CSLB C-10 Electrical ContractorWiring Methods and MaterialsMedium
A single-gang nonmetallic device box is marked with an interior volume of 18 cubic inches. It contains one duplex receptacle, two 14/2 NM-B cables (4 current-carrying 14 AWG conductors), and one internal cable clamp. Using NEC Table 314.16(B), where each 14 AWG conductor requires 2.0 in³ per allowance, what is the total required volume, and does the box comply?
- A14 in³ required; the box complies
- B16 in³ required; the box complies
- C18 in³ required; the box does not comply
- D20 in³ required; the box does not comply
Show answer & explanationAnswer & explanation
Correct answer: A. 14 in³ required; the box complies
Conductors: 4 × 2.0 = 8.0 in³. Cable clamp: 1 allowance × 2.0 = 2.0 in³. Receptacle yoke: counts twice the largest conductor size, so 2 × 2.0 = 4.0 in³. Total = 8.0 + 2.0 + 4.0 = 14 in³, which is less than the 18 in³ marked box volume, so it complies.
Why the other options are wrong
- B. 16 in³ overcounts one of the allowances.
- C. 18 in³ would exactly equal capacity but is not the actual calculated requirement.
- D. 20 in³ exceeds both the calculation and the box's marked volume.
Box Fill with Clamp, Device, and Conductors
Box fill totals conductor volume, one allowance for internal clamps, and double allowance for each device yoke, all based on the largest conductor present.
- Conductors: 1 allowance each based on size
- Internal clamp: 1 single allowance total (largest conductor)
- Device yoke: 2 allowances per yoke (largest conductor)
Memory trick: Wires count once, clamps count once, devices count twice.