2012
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Is it code-compliant to install Type NM cable exposed in an attached residential garage? How about a detached garage? What about a detached residential building such as a storage shed? BG
Thank you for the inquiry. For clarity, I will frame my response. First, I will substitute the term residential with the term dwelling which is defined in NEC Article 100. Second, I will consider the attached garages or detached garages and storage sheds to be directly associated with one- and two-family dwellings. Third, all structures considered will be of Types III, IV, and V construction. For this discussion, I will focus my response to the language of the 2011 National Electrical Code and before I begin, I will state my understanding of your question. You wish to know if nonmetallic-sheathed cable is permitted to be installed as an exposed wiring method in an attached or detached garage or in a detached building such as storage shed. Whenever there is a need to determine the permitted uses of any wiring method, I begin by reviewing the XXX.10 Uses Permitted section for that specific wiring method. I will start by reviewing 334.10 which generally states that nonmetallic-sheathed cable is permitted to be used in accordance with its following five (5) listed items. For this discussion we will only consider the language in list items (1) and (3).
The language of 334.10(1) permits the use of nonmetallic-sheathed cable in one- and two-family dwellings including their attached or detached garages and their storage buildings. This section was revised for the 2011 edition of the NEC by inserting the words "and their attached or detached garages, and their storage buildings” after the word "dwellings.” The new language was added to make it clear that the use of nonmetallic-sheathed cable as an exposed wiring method in one- and two-family dwellings, including their attached and detached garages and storage sheds, is permitted. To further support this conclusion, I look at the previous editions’ language in 334.10(1) which stated that nonmetallic-sheathed cable was permitted in one- and two-family dwellings. This language did not prohibit exposed cable installations in a dwelling or its attached garages, but it was silent in regard to installations in detached garages or other detached structures associated with a dwelling.
Under previous editions of the Code, installations in detached structures associated with dwellings were guided by the language of 334.10(3). That section addresses installations of nonmetallic-sheathed cable in other structures which are not dwellings and the language gives us two basic requirements. (1) It restricts the use of the cable to those structures that conform to Types III, IV, and V construction and (2) it requires that the cable be concealed within walls, floors, and ceilings that provide a thermal barrier of material that has at least a 15-minute finish rating.
Keeping all this in mind and upon review of sections of 334.10(3) and the revised language of 334.10(1), it is clear that nonmetallic-sheathed cable is permitted to be installed exposed in one- and two-family dwellings, their attached and detached garages, and their storage buildings. It is additionally important to note that regardless of the building’s condition of occupancy where the cable is installed exposed, it must comply with the provisions of 334.15; and if it is determined that the installation is subject to physical damage, then guarding in accordance with 334.15(B) must be considered.
Additional sections of the Code relevant to the installation and routing of the cable will provide guidance for a compliant installation (see 300.4, for example). It is also helpful to note that structures are grouped into five general types. These types are summarized in the Informative Annex E titled, "Types of Construction,” which may be found on page 814 of the 2011 NEC (soft cover). I hope this information is helpful.
IAEI Principal Member, CMP-7
Focus on the Code