Jim Dollard has an extensive background in codes and standards. If you have a query about the National Electrical Code (NEC), Jim will help you solve it. Questions can be sent to [email protected]. Answers are based on the 2014 NEC.


Point of supply


During a recent NEC training session, we had an interesting discussion about where a conductor must be protected by a fuse or circuit breaker. However, I am still confused. The question is about the wording in 240.21. This requirement states that each ungrounded conductor must have protection at the point where the conductors receive their supply. What if a disconnect is added to the circuit? Does that require a circuit breaker or fuse to protect the conductors on the load side of the disconnect? The rest of the section refers to the tap rules and states that I cannot supply another conductor from the tap unless it is through an overcurrent protective device. What does that really mean?


Article 240—Overcurrent Protection is separated into nine parts for clarity and usability. To answer your questions, we must start in Part I, General. Section 240.4 contains the general requirements for conductor protection. The general rule for conductors, other than flexible cords, cables and fixture wire, is to provide overcurrent protection in accordance with the conductor ampacity in 310.15. Multiple subdivisions in 240.4 recognize other methods, including 240.4(E), which references multiple NEC sections for tap conductors, including 240.21. Part II of Article 240 addresses the location of the overcurrent protection. The parent text of 240.21 states that the overcurrent protection must be located at “the point where the conductors receive their supply except as specified in 240.21(A) through (H).” This requirement’s intent is to ensure that the entire length of the ungrounded conductor is protected. For example, a branch circuit is installed using copper 12 AWG conductors. The ungrounded conductor is protected with a 20-ampere (A) circuit breaker and is run into a junction box. In the box, the branch circuit is spliced to two 12/2 copper cable assemblies. The conductors within the cable assemblies meet the requirements of 240.21 because they are protected at their rated ampacity (240.4) at the point they receive their supply (240.21) by the 20A circuit breaker located upstream. There is no requirement that every ungrounded conductor must terminate in an overcurrent protective device. The requirement is that adequate overcurrent protection is located at the point of termination or elsewhere upstream.


The last sentence in 240.21 essentially prohibits “tapping a tap.” For example, the provisions of 240.21(C)(6) address the rules for transformer secondary conductors not longer than 25 feet. Section 240.21(C)(6)(2) requires the tap conductors to terminate in an overcurrent protective device. This prohibits running transformer secondary conductors into a wireway to supply multiple circuits. The conductors must first terminate in an overcurrent protective device that limits the load current to not more than the conductor ampacity that 310.15 permits. In this case, as well as for feeder taps, the provisions of 240.4(B) may not be applied.


Arc-resistant switchgear


The new informational note in Article 100, after the definition of switchgear, states that the equipment is available in non-arc-resistant or arc-resistant construction. We have never installed or even seen an arc-resistant piece of switchgear. Outside of an NFPA 70E class and this informational note, I have never heard of it. How does it work, and, if it is safer, why is it not mandatory that all switchgear be arc-resistant?


Arc-resistant switchgear channels arc energy inside the equipment up and away from where people operating the equipment would be located. So it is typically specified where operator safety is a concern. You will recognize this type of equipment because it is required to have a separate label stating that the switchgear is certified as “arc resistant.” There is no blanket requirement for arc-resistant switchgear because other methods may be employed to reduce exposure to injury from arc energy, including, but not limited to, energy-reduction maintenance switches (see 240.87), remote-racking devices, zone-selective interlocking, and remote operation of circuit breakers.


Ampacity-derating requirements


Why does the NEC require spare conductors to be counted as current-carrying conductors for derating purposes? It seems to be a bit ridiculous when you look at the branch-circuit loads in a commercial occupancy and apply all of the ampacity-derating factors. For more than 35 years, we have been doing a substantial amount of work for a large mechanical contractor with many rooftop installations. We are fortunate to have a great relationship with the local township electrical inspector. He has been a big help with new changes in the NEC and keeps us straight. We are now required to install 3 AWG copper circuit conductors where we were once permitted to use 6 AWG copper conductors. The 6 AWG installations are still in place, and we have never experienced an issue with respect to sunlight on raceways containing circuit conductors. In a few cases, we have actually had to splice back to smaller conductors inside the equipment to make the termination. Is there a logical explanation for this?


The stated purpose of the NEC is the practical safeguarding of people and property from potential electrical hazards. All of the requirements therein are based on that premise. While some rules may seem excessive, there was technical substantiation to support the requirement. The definition of the term “ampacity” in Article 100 gives us the basis for derating factors in the NEC. The intent is to ensure conductors do not carry more current under given conditions of use that would result in a level of heat exceeding the conductor-temperature rating. One of the heat sources that is considered is the heat created by current flow through conductors. It is logical to assume spare conductors will be used as current-
carrying conductors at some point, and that is the reason for including spares in the calculation.


While your question does not specify a conductor type for the rooftop installation, the vast majority of conductors installed in raceway will have a marked temperature rating of 90°C or 194°F. This means that—after we apply all of the rules in 310.15, including the rooftop adders—the temperature of the conductor must not exceed 194°F. A technical committee accepted the rooftop adders, and their actions were based on technical substantiation available at that time. This issue has received a lot of attention over the last few years, and the new exception in 310.15(B)(3)(c) is significant. It exempts type XHHW-2 insulated conductors from rooftop-derating factors. Data provided showed that, even when under stress, XHHW-2 type insulation subjected to the heat shock test (typically run for one hour) for 60 days performed extremely well. We are finding that the insulation types we use today are extremely robust and perform far better than ever anticipated and that many derating rules existed prior to these newer insulation types. The 2017 NEC revision cycle will most certainly involve some type of action with respect to the rooftop-derating factors. I urge you to get involved in the Code-revision process. Everyone can have an impact on the next edition of the NEC.


Locating a panelboard


The drawings for a condominium project we just completed showed the panelboard location in a clothes closet in some units and over a small countertop in another type of unit. We informed the owner that the NEC did not permit both locations noted on the drawings. These units get progressively smaller, and there is just not much real estate left to locate the panelboard. The architect would not permit the panelboard in the limited living space. Our only feasible option was to install it on the stairway landing.
There was plenty of working space. Is
that permitted?


Yes. Section 240.24(F) prohibits the installation of overcurrent devices (circuit breakers, in this case) to be installed over the “steps of a stairway.” The intent here is to prohibit installing a panelboard over the steps themselves, which would require an installer/maintainer to straddle multiple steps while performing maintenance. The language was carefully chosen to permit such an installation provided the requirements of Section 110.26 are met.