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 2023 NEC.

Emergency shutoff

Working on a gas station project, there are no structures less than 100 feet away on which to mount the emergency shutoff. Is there an exception that would allow it at 125 feet away from dispensers?

No, see Section 511.4(A), which prohibits the emergency shutoff device from being closer than 20 feet or more than 100 feet from the gas pumps (dispensers). In this case, you will need to build a support structure (unistrut or other) no closer than 20 feet or no more than 100 feet from the gas pumps on which to mount the shutoff device.

Field evaluation of luminaires

We installed about 250 unlisted, specialty lighting fixtures in a high-end hotel. The inspector is requiring a field evaluation report for each fixture. Is that necessary? What will this cost?

The inspector is correct. See Section 410.6, which requires luminaires, lampholders and retrofit kits to be listed. In this case, they must be “field labeled,” which is defined in Article 100. The luminaires must be evaluated to determine compliance with applicable product standards. A field label is required and a field evaluation report must be provided. See Section 110.3(C), which requires that this evaluation be performed by a recognized, qualified electrical testing laboratory, and the evaluation must determine compliance with applicable product standards recognized as achieving equivalent and effective safety for equipment installed to comply with the NEC. An informational note following 110.3(C) provides guidance in this situation. The cost of a field evaluation can only be provided by a recognized, qualified electrical testing laboratory. It is important to note that this additional cost can be eliminated by using listed luminaires.

Meeting or exceeding breaker ratings

A city plans examiner red-flagged our drawings for equipment replacement in an industrial facility. The equipment being replaced was a 225-kVA transformer, stepping 480V down to 208/120V. The existing transformer secondary conductors (2 x (4) 600 kcmil aluminum) supplied a small MLO switchboard with seven 3-pole circuit breakers. The ampacity of the existing secondary conductors exceeds the rated transformer secondary current. We have to upsize the secondary conductors to meet or exceed the sum of the seven circuit breaker ratings. Why? Is the plans examiner correct?

This question is about conductor protection. Sections 240.4(F) and 240.21(C)(1) are clear and explain in detail that conductors supplied by a wye-connected transformer secondary are not considered to be protected by the transformer primary overcurrent protective device (OCPD) under any circumstance. The general rules for transformer secondary conductors are located in 240.21(C)(2), the 10-foot rule, and 240.21(C)(6), the 25-foot rule. These requirements mandate the transformer secondary conductors terminate in an OCPD. Keep in mind that multiple sets of transformer secondary conductors are permitted under these requirements (see the first sentence in the parent text of 240.21(C)), provided that each set of conductors terminates in an OCPD.

An industrial installation is not defined in the NEC. Industrial installations typically have raw materials coming into the venue and finished products leaving it. In this case, 240.21(C)(3) applies and permits the transformer secondary conductors to supply main lug terminals in a switchgear or switchboard supplying multiple OCPDs in accordance with the four list items provided. List item 240.21(C)(3)(2) requires the ampacity of the transformer secondary conductors be not less than the secondary current rating of the transformer, and the sum of the ratings of the OCPDs supplied in the MLO switchboard cannot exceed the ampacity of the secondary conductors.

While you did not provide the ampacity rating of the seven circuit breakers in the existing panelboard, it is apparent that value is larger than the ampacity of the existing conductors. The rule’s intent is to prevent overloading the transformer secondary conductors. It is possible that without termination in a single OCPD, the load could exceed the conductor ampacity without opening the required OCPD protecting the transformer (Table 450.3(B)), which could be set as high as 250% of the rated transformer primary current.

Difference between room and suite

I am confused on how to distinguish between a guest room and guest suite. Is it always that a cooktop is in the room? Are they wired differently?

No, cooking equipment is not part of the determination between a guest room or guest suite. See the defined terms in Article 100. Both are accommodations that contain living, sleeping, sanitary and storage facilities within a compartment. To be classed as a guest suite, there must be two or more contiguous rooms comprising a compartment, with or without doors between the rooms. The requirements are the same for guest rooms or suites. 

Where either a guest room or guest suite contains permanent provisions for cooking, Section 210.17 applies, and all branch circuits must be installed to meet the rules for dwelling units.

Building Type?

Is Type NM cable permitted in commercial spaces? We are seeing it more and more in existing installations. Where in the NEC can we find if this is limited to residential only?

The use of Type NM cable is not based on residential, commercial or other occupancy classifications; it is based on sections 334.10 for uses permitted and 334.12 for uses not permitted. In general, 334.10 permits Type NM in any one- and two-family dwelling and associated garages and storage buildings, multifamily dwellings and their detached garages permitted to be of types III, IV and V construction and other structures (could be a commercial space) permitted to be of types III, IV and V construction. In these other structures, Type NM must be concealed within walls, floors or ceilings that provide a thermal barrier of material that has at least a 15-minute finish rating as identified in listings of fire-rated assemblies.

It is necessary to determine the building type. New construction documents will include this information. Where an existing building is encountered and the building type is questionable, I suggest that you refer to informative Annex E for information on types of construction, and where necessary, contact the building code official to determine building type.

Electric range demand

Where a single 48-inch-wide household electric range/oven is rated at 20.3 kW, how do I derate for demand? Table 220.55 does not go over 12 kW.

When applying any table in the NEC, it is extremely important to read all table notes. These are an enforceable part of the Code and contain mandatory text. In this case, Note 1 to Table 220.55 applies, and it addresses ranges over 12 kW through 27 kW that are all of the same rating. This note applies to a single range or multiple  ranges of the same rating. The maximum demand in kilowatts from column C can be used by increasing this value by 5% for each additional kilowatt of rating or major fraction thereof by which the rating of individual ranges exceeds 12 kW. Therefore 20 – 12 = 8 kW over the column value of 12 kW. 8 x 5% = 40%. The column C value for one range is 8 kW and must be increased by 40%. Your permitted demand for one 20.3 kW range is 8 x 1.4 = 11.2 kVA or 11,200 VA.

GEC through ventilation openings

Is it permitted to take the grounding electrode conductor  through the vents on the bottom of a transformer? I’ve done it this way for years and co-workers tell me it’s wrong.

Requirements for the installation of GECs are located in Section 250.64. Enclosures, including, but not limited to, transformers, switchboards and motor control centers all have ventilation openings. See Section 250.64(G), which prohibits the installation of a GEC through these openings.

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