Dwelling Unit Receptacles, NEC Annex Explanations and More

If you have a problem related to the National Electrical Code (NEC), are experiencing difficulty in understanding a Code requirement, or are wondering if or why such a requirement exists, ask Charlie, and he will let the Code decide. Questions can be sent to letthecodedecide@earthlink.com.

High dwelling-unit receptacle
Does a dwelling-unit receptacle installed more than 51⁄2 feet above the floor need to be tamper-resistant?
Yes, it does if the receptacles are installed in any of the areas specified in 210.52. NEC 406.11 requires that, in all areas specified in 210.52, all 125-volt (V), 15- and 20-ampere (A) receptacles must be listed as tamper-resistant receptacles.

Adding to already-calculated loads
Please explain Example D1(b) in Annex D.
This example shows the calculations required where additional loads are included with the original loads already calculated in Example No. D1(a). See the table below. The feeder current from D1(a) is 78A each on Line A and B with a neutral load of 61A.

No neutral load is calculated for the 10A dishwasher since it is balanced out with the 12A air conditioning unit using the larger of the two appliances for unbalance.

To calculate the requirement for the largest motor as required by 430.24, we must use the largest motor on Line A, which is 12A 0.25 = 3A on Line A and 3A for the neutral. For the largest motor on Line B we use 8A 0.25 = 2A. No neutral load is required for the Line B motor because it is balanced out with the 12A motor on Line A.

Grounding conductors
I was taught that it was permissible to not count the grounding conductor when calculating conduit fill because it is not a current-carrying conductor. Doesn’t the grounding conductor carry the unbalance current in a multiwire circuit and the return current in a two-wire circuit?
A grounding conductor (equipment-grounding conductor) is the conductor used to connect noncurrent-carrying metal parts of equipment together and to the system-grounded conductor (neutral). The grounded conductor (neutral) is the intentionally grounded conductor.

The grounding conductor is not a current-carrying conductor, but it takes up space and is required to be counted when calculating conduit fill. See Chapter 9, Note 3 to Table 1.

It is the intentionally grounded conductor (neutral) that carries the unbalanced current from other conductors of the same circuit.

Perhaps what you were taught was that, when calculating the adjustment factors in 310.15(B)(2), it is permitted by 310.15(B)(4)(a) to not count a grounded-neutral conductor that carries only the unbalanced current.

Swapping receptacles
Does the NEC allow you to change an old two-prong 120V ungrounded receptacle to a three-prong 120V grounded receptacle when the wiring method inside the metal outlet box is the old-style BX, manufactured before 1959 with no metal bonding strip inside cable?

BX is armored cable, and the requirements are shown in NEC Article 320. The old cable you are questioning has no provisions for grounding but can be used with grounding-type receptacles in accordance with NEC 406.3(D)(3)(b), which permits a nongrounding-type receptacle to be replaced with a ground-fault circuit interrupter-type receptacle. These receptacles must be marked “No Equipment Ground.” Grounded receptacles can be installed downstream supplied from the ground-fault circuit interrupter (GFCI) receptacle. An equipment-grounding conductor must not be connected from the GFCI-type receptacle to any outlet supplied from the GFCI receptacle.

Derating cables
What is the derating requirement for Type NM cables?
Derating of conductors in Type NM cables is addressed in NEC 334.80. Where more than two NM cables containing two or more current-carrying conductors are installed, without spacing between the cable and through the same opening in wood framing that is to be fire or draft-stopped using thermal insulation, caulk or sealing foam, the allowable ampacity of each conductor must be adjusted in accordance with Table 310.15(B)(2)(a).
NEC 300.21 requires that openings around electrical penetrations through fire-rated walls, partitions, floors or ceilings must be fire-stopped using approved methods to maintain the fire-resistant rating.

Where more than two NM cables containing two or more current-carrying conductors are installed in contact with thermal insulation without maintaining spacing between the cables, the allowable ampacity of each conductor must be adjusted in accordance with Table 310.15(B)(2)(a).
Where multiconductor cables are installed without maintaining spacing for a continuous length longer than 24 inches, the allowable ampacity of each conductor shall be reduced as shown in Table 310.15(B)(2)(a).

Sound system power
In connecting power to a permanently installed sound system rack, can I wire three or four branch circuits directly to the power distribution inside the rack? Am I required to provide a disconnecting means other than the dedicated branch-circuit breakers?
NEC 640.9(A)(1) requires the power wiring to comply with the requirements of Chapter 1 through 4, except as modified by Article 640. You are permitted to install a distribution panel inside the rack where you can wire branch circuits as needed. The branch-circuit breaker supplying the distribution panel is the only required disconnecting means.

Copper feeders
What size copper feeders are required for a 120/208V, 125A service? We submitted 1 AWG THHN but are being told it must be 1/0 because of NEC 110.14(c)(1).
NEC 110.14(C)(1) states, when selecting a conductor, the temperature rating of the conductor used to determine the conductor ampacity must be coordinated with the lowest temperature rating of any device or equipment to which it is terminated. NEC 110.14(C)(1)(b) permits—for termination provisions of equipment for circuits rated over 100A—the use of conductors rated 75°C. The conductor ampacities must be based on Table 310.16.

This means that, if the equipment is not marked with a temperature rating but is marked or rated for 100A or less, the provisions of 110.14(C)(1)(a) must be used. If the equipment is not marked with a temperature rating but is marked for circuits rated over 100A, 110.14(C)(1)(b) must be used.
I believe that if the service equipment is rated for 75°C, your use of 1 AWG THHN is correct.

Grounding metal lighting poles
In reference to grounding a metal lighting pole, is that pole considered a structure, and as such, does it need a grounding electrode?
The definition of a structure according to NEC Article 100 is “That which is built or constructed.” I don’t believe the manufacturing process can be described in that manner. A grounding electrode is permitted but not required for a metal lighting pole. Some designers believe that a grounding electrode is necessary at metal lighting poles for protection from lightning. NEC 250.54 covers auxiliary grounding electrodes and emphasizes that when used, they do not take the place of an equipment-grounding conductor and that the earth shall not be used as an effective ground-fault-current path.

TROUT answers the Code Question of the Day on the NECA Web site. He can be reached at letthecodedecide@earthlink.net.

About the Author

Charlie Trout

Code Contributor
Charlie Trout is most known for his work with the National Electrical Code (NEC). He helped write the NEC Since 1990; he was a member of NECA’s National Codes & Standards Committee and chairman of the National Fire Protection Association (NFPA)’s Cod...

Stay Informed Join our Newsletter

Having trouble finding time to sit down with the latest issue of
ELECTRICAL CONTRACTOR? Don't worry, we'll come to you.