NEC Box Fill Calculator
Calculate box fill per NEC 314.16 — all 5 counting rules from Table 314.16(B). Free, no account required.
1. Select Your Box
2. Conductors
Each conductor entering or passing through the box counts as 1 volume allowance per its size.
3. Clamps, Supports & Devices
Clamps and supports each add 1 volume allowance (largest conductor). Each device on a yoke adds 2.
Each device = 2 volume allowances based on largest conductor
4. Equipment Grounding Conductors
NEC 2023: Up to 4 EGCs = 1 volume allowance. Each EGC beyond 4 = additional ¼ volume.
FAIL
Box is too small — select a larger box or reduce conductors/devices.
Volume Breakdown — NEC 314.16(B)
132.5% of box volume used
NEC 314.16(B) — The 5 Counting Rules
1. Conductors: Each conductor entering or passing through = 1 volume allowance based on its own size.
2. Cable clamps: All internal clamps combined = 1 volume allowance based on the largest conductor.
3. Support fittings: All studs/hickeys combined = 1 volume allowance based on the largest conductor.
4. Devices: Each device on a yoke or strap = 2 volume allowances based on the largest conductor connected.
5. EGCs: Up to 4 EGCs = 1 volume allowance (largest EGC). Each beyond 4 = additional ¼ volume (NEC 2023).
NEC Table 314.16(B) — Volume Allowances
| Wire Size | Volume (in³) |
|---|---|
| 18 AWG | 1.50 |
| 16 AWG | 1.75 |
| 14 AWG | 2.00 |
| 12 AWG | 2.25 |
| 10 AWG | 2.50 |
| 8 AWG | 3.00 |
| 6 AWG | 5.00 |
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This calculator uses NEC 2023 (NFPA 70) Table 314.16(A) and 314.16(B) values. Always verify with your local AHJ and the current NEC edition adopted in your jurisdiction.