Box Fill Calculator
Calculate the required cubic-inch volume for a junction or device box per NEC 314.16(B). Enter the conductor counts, device yokes, clamps, support fittings, and grounding conductors — the calculator sums the volume allowances and compares the total to your box.
Each conductor counts as 1×. Conductors that pass through unbroken count once. (314.16(B)(1))
Each yoke counts as 2× the largest conductor. (314.16(B)(4))
Each fitting (hickey, fixture stud) = 1×. (314.16(B)(3))
Each additional EGC set = 0.25× of the largest conductor.
Required
20.25 in³
Available
21.00 in³
Headroom
0.75 in³
✓ Box volume is sufficient — install meets NEC 314.16.
Show calculation breakdown
- 6 × 12 AWG insulated conductors: 13.50 in³
- 1 × device yoke (2× 12 AWG each): 4.50 in³
- Equipment grounding conductor(s) (1× 12 AWG total): 2.25 in³
- Total required: 20.25 in³
How the counts work (314.16(B))
- (B)(1) Conductor fill — each insulated conductor entering, leaving, or terminating in the box counts as 1× based on its size.
- (B)(2) Clamp fill — one or more cable clamps inside the box = 1× the largest conductor entering.
- (B)(3) Support fittings — each fixture stud or hickey = 1× the largest conductor.
- (B)(4) Device yoke — each yoke or strap (with a switch, receptacle, etc.) = 2× the largest conductor connected to it.
- (B)(5) Equipment grounding conductors — all of them together = 1×; an additional set of isolated/equipment grounds adds 0.25×.
Required (in³) = Σ (count · volume_per_conductor) volume_per_conductor — Table 314.16(B)
Test your knowledge
Educational tool only. Verify against the current NEC. Not a substitute for a licensed engineer.