Your most pressing National Electrical Code (NEC) questions answered
All questions and answers are based on the 2011 NEC.
Q. How do I size the equipment grounding conductors for a branch circuit or feeder if the conductors are increased in size for any reason?
A. Ungrounded conductors are sometimes increased in size to accommodate conductor voltage drop, harmonic current heating, short circuit rating, or simply for future capacity. If ungrounded conductors are increased in size from the minimum size, equipment grounding conductors must be proportionately increased in size according to the circular mil area of the ungrounded conductors [250.122(B)].
Here’s an example to further explain this concept. If the ungrounded conductors for a 40A circuit are increased in size from 8 AWG to 6 AWG, what size circuit equipment grounding conductor must be used?
The circular mil area of 6 AWG is 59% more than 8 AWG (26,240 circular mil as compared to 16,510 circular mil) [Chapter 9, Table 8]. According to Table 250.122, the circuit equipment grounding conductor for a 40A overcurrent device will be 10 AWG (10,380 circular mil), but the circuit equipment grounding conductor for this circuit must be increased in size by a multiplier of 1.59.
Conductor size = 10,380 circular mil × 1.59 = 16,504 circular mil
Thus, you must use an 8 AWG grounding conductor (Chapter 9, Table 8).
Q. What is the Code rule regarding mixing different voltage systems, such as 120V and 480V in the same raceway or enclosure?
A. Power conductors of AC and DC systems rated 600V or less can occupy the same raceway, cable, or enclosure if all conductors have an insulation voltage rating not less than the maximum circuit voltage [300.3(C)(1)], as shown in Fig. 1.
Control, signal, and communications wiring must be separated from power and lighting circuits so the higher-voltage conductors do not accidentally energize the control, signal, or communications wiring. See the following references in the NEC:
- CATV Coaxial Cable, 820.133(A)
- Class 1, Class 2, and Class 3 Control Circuits, 725.48 and 725.136(A)
- Communications Circuits, 800.133(A)(1)(c)
- Fire Alarm Circuits, 760.136(A)
- Instrumentation Tray Cable, 727.5
- Sound Circuits, 640.9(C)
Class circuit conductors can be installed with associated power conductors [725.48(B)(1)] if all conductors have an insulation voltage rating not less than the maximum circuit voltage [300.3(C)(1)].
A Class 2 circuit that has been reclassified as a Class 1 circuit [725.130(A) Ex 2] can be installed with associated power conductors [725.48(B)(1)] if all conductors have an insulation voltage rating not less than the maximum circuit voltage [300.3(C)(1)].
300.3(C)(1) Note 2: PV system conductors, both direct current and alternating current, are permitted to be installed in the same raceways, outlet and junction boxes, or similar fittings with each other, but they must be kept entirely independent of all other non-PV system wiring [690.4(B)].
Q. Is AFCI protection required in nondwelling occupancies?
A. No. Here is the rule for AFCI protection, and it applies only to dwellings: 120V branch circuits in dwelling units supplying outlets in family rooms, dining rooms, living rooms, parlors, libraries, dens, bedrooms, sunrooms, recreation rooms, closets, hallways, or similar rooms or areas must be protected by a listed AFCI device of the combination type [210.12].
The 120V circuit limitation means AFCI protection isn’t required for equipment rated 230V, such as a baseboard heater or room air conditioner.
Note 3: See 760.41(B) and 760.121(B) for power-supply requirements for fire alarm systems.
Smoke alarms connected to a 15A or 20A circuit of a dwelling unit must be AFCI protected if the smoke alarm is located in one of the areas specified in 210.12(A). The exemption from AFCI protection for the “fire alarm circuit” contained in 760.41(B) and 760.121(B) doesn’t apply to the single- or multiple-station smoke alarm circuit typically installed in dwelling unit bedroom areas. This is because a smoke alarm circuit isn’t a fire alarm circuit as defined in NFPA 72, National Fire Alarm Code. Unlike single- or multiple-station smoke alarms, fire alarm systems are managed by a fire alarm control panel.
Q. What are the requirements for installing receptacles at a dwelling unit kitchen island?
A. At least one receptacle outlet must be installed at each island countertop space with a long dimension of 2 ft or more, and a short dimension of 1 ft or more [210.52(C)(2)]. The Code doesn’t require more than one receptacle outlet in an island or peninsular countertop space, regardless of the length of the countertop, unless the countertop is broken as described in 210.52(C)(4).
Q. How are branch circuits sized for motors?
A. Conductors to a single motor must be sized not less than 125% of the motor FLC rating as listed in Table 430.247 Direct-Current Motors, Table 430.248 Single-Phase Motors, or Table 430.250 Three-Phase Motors [430.22].
Q. Must all conductors [ungrounded and neutral] of a circuit be run in the same raceway?
A. All conductors of a circuit must be installed in the same raceway, cable, trench, cord, or cable tray, except as permitted by (1) through (4) [300.3(B)]. Conductors installed in parallel in accordance with 310.10(H) must have all circuit conductors within the same raceway, cable tray, trench, or cable [300.3(B)(1)].
Exception: Parallel conductors run underground can be installed in different raceways (Phase A in raceway 1, Phase B in raceway 2, and so forth) if, in order to reduce or eliminate inductive heating, the raceway is nonmetallic or nonmagnetic and the installation complies with 300.20(B). See 300.5(I) Exception No. 2.
All conductors of a circuit must be installed in the same raceway, cable, trench, cord, or cable tray to minimize induction of the heating of ferrous metal raceways and enclosures, and to maintain a low-impedance ground-fault current path [250.4(A)(3)].
Q. What is the Code rule regarding neutral conductors at switches?
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 is unfinished on one side, as shown in Fig. 2.
Note: The purpose of the neutral conductor is to complete a circuit path for electronic lighting control devices.
Q. What are the overcurrent protection requirements for transformers under 600V?
A. The primary winding of a transformer of 600V and less must be protected against overcurrent in accordance with the percentages listed in Table 450.3(B) and all applicable notes [450.3(B)].