Jim Dollard has an extensive background in codes and standards. Send questions about the National Electrical Code (NEC) to Jim at [email protected]. Answers are based on the 2020 NEC.

NM vs. UF cable in a barn

During township plan review, we were told that type NM cable was not permitted in a barn used for horses, etc., and that we had to use type UF cable. Why is type NM cable not permitted? We see that type NMC is permitted, but we could not find any to purchase.

A barn used for horses and other livestock as stated in your question falls under the scope of Article 547, Agricultural Buildings. Section 547.5 provides requirements for wiring methods, and 547.5(A) permits multiple wiring methods including, but not limited to, types UF and NMC cable. The standard type of NM cable (the most common) used today is type NMB. See Section 334.10(A), which provides uses permitted for type NM cable (NMB). Uses permitted for type NMC cable are addressed in 334.10(B) and include dry, moist, damp or corrosive locations, such as those common in an agricultural building. Type NM cable is permitted only in normally dry locations. An internet search failed to locate any type NMC cable readily available, but there were several cable assemblies on the market that are dual-marked as Type UF/NMC.

Explosion-proof flexible fittings

Where flexibility is required in a Class I, Division I location, the Code permits flexible fittings listed for the location. No wiring method is mentioned, but there is permission for flexible cord, TC-ER-HL and even Type P cable. Where flexibility is required in a Class I, Division II location, the same permission for flexible fittings listed for the location exists and flexible metal conduit with listed fittings. I’m confused; what wiring method goes with flexible fittings listed for the location?

Section 501.10 contains requirements for permitted wiring methods in Class I locations, and 501.10(A)(2) contains permitted wiring methods for Class I, Division I locations that require flexibility, such as a motor, and the first method listed is “flexible fittings listed for the location.” This text does not refer to a flexible raceway or wiring method and listed fittings. These fittings are manufactured in short lengths in a single assembly. They are sometimes referred to as “explosion-proof flex” fittings. These fittings are listed for the location (Class I, Division I) and will also be listed for the group classification as required in 500.6(A). These fittings are typically seen at motor terminations in a Class I, Division I location.

Exit sign location

The fire marshal denied a certificate of occupancy due to the location of multiple exit signs. We located the signs per the engineered drawings and already had our final electrical inspection approved. The electrical inspector told us that they have purview over the electrical installation, but not the exact location of exit signs. Is that right?

Yes, in most cases an electrical inspector enforces only the NEC , and the fire marshal inspects many other life safety requirements such as means of egress. The NEC has purview over the electrical installation of emergency systems (see Article 700) but does not have purview over the location of exit signs. The Life Safety Code, NFPA 101, governs the location of exit signs in Section 7.10, Marking of Means of Egress.

EGC over 1,000V

Section 250.180 requires that circuits over 1,000V be installed in accordance with all of Article 250, including Part X for high-voltage. A drawing was submitted for a 5-kilovolt feeder, fused at 30A with a 10 AWG copper equipment grounding conductor (EGC). The plan reviewer required 6 AWG copper. We modified the drawing but disagree. Who is correct?

Part X of Article 250 applies to the grounding of systems and circuits over 1,000V. As you stated, the first section in this part (250.180) requires compliance with all of the preceding requirements in Article 250 as supplemented or modified in Part X. See Section 250.190(C) for EGC requirements for circuits over 1,000V. Section 250.190(C)(1) requires that, in general, EGCs for circuits over 1,000V shall not be smaller than 6 AWG copper or 4 AWG aluminum.

EMT as EGC?

During the replacement of an existing panelboard in a commercial venue, the electrical inspector informed us that we would not get a final inspection approved unless we installed EGC in the existing feeder because it was a parallel feeder. The feeder is installed in two 4-inch conduits, electrical metallic tubing (EMT). Was he correct?

No, in this case the inspector was incorrect. EMT is recognized in the NEC as an EGC. See Section 250.118, which permits the EGC to be run with or enclose the circuit conductors. Metal raceways including, but not limited to, EMT, IMC and RMC enclose the circuit conductors and provide an effective ground fault return path. The fact that this was a parallel feeder has no relevance whatsoever. Boilerplate specifications typically require a wire-type EGC to be installed in EMT. This regular practice leads many individuals to incorrectly believe that it is an NEC requirement.

Open knockouts in panelboard

It is typical for us to reuse panelboards in temporary installations. We recently sent a few to a job site, and the general contractor took issue with a couple of 4-inch box covers used to cover unused knockout holes and wanted KO blanks installed instead. Can we use a blank cover in that manner?

Yes, the NEC does not prohibit the use of a blank box cover for covering unused holes. Unused openings must be closed. Section 110.12(A) requires that all openings other than those intended for the operation of equipment, intended for mounting purposes or permitted as part of the design for listed equipment, be closed to afford protection substantially equivalent to the wall of the equipment. The use of a 4-inch blank box cover would certainly be considered as “substantially equivalent to the wall of the equipment.” Individual KO blanks may also be used, but are not the only option.

Transformer overcurrent protection

Why does 450.3 permit overcurrent protection on the primary side to go up to 250% on a three-phase transformer? I understand that secondary protection is required, but isn’t it always there? Why would anyone do that with larger conductors, etc.? Secondary conductors are always protected anyway.

Section 450.3 requirements are only limited to the transformer’s protection. Requirements for protection of conductors on the primary and secondary of the transformer are located in, but not limited to, sections 240.4 and 240.21. The permission in Table 450.3(B) to protect a transformer with an OCPD on the primary at not over 250% rated primary current, provided secondary overcurrent protection is provided not to exceed 125% of rated secondary current, exists to allow flexibility in design. For example, two transformers of equal rating could be supplied from a single OCPD, requiring a larger device on the primary. Transformer secondary conductors must be protected in accordance with 240.21(C).

SUSE-rated ATS

Where is the requirement for an automatic transfer switch (ATS) used for optional standby in a dwelling unit to be rated as suitable for use in service equipment (SUSE)? What if the ATS is not service supplied?

An ATS used in an optional (or other) standby system may be designed and installed in multiple scenarios. An ATS is required to be SUSE-rated only where it is used as service equipment. See Section 230.66(A), which requires service equipment rated at 1,000V or less to be: (1) marked as SUSE and (2) listed or field evaluated. This applies to any equipment used as service equipment, including an ATS. Where an ATS is not installed as service equipment, 230.66 does not apply and the ATS is not required to be SUSE-rated. Note that 230.66 does not consider meter sockets as service equipment, but they must be listed and rated for the service voltage/current.