Electrical Box Fill Calculator
Calculate box fill volume per NEC 314.16(B) and check against box capacity.
| Conductor Volume | 8.00 cu in |
|---|---|
| Device Volume | 4.00 cu in |
| Clamp Volume | 0.00 cu in |
| Grounding Volume | 2.00 cu in |
| Total Fill Required | 14.00 cu in |
| Remaining Capacity | 4.00 cu in |
| Box Used | 77.8% % |
· Volume per 14 AWG conductor: 2 cu in.
· Conductors: 4 x 2 = 8.00 cu in.
· Devices (switches + receptacles): 1 x 4.00 = 4.00 cu in.
· Grounding conductors: 1 allowance = 2.00 cu in.
How It Works
NEC 314.16 sets limits on how many conductors and fittings can fit in a box based on its volume. Each item consumes a volume allowance equal to the volume listed for the largest conductor in the box: - Each conductor = 1 allowance - Each device (switch, receptacle) = 2 allowances - All internal cable clamps combined = 1 allowance - All grounding conductors combined = 1 allowance Volume allowances by gauge (cu in): 18 AWG = 1.5, 16 AWG = 1.75, 14 AWG = 2.0, 12 AWG = 2.25, 10 AWG = 2.5, 8 AWG = 3.0, 6 AWG = 5.0. Box volume is stamped on standard boxes. When in doubt, look at the manufacturer's listing.
Example
4 conductors x 2.0 = 8 cu in. 1 switch x 2 x 2.0 = 4 cu in. 1 grounding allowance = 2.0 cu in. Total = 14 cu in in an 18 cu in box. Passes.
Frequently Asked Questions
Do equipment grounding conductors count the same as current-carrying conductors?
No. All grounding conductors in the box together count as one volume allowance, regardless of how many there are.
What about pigtails and wire nuts?
Short pigtails (under 12 inches) that originate inside the box and do not pass through the box wall are counted as conductors. Wire connectors themselves are not counted separately.
What is the box volume for a standard single-gang plastic box?
Common single-gang old-work boxes are 18 or 20 cubic inches. Metal single-gang boxes are typically around 13-14 cubic inches. Check the marking on the box.