NEC Questions and Answers – Based on the 2011 NEC
September 2011 - Part 1 of 2

By Mike Holt for EC&M Magazine

Here’s the follow up to yesterday’s newsletter. This includes all of the answers to the questions sent, so you can see how you did.

Q1. Are outdoor receptacles at a dwelling unit required to be tamper resistant?

A1. Yes. According to 406.12:

All nonlocking type 15A and 20A, 125V receptacles in the following areas of a dwelling unit [210.52] must be listed as tamper resistant.

•    Wall Space—210.52(A)

•    Small-Appliance Circuit—210.52(B)

•    Countertop Space—210.52(C)

•    Bathroom Area—210.52(D)

•    Outdoors—210.52(E)

•    Laundry Area—210.52(F)

•    Garage and Outbuildings—210.52(G)

•    Hallways—210.52(H)

Ex: Receptacles in the following locations aren’t required to be tamper-resistant:

(1) Receptacles located more than 5½ ft above the floor.

(2) Receptacles that are part of a luminaire or appliance.

(3) A receptacle located within dedicated space for an appliance that in normal use isn’t easily moved from one place to another.

(4) Nongrounding receptacles used for replacements as permitted in 406.4(D)(2)(a).

 

Q2. Are outlet boxes allowed to be supported by the ceiling framing members or must independent support wires be added?

A2. Outlet boxes can be supported to the structural or supporting elements of a suspended ceiling, if securely fastened by one of the following methods [314.23(D)]:

(1) Ceiling-Framing Members. An outlet box can be secured to suspended-ceiling framing members by bolts, screws, rivets, clips, or other means identified for the suspended-ceiling framing member(s).

Comment: If framing members of suspended-ceiling systems are used to support luminaires, they must be securely fastened to each other and must be securely attached to the building structure at appropriate intervals. In addition, luminaires must be attached to the suspended-ceiling framing members with screws, bolts, rivets, or clips listed and identified for such use [410.36(B)].

(2) Independent Support Wires. Outlet boxes can be secured, with fittings identified for the purpose, to the ceiling-support wires. If independent support wires are used for outlet box support, they must be taut and secured at both ends [300.11(A)].

Comment: See 300.11(A) on the use of independent support wires to support raceways and cables.

Q3. When is the neutral considered to be a current carrying conductor for the purposes of ampacity adjustment?

A3. There are several cases when the neutral is considered to be current carrying according to 310.15(B)(5):

• The neutral conductor of a 3-wire, single-phase, 120/240V system, or 4-wire, three-phase, 120/208V or 277/480V wye-connected system, isn’t considered a current-carrying conductor for conductor ampacity adjustment of 310.15(B)(3)(a) [310.15(B)(5)(a)].
• The neutral conductor of a 3-wire circuit from a 4-wire, three-phase, 120/208V or 277/480V wye-connected system is considered a current-carrying conductor for conductor ampacity adjustment of 310.15(B)(3)(a) [310.15(B)(5)(b)].

Comment: When a 3-wire circuit is supplied from a 4-wire, three-phase, 120/208V or 277/480V wye-connected system, the neutral conductor carries approximately the same current as the ungrounded conductors.

• The neutral conductor of a 4-wire, three-phase, 120/208V or 277/480V wye-connected system is considered a current-carrying conductor for conductor ampacity adjustment of 310.15(B)(3)(a) if more than 50 percent of the neutral load consists of nonlinear loads [310.15(B)(5)(c)].

Comment: Nonlinear loads supplied by a 4-wire, three-phase, 120/208V or 277/480V wye-connected system can produce unwanted and potentially hazardous odd triplen harmonic currents (3rd, 9th, 15th, and so on) that can add on the neutral conductor. To prevent fire or equipment damage from excessive harmonic neutral current, the designer should consider increasing the size of the neutral conductor or installing a separate neutral for each phase. For more information, visit www.MikeHolt.com, click on the “Technical” link, then the “Power Quality” link. Also see 210.4(A) Note, 220.61 Note 2, and 450.3 Note 2.

 

Q4. Does the Code allow the installation of cord-connected equipment above a suspended ceiling?

A4. Unless specifically permitted in 400.7, flexible cords must not be concealed by walls, floors, or ceilings, or located above suspended or dropped ceilings [400.8(5)].

Comments::

•  Flexible cords are permitted under a raised floor (with removable panels) used for environmental air, because this area isn’t considered a concealed space. See the definition of “Exposed” in Article 100.

•  Receptacles are permitted above a suspended ceiling, but a flexible cord isn’t. Why install a receptacle above a ceiling if the flexible cord isn’t permitted in this space? Because the receptacle can be used for portable tools; it just can’t be used for cord-and-plug-connected equipment fastened in place, such as a projector.

Q5. How many general use receptacles are allowed per circuit for dwelling units?

A5. For dwelling units, 210.11(A) requires that the minimum number of general lighting and general-use receptacle branch circuits must be determined by dividing the total calculated load in amperes by the ampere rating of the circuits used.

Question: How many 15A, 120V circuits are required for the general lighting and general-use receptacles for a dwelling having floor area of 1500 ft2, exclusive of an unfinished cellar not adaptable for future use [Example D1(a) in Annex D].

(a) 1              (b) 2             (c) 3           (d) 4

Answer: (d) 4

Step 1:        Determine the total VA load:

VA = 1,500 sq ft x 3 VA per sq ft [Table 220.12]
VA = 4,500 VA

Step 2:        Determine the amperes:

I = VA/E
I = 4,500VA/120V
I = 38A

Step 3:        Determine the number of circuits:

Number of Circuits = 38A/15A
Number of Circuits = Three 15A, or two 20A, 120V

Comment: There’s no limit to the number of receptacles on a circuit in a dwelling unit.

If the load is calculated on the volt-amperes/square foot, the wiring system must be provided to serve the calculated load, with the loads evenly proportioned among multioutlet branch circuits within the panelboard [210.11(A)].