Practice Questions: Box Fill
Real questions from our bank, all referenced to NEC 2026. Tap “Show explanation” after you answer.
You are installing a 4" × 2-1/8" deep device box. The box contains three 12 AWG conductors, two 12 AWG equipment grounding conductors, one internal cable clamp, and one single-gang device (yoke). Per NEC 314.16, how many cubic inches of volume is required?
- A13.50 cubic inches
- B15.75 cubic inches
- C18.00 cubic inches
- D20.25 cubic inches
Show explanation
Per NEC Table 314.16(B), 12 AWG = 2.25 cu in each. Box fill: 3 conductors (3 × 2.25 = 6.75) + 1 EGC count (1 × 2.25 = 2.25) + 1 clamp count (1 × 2.25 = 2.25) + 1 device/yoke (2 × 2.25 = 4.50) = 15.75 cu in. Note: Per 314.16(B)(4), each device or equipment yoke counts as 2 conductor volumes.
How much volume must be allotted for each 12 AWG conductor in an outlet box, per NEC Table 314.16(B)?
- A2.00 cubic inches
- B2.25 cubic inches
- C2.50 cubic inches
- D3.00 cubic inches
Show explanation
Per NEC Table 314.16(B), each 12 AWG conductor is allotted 2.25 cubic inches of free volume. The free-volume requirements are additive for every conductor counted in the box.
An outlet box contains one or more internal cable clamps. Per NEC 314.16(B)(2), how are the clamps counted in the box fill calculation?
- AOne conductor volume allowance, based on the smallest conductor in the box
- BOne conductor volume allowance, based on the largest conductor in the box
- CTwo conductor volume allowances, based on the largest conductor in the box
- DThey are not counted in the box fill calculation
Show explanation
Per NEC 314.16(B)(2), where one or more internal cable clamps are present in the box, a single volume allowance in accordance with Table 314.16(B) is required, based on the largest conductor present in the box.
A box contains one fixture stud and one hickey. How many conductor volume allowances must be added for these fittings per NEC 314.16(B)(3)?
- A1 allowance
- B2 allowances
- C3 allowances
- D0 allowances — support fittings are exempt
Show explanation
Per NEC 314.16(B)(3), each fixture stud or hickey in a box counts as a single conductor volume allowance based on the largest conductor present. Two fittings require two allowances.
A device box contains one duplex receptacle mounted on a single yoke. How many conductor volume allowances must be added for the device per NEC 314.16(B)(4)?
- AOne, based on the largest conductor connected to the device
- BTwo, based on the largest conductor connected to the device
- CTwo, based on the smallest conductor connected to the device
- DZero — devices are not counted
Show explanation
Per NEC 314.16(B)(4), each yoke or strap containing one or more devices or equipment requires a double volume allowance, sized based on the largest conductor connected to any device on that yoke.
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