Electrical Box Fill Calculator
Calculate box fill per NEC 314.16 to ensure your electrical boxes have adequate volume for all conductors and devices.
Results
Visualization
How It Works
NEC 314.16 requires that every electrical box has enough volume for all the conductors, devices, clamps, and fittings inside it. Overfilled boxes cause damage to wire insulation and create fire hazards.
The Formula
Total Volume = (Conductors × Allowance) + (1 × Allowance for all grounds) + (Devices × 2 × Allowance) + (1 × Allowance if clamps present)
Variables
- Allowance — Volume per conductor based on largest wire: 14 AWG=2.00, 12 AWG=2.25, 10 AWG=2.50, 8 AWG=3.00 cu in
- Conductors — Count of hot and neutral wires entering the box
- Grounds — All ground wires count as one conductor regardless of quantity
- Devices — Each switch or receptacle counts as 2x the largest wire allowance
Example
Four 12 AWG conductors, 2 grounds, 1 device, 1 clamp in a 4x2-1/8 square box: (4x2.25) + (1x2.25) + (1x2x2.25) + (1x2.25) = 9.0 + 2.25 + 4.5 + 2.25 = 18.0 cu in. Box = 30.3 cu in. PASS.
Tips
- All ground wires together count as only one conductor volume allowance.
- Internal cable clamps collectively count as one conductor volume allowance.
- Each yoke (device) counts as two times the volume of the largest conductor.
- Pigtails and wire nuts inside the box do not require additional volume allowance.
- When a box is too small, use the next size up or add an extension ring.