The wiring method used in Figure 124.103 is 14/3 w/ground AWG MC cable. Which of the device boxes listed below is the smallest you can use and not violate the NEC? Each cable contains an EGC.

The wiring method used in Figure 124.103 is 14/3 w/ground AWG MC cable. Which of the device boxes listed below is the smallest you can use and not violate the NEC? Each cable contains an EGC.

• A. 3 × 2 × 2-1/2 in.
• B. 3 × 2 × 2 in.
• C. 3 × 2 × 2 3/4 in.
• D. 3 × 2 × 3 in.

Answer: (C)

Box fill is the key idea here. The NEC requires a box to have enough cubic inches to harbor all conductors that end or splice inside it, plus any internal hardware like device yokes, clamps, and so on. For 14 AWG copper, each insulated conductor that originates outside the box counts as one fill unit, and all equipment grounding conductors in the box together count as a single fill unit. Each device yoke in the box also adds one fill unit. The volume per fill unit for 14 AWG is 2.0 cubic inches. In this scenario, there are two MC cables with 14/3 w/ground entering the box. That gives six insulated conductors (three from each cable). The equipment grounding conductors from both cables exist, but all grounds together count as one fill unit. There is one device mounted in the box, so its yoke adds one fill unit. Total fill units = 6 (insulated conductors) + 1 (EGCs) + 1 (device yoke) = 8. Multiply by 2.0 cu in per unit = 16 cubic inches required. Among the listed boxes, the volumes are 12, 15, 16.5, and 18 cubic inches. The smallest box that meets or exceeds 16 cubic inches is the one with 16.5 cubic inches, which corresponds to a depth of 2 3/4 inches. So the smallest suitable box is the 2 3/4 inch deep box. If there were clamps or more devices, the required volume would increase accordingly.

Box fill is the key idea here. The NEC requires a box to have enough cubic inches to harbor all conductors that end or splice inside it, plus any internal hardware like device yokes, clamps, and so on. For 14 AWG copper, each insulated conductor that originates outside the box counts as one fill unit, and all equipment grounding conductors in the box together count as a single fill unit. Each device yoke in the box also adds one fill unit. The volume per fill unit for 14 AWG is 2.0 cubic inches.

In this scenario, there are two MC cables with 14/3 w/ground entering the box. That gives six insulated conductors (three from each cable). The equipment grounding conductors from both cables exist, but all grounds together count as one fill unit. There is one device mounted in the box, so its yoke adds one fill unit.

Total fill units = 6 (insulated conductors) + 1 (EGCs) + 1 (device yoke) = 8. Multiply by 2.0 cu in per unit = 16 cubic inches required.

Among the listed boxes, the volumes are 12, 15, 16.5, and 18 cubic inches. The smallest box that meets or exceeds 16 cubic inches is the one with 16.5 cubic inches, which corresponds to a depth of 2 3/4 inches.

So the smallest suitable box is the 2 3/4 inch deep box. If there were clamps or more devices, the required volume would increase accordingly.