Electrical Box Fill Calculator
NEC 314.16 box fill — conductors, device yokes, grounds, and clamps vs. box volume.
Conductors entering/spliced in box
Devices, grounds & clamps
How this is calculated
NEC 314.16(B) assigns a volume allowance per item in the box, based on Table 314.16(B):
| Wire size | Volume allowance |
|---|---|
| 14 AWG | 2 in³ |
| 12 AWG | 2.25 in³ |
| 10 AWG | 2.5 in³ |
| 8 AWG | 3 in³ |
| 6 AWG | 5 in³ |
- Each conductor originating outside the box and terminated or spliced inside counts once, by its own size.
- Each device yoke (switch/receptacle) counts as 2 allowances, sized by the largest conductor connected to it.
- All equipment grounding conductors together count as 1 allowance for up to four grounding conductors, sized by the largest one; each grounding conductor beyond four adds a quarter allowance (NEC 2020 and 2023).
- Internal cable clamps count as 1 allowance total, sized by the largest conductor in the box.
The sum must not exceed the box's cubic-inch volume from NEC Table 314.16(A).
In-depth guide: How to calculate box fill under NEC 314.16
How this calculator works
NEC 314.16: Table 314.16(A) box volumes, Table 314.16(B) volume per conductor and the 314.16(B)(1)–(5) counting rules for devices, grounds and clamps.
Worked example
A 3×2×2" device box (10.0 cu in) with four 12 AWG conductors, one receptacle, and a ground.
Conductors: 4 × 2.25 = 9.0 cu in. The device yoke counts double: 2 × 2.25 = 4.5 cu in. All grounds together: 1 × 2.25 = 2.25 cu in. Total required: 15.75 cu in.
15.75 is more than the box's 10.0 cu in, so it fails. A 4" square box 1-1/2" deep (21.0 cu in) with a mud ring, or a deeper device box, fixes it.
Frequently asked questions
Do pigtails count in box fill?
No. A conductor that starts and ends inside the box, such as a pigtail, does not count. Only conductors that come from outside the box and are spliced or terminated inside are counted.
How do grounds count?
Under NEC 2020 and 2023, up to four equipment grounding conductors count as a single allowance, based on the largest ground in the box, and each one beyond four adds a quarter allowance. Six 12 AWG grounds = 1.5 × 2.25 = 3.375 cu in. (The 2017 edition added a full allowance for an isolated ground instead.)
Why does a switch or receptacle count twice?
NEC 314.16(B)(4) assigns each device yoke two volume allowances, based on the largest conductor connected to it, because the device takes up a large share of the box.
Where do I find the box volume?
Use the volume stamped on the box by the manufacturer, or the standard volumes in NEC Table 314.16(A). Plaster rings, extension rings and raised covers marked with their volume can be added to the box volume.
Common mistakes
- Counting a device as one conductor. A switch or receptacle yoke counts as two allowances, sized by the largest conductor connected to it (314.16(B)(4)).
- Counting pigtails. Conductors that start and end inside the box are not counted. Only conductors entering from outside, or passing through, count.
- Using the empty-box conductor count. The conductor counts in Table 314.16(A) assume nothing else is in the box. With a device, clamps or grounds, subtract their allowances first.
- Applying the old ground rule. Under the 2020 and 2023 NEC, more than four grounding conductors add a quarter allowance each. Enter the number of grounds to include it.
What it does not cover
Support fittings, fixture studs, hickeys and non-standard box volumes (add those allowances per NEC 314.16(B)).
Sources
- NFPA 70, National Electrical Code (free read-only access)
- EC&M, Code Quiz: box volume calculations and EGCs (2020 NEC change)
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