Box Fill Calculator
Overcrowded electrical boxes are a code violation and a fire hazard. The National Electrical Code (NEC) Section 314.16 specifies exactly how much space each conductor, device, and fitting needs inside a box. This calculator adds up all the volume allowances and recommends a properly sized box.
How NEC Box Fill Works
Every item inside an electrical box takes up space, and the NEC assigns a specific volume allowance to each one based on wire gauge. The total of all allowances must not exceed the box's rated volume.
Volume Allowances (NEC Table 314.16(B))
| Wire Gauge | Volume per Conductor |
|---|---|
| 14 AWG | 2.00 in3 |
| 12 AWG | 2.25 in3 |
| 10 AWG | 2.50 in3 |
| 8 AWG | 3.00 in3 |
| 6 AWG | 5.00 in3 |
Counting Rules
The NEC has specific rules for counting what goes in the box:
- Current-carrying conductors: Each hot and neutral wire counts as 1 volume allowance
- Equipment grounds: ALL ground wires together count as just 1 volume allowance (based on the largest ground conductor)
- Devices: Each switch or receptacle yoke counts as 2 volume allowances
- Internal clamps: ALL internal cable clamps count as 1 volume allowance
- Support fittings: Fixture studs and hickeys count as 1 volume allowance
- Pigtails and wire nuts: Wires that originate and terminate within the box are not counted
Common Box Sizes
| Box Type | Volume | Typical Use |
|---|---|---|
| Single-gang device | 12.5-18.0 in3 | Switches, receptacles |
| 4" octagonal | 15.5-21.5 in3 | Ceiling fixtures |
| 4-11/16" square | 25.5-42.0 in3 | Multiple device, junction |
Important Notes
- Wire nuts and crimp connectors do not count toward box fill
- Wires passing straight through (no splice, no termination) count as 1 conductor
- When mixing wire sizes, use the volume for each wire's actual gauge
- This calculator assumes all conductors are the same gauge for simplicity