Your most pressing National Electrical Code (NEC) questions answered
All questions and answers are based on the 2011 NEC.
Q. What is the Code rule regarding enclosures that contain splices, taps, and feed-through conductors?
A. Cabinets, cutout boxes, and meter socket enclosures can be used for conductors as feeding through, spliced, or tapping off to other enclosures, switches, or overcurrent devices where all of the following conditions are met [312.8]:
- The total area of the conductors at any cross section doesn’t exceed 40% of the cross-sectional area of the space [312.8(A)].
- The total area of conductors, splices, and taps installed at any cross section doesn’t exceed 75% of the cross-sectional area of that space [312.8(B)].
- A warning label on the enclosure identifies the disconnecting means for feed-through conductors [312.8(C)], as shown in Fig. 1.
Q. What is the rule on having a neutral conductor at light switch locations?
A. Switches controlling line-to-neutral lighting loads must have a neutral provided at the switch location [404.2(C)].
Exception: The neutral conductor isn’t required at the switch location if:
- The conductors for switches enter the device box through a raceway that has sufficient cross-sectional area to accommodate a neutral conductor.
- Cable assemblies for switches enter the box through a framing cavity that’s open at the top or bottom on the same floor level, or switches enter the box through a wall, floor, or ceiling that’s unfinished on one side.
Note: The purpose of the neutral conductor is to complete a circuit path for electronic lighting control devices.
Q. Where are tamper-resistant receptacles required?
A. All nonlocking type 15A and 20A, 125V receptacles in the following areas of a dwelling unit [210.52] must be listed as tamper-resistant [406.12].
- 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)
Exception: Receptacles in the following locations aren’t required to be tamper-resistant:
- Receptacles located more than 5½ ft above the floor.
- Receptacles that are part of a luminaire or appliance.
- A receptacle located within dedicated space for an appliance that in normal use isn’t easily moved from one place to another.
- Nongrounding receptacles used for replacements as permitted in 406.4(D)(2)(a).
Nonlocking type 15A and 20A, 125V receptacles in guest rooms and guest suites must be listed as tamper-resistant [406.13]. In addition, nonlocking type 15A and 20A, 125V receptacles in child care facilities must be listed as tamper-resistant [406.14].
Q. What are the NEC rules for securing and supporting raceways?
A. Raceways, cable assemblies, boxes, cabinets, and fittings must be securely fastened in place. The ceiling-support wires or ceiling grid must not be used to support raceways and cables (power, signaling, or communications). However, independent support wires that are secured at both ends and provide secure support are permitted [300.11(A)].
Outlet boxes [314.23(D)] and luminaires can be secured to the suspended-ceiling grid if securely fastened to the ceiling-framing members by mechanical means such as bolts, screws, or rivets, or by the use of clips or other securing means identified for use with the type of ceiling-framing member(s) used [410.36(B)].
Electrical wiring within the cavity of a fire-rated floor-ceiling or roof-ceiling assembly can be supported by independent support wires attached to the ceiling assembly. The independent support wires must be distinguishable from the suspended-ceiling support wires by color, tagging, or other effective means [300.11(A)(1)].
Wiring in a nonfire-rated floor-ceiling or roof-ceiling assembly can be supported by independent support wires attached to the ceiling assembly. The independent support wires must be distinguishable from the suspended-ceiling support wires by color, tagging, or other effective means [300.11(A)(2)].
Raceways must not be used as a means of support for other raceways, cables, or nonelectrical equipment, except [300.11(B)]:
- If the raceway or means of support is identified for the purpose [300.11(B)(1)].
- Class 2 and 3 cable can be supported by the raceway that supplies power to the equipment controlled by the Class 2 or 3 circuit [300.11(B)(2)].
- Raceways are permitted as a means of support for threaded boxes and conduit bodies in accordance with 314.23(E) and (F), or to support luminaires in accordance with 410.36(E) [300.11(B)(3)].
Q. Can you explain how to size 90°C conductors terminating on 75°C rated equipment?
A. Conductors are to be sized using their ampacity from the insulation temperature rating column of Table 310.15(B)(16) that corresponds to the lowest temperature rating of any terminal, device, or conductor of the circuit [110.14(C)]. Unless the equipment is listed and marked otherwise, conductor sizing for equipment terminations must be based on Table 310.15(B)(16) in accordance with (a) or (b):
(a) Equipment Rated 100A or Less [110.14(C)(1)(a)].
- Conductors must be sized using the 60°C temperature column of Table 310.15(B)(16) [110.14(C)(1)(a)(1)].
- Conductors terminating on terminals rated 75°C are sized in accordance with the ampacities listed in the 75°C temperature column of Table 310.15(B)(16) [110.14(C)(1)(a)(3)], as shown in Fig. 2.
(b) Equipment Rated Over 100A.
- Conductors must be sized using the 75°C temperature column of Table 310.15(B)(16) [110.14(C)(1)(b)(1)].
- Separate connector provisions. Conductors can be sized to the 90°C column of Table 310.15(B)(16) if the conductors and pressure connectors are rated at least 90°C [110.14(C)(1)(b)(2)].
Q. What are the installation requirements for equipment disconnecting means?
A. Switches and circuit breakers used as switches must be capable of being operated from a readily accessible location. They must also be installed so the center of the grip of the operating handle of the switch or circuit breaker, when in its highest position, isn’t more than 6 ft, 7 in. above the floor or working platform [240.24(A) and 404.8(A)].
The disconnecting means for a mobile home must be installed so the bottom of the enclosure isn’t less than 2 ft above the finished grade or working platform [550.32(F)].
Exception No. 1: On busways, fusible switches and circuit breakers can be located at the same level as the busway where suitable means is provided to operate the handle of the device from the floor.
Exception No. 2: Switches and circuit breakers used as switches can be mounted above 6 ft - 7 in. if they’re next to the equipment they supply, and are accessible by portable means [240.24(A)(4)].
Q. What are the grounding requirements for a building supplied by a feeder?
A. Each building/structure’s disconnect must be connected to an electrode of a type identified in 250.52 [250.32(A)]. The grounding of the building/structure disconnecting means to the earth is intended to help in limiting induced voltages on the metal parts from nearby lightning strikes [250.4(A)(1)].
The Code prohibits the use of the earth to serve as an effective ground-fault current path [250.4(A)(5) and 250.4(B)(4)].
Exception: A grounding electrode isn’t required where the building/structure is served with a 2-wire, 3-wire, or 4-wire multiwire branch circuit.