Box Fill Calculator
Calculate NEC 314.16 box fill from conductors, clamps, support fittings, devices, and grounds. Get the required box volume, fill %, pass/fail, and the minimum box size. Free, no sign-up.
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Explanation
Box fill is the calculation that checks whether an outlet, device, or junction box is large enough for everything inside it. Under NEC 314.16, each conductor, clamp, support fitting, device, and grounding conductor is converted to a volume allowance in cubic inches, and their total must not exceed the box volume. This calculator adds up the allowances from Table 314.16(B), reports the required volume, and checks it against a standard or marked box.
The five counting rules of 314.16(B)
NEC 314.16(B) breaks the box contents into five categories. Conductors are counted at their own size; everything else is counted using the volume allowance of the largest conductor in the box. The required box volume is the sum of all five:
Each term uses the per-conductor allowance from Table 314.16(B): conductors at their own size, and the clamp, support, device, and ground terms at the largest conductor’s allowance. A device yoke counts as two allowances, so it is multiplied by 2.
| Section | Item | Allowance |
|---|---|---|
| (B)(1) | Each conductor entering and terminating, splicing, or passing through | 1× (own size) |
| (B)(2) | One or more internal cable clamps | 1× (largest) |
| (B)(3) | Each fixture stud or hickey (support fitting) | 1× each (largest) |
| (B)(4) | Each device yoke or strap (switch, receptacle) | 2× each (largest) |
| (B)(5) | All equipment grounding conductors together | 1× (largest EGC) |
Conductors that originate and terminate entirely within the box — such as pigtails and made-up jumpers — are not counted at all. A conductor looped or coiled through the box without a splice, at least twice the minimum free-conductor length, counts as two.
Volume allowance per conductor size
The per-conductor allowances come from NEC Table 314.16(B). To translate a wire size into its diameter, cross-sectional area, or ampacity for related calculations, use the AWG wire size chart.
| Conductor size (AWG) | Volume allowance (in³) |
|---|---|
| 18 | 1.50 |
| 16 | 1.75 |
| 14 | 2.00 |
| 12 | 2.25 |
| 10 | 2.50 |
| 8 | 3.00 |
| 6 | 5.00 |
Standard box volumes — Table 314.16(A)
If a box has no marked volume, use the value for its trade size from NEC Table 314.16(A). Non-standard and listed boxes are durably marked with their volume in cubic inches, which takes precedence.
Notes and limitations
This tool applies the box-fill rules of NEC 314.16(A) and (B) for splice and outlet boxes not over 1650 cm³ (100 in³). It does not size pull or junction boxes for raceways 4 in and larger under 314.28, and it does not check conductor ampacity or the number of current-carrying conductors. For cables routed in trays instead of boxes, see the cable tray fill calculator. Always verify the final design against the current NEC and the authority having jurisdiction.