Protecting conductors

For a 1,200-ampere (A), three-phase, 120/208-volt (V) service, can I run three sets of 500 kcmil underground from the transformer pad to the main 1,200A switch, or does it have to be 600 kcmil?

Table 310.16 shows a 500 kcmil copper conductor with a 75-degree rating as having an ampacity of 380A. Three sets run in parallel is 1,140A. NEC 240.4(B) permits the use of the next higher standard overcurrent device rating above the ampacity of the conductors being protected, if the next higher standard rating does not exceed 800A. NEC 240.4(C) requires, where the overcurrent device is rated more than 800A, the ampacity of the conductors it protects shall be equal to or greater than the rating of the overcurrent device. Therefore, you cannot run three sets of 500 kcmil copper conductors for a 1,200A system.

Undercabinet lighting termination

I wired my house for fluorescent lights under the kitchen cabinets. I left the wire straight out, and the inspector said I needed a box on the end of the wire. I told him the fluorescent light will be the box since the wiring terminates in the lighting fixture. Am I wrong? How else can you wire undercabinet lights for the counter?

Apparently, the inspector is asking that a box be furnished to terminate this fixture feed because you don’t have the fixtures and are asking for an occupancy permit before you get them.

20A circuits, 15A receptacles

Are 20A circuits required for all receptacle outlets in a commercial building? If I use 20A circuits, can 15A receptacles be connected to them?

There’s no NEC requirement that the minimum rating of receptacle circuits in commercial building be rated 20A. However, many local ordinances require 20A circuits in commercial buildings, so check with the authority having jurisdiction (AHJ) for the requirements of the municipality where you are working. NEC Table 210.21(B)(3) permits receptacles rated 15A to be used on 20A circuits that supply two or more receptacles or outlets.

Calculating conductor size

Article 250.122 (B)—Size of Equipment Grounding Conductors–Increased in Size reads, “Where ungrounded conductors are increased in size, equipment grounding conductors, where installed, shall be increased in size proportionately according to circular mil area of the ungrounded conductors.” If I install No. 1 copper conductors in lieu of No. 3 copper conductors (to compensate for voltage drop) for a circuit protected by a 100A overcurrent device, what size of copper equipment grounding conductor is required, and how do you calculate it?

Go to Table 8 in the NEC. A 3 AWG conductor has a cross-sectional area of 52,620 circular mils. You are increasing the size to a 1 AWG, which has a circular mils area of 83,690. Dividing 83,690 by 52,620 equals an increase of 1.60 in the circular mil area. The equipment grounding (bonding) conductor required by Table 250.122 for a 100A overcurrent device is an 8 AWG copper conductor, which Table 8 indicates as being 16,510 circular mils. Multiply 16,510 by 1.60 to increase the size proportionately, according to the increase in size in circular mil area of the ungrounded conductors and find 26,416 as the required circular mil area for the equipment grounding (bonding) conductor. Table 8 shows that a 4 AWG conductor has a circular mil area of 41,740. A 6 AWG conductor has a cross-sectional area of 26,240 circular mils, but don’t be tempted.

Mind the gap

Why is it so important that the maximum gap around a box or fitting be no more than ¹/₈ inch?

This requirement permits gaps or open spaces not greater than 1/8 inch at the edge of the box or fitting, which can be closed by the device faceplate. This requirement addresses a safety issue. If the gap around the box extends beyond the outside edge of the faceplate, someone could accidently access live parts.

Grounding old switches

I replaced some switches in an old residence and found that they used NM cable without a ground wire. How do I complywith the requirement that switches must be grounded?

NEC 404.9(B) requires snap switches to be effectively grounded and provide a means to ground metal faceplates. There is an exception to this requirement that, where the wiring method does not provide an equipment ground, a snap switch without a grounding connection is permitted for replacement purposes only. If you employ this exception, a nonconducting faceplate generally must be used.

Transformer accessibility

I installed a 75 kVA, 480-120/208V transformer hanging from the bar joists in a warehouse. Now they are making a lunchroom in that area and are putting a dropped ceiling below the transformer. I am told that the transformer has to be readily accessible and must be moved. Is this correct?

Partly. Transformers installed in hollow spaces not permanently closed in by structure are not required by 450.13(B) to be readily accessible. However, only dry-type transformers, 600V or less and not exceeding 50 kVA may be installed in these hollow spaces if they meet 450.9’s ventilation requirements and 450.21(A)’s separation from combustible materials requirements. Your transformer exceeds 50 kVA and must be moved, or a portion of the lunchroom ceiling must remain open. (For more on dry-type transformers, see Code Comments, page 130.)

Connected to the earth

What does it mean to say the Code requires grounded electrical systems to be connected to earth in a manner that will stabilize the voltage to earth during normal operation?

An ungrounded conductor can reach any voltage within that system with respect to earth. When one conductor is grounded, a fixed potential to ground is established. For example, if you ground the common conductor of a single-phase, 3-wire, 120/240V transformer secondary, the highest voltage to ground that any conductor in that system can reach is 50 percent of the transformer secondary potential (or 120V) because the grounded conductor is tapped at midpoint of the 240V secondary winding.

Heavy-duty lampholders

Can I put standard four-lamp T8 fluorescent fixtures on a 30A circuit without individual fusing?

NEC 210.23(B) permits the use of 30A circuits to supply fixed lighting units with heavy-duty lampholders in other than a dwelling unit. However, the terms “heavy-duty” and “heavy-duty lampholders” are not defined in Article 100 of the NEC, nor as far as I can determine in UL standards. “Heavy duty lampholder” is used in 210.21(A), which states, where “connected to a branch circuit having a rating in excess of 20A, lampholders shall be of the heavy-duty type. A heavy-duty lampholder shall have a rating of not less than 660W if of the addendum type, or not less than 750W if of any other type.” This may qualify as a definition. I do not believe that the fixtures you are questioning would qualify as having heavy-duty lampholders and are limited to 15- or 20A circuits.

Installing breaker locks

If I install a breaker lock, will it still trip under overload?

Circuit breakers are trip-free, so if the handle is in the on position, the internal mechanism will trip to the off position under overload. The breaker lock just prevents manual operation of the circuit breaker.

Multiwire branch circuits

If the tab is broken to make a two-circuit receptacle, should the two circuits be placed on a breaker with a common trip?

NEC 210.4(B) requires that, where a multiwire branch circuit supplies more than one device on the same yoke, a means must be provided to disconnect simultaneously all ungrounded conductors supplying those devices at the point where the branch circuit originates. A multiwire branch circuit consists of two or more ungrounded conductors of opposite polarity sharing one grounded conductor. A two-pole circuit breaker will comply.

Nice catch

In the July Code FAQs, there was a question about sizing the overcurrent protection for a hot water heater. The graphic that was included on the center of the page shows a gas water heater that typically does not require an electrical connection. I enjoy reading your articles and look forward to each issue of Electrical Contractor.

Very observant. I’d like to say I was checking on your attention to detail, but we slipped up and apologize for our error. Stay with us. We need you.