Box Fill Calculator
Conductors and devices to box fill verdict.
What it does
Crammed boxes overheat, arc, and fail inspection on arithmetic alone. Box fill adds conductor volumes by gauge with double-count devices plus clamps and grounds, then compares against the stamped box size. Counting beats stuffing.
The tool above takes wire counts per gauge, device yokes, clamp and ground flags, and box volume, then prints required volume with a fits-or-overfilled verdict. Volumes follow NEC allowances with the largest-wire rule for extras.
Electrical code resources with safety-foundation guides publish these allowances: 2.0, 2.25, 2.5, 3.0, and 5.0 cubic inches from 14 to 6 AWG, devices at twice largest, clamps and grounds at once largest each.
Size up when close. Deep boxes, extensions, and split circuits all beat forced bundles, and derating plus conduit fill still apply on loaded runs.
How to use
- Enter conductor counts by gauge.
- Enter device yoke count.
- Mark clamps and grounds present.
- Enter stamped box volume.
- Read required volume and verdict.
- Upsize or split when over.
Formula
- wires
- Conductor counts by gauge.
- devices
- Device yokes on the box.
- box
- Stamped box volume in cu in.
Volumes follow NEC conductor allowances by gauge with devices at twice the largest connected. Clamps and grounds each take one largest allowance. Compare against stamped box volume; over means upsize.
Worked example
Example: 4x12 plus device
Four 12s, one device, clamps, grounds, 21 cu in box.
- Four 12 AWG at 2.25 is 9.0 cu in.
- One device counts 2 x 2.25 = 4.5.
- Clamps add 2.25, grounds add 2.25.
- Required: 9 + 4.5 + 2.25 + 2.25 = 18.0.
- Box is 21 cu in: fits.
Reference table
| Gauge | Each | Note |
|---|---|---|
| 14 AWG | 2.0 | Smallest counted |
| 12 AWG | 2.25 | Common branch |
| 10 AWG | 2.5 | 30-amp runs |
| 8 AWG | 3.0 | Large branch |
| 6 AWG | 5.0 | Feeders |
Limits
- Volume only. Derating and conduit separate.
- Grounds simplified to one allowance.
- Box volume must come from its stamp.
- Not a permit drawing.
Common mistakes
- Forgetting devices count double.
- Skipping clamps and grounds.
- Guessing box volume from memory.
- Mixing gauges in one count.
- Forcing overfilled boxes shut.
Try it yourself
Quick checks. Answers sit tucked under each question.
Five 14s, 1 device, clamps, 18 box?
Copper is 5 x 2.0 = 10. Devices 1 x 2 x 2.0 = 4. Clamps 2.0. Required 16.0 in 18: fits.
Six 12s, 2 devices, clamps, grounds, 21 box?
Copper is 6 x 2.25 = 13.5. Devices 2 x 2 x 2.25 = 9. Clamps plus grounds 4.5. Required 27.0 over 21: overfilled.
Questions
What volume does each wire count?
2.0 for 14 AWG, 2.25 for 12, 2.5 for 10, 3.0 for 8, 5.0 for 6. Each conductor counts once per gauge; devices count twice the largest connected.
How do devices count?
Two allowances of the largest connected conductor. A duplex on 12s eats 4.5 cu in before a single wire lands.
What about clamps and grounds?
One allowance of the largest conductor for internal clamps, and one for grounds together. Four or fewer grounds share one; pigtails under 12 inches ride free.
Where is the box volume?
Stamped inside single-gang boxes or computed from dimensions for big pulls. Metal single-gangs near 18, deep doubles near 34; read the stamp, not memory.
What if the math overflows?
Splice in a bigger box, split the circuit, or drop a device. Crammed boxes overheat and fail inspection regardless of neatness.
Does this cover conduit too?
No. Box fill covers conductor volume; conduit fill, derating, and support follow their own articles. Pass each separately.