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NEC Art.314
Validated against the standard.
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How it works
Enter your values
Type in your inputs — the app fills code tables and constants for you, so there's nothing to look up.
Get a code-checked result
Instant answer with a clear PASS / FAIL and the exact NEC Art.314 reference behind every number.
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Keep a history of calculations and export a clean PDF report to share or attach to a permit (Pro).
What is box fill?
A box fill calculation makes sure an outlet or junction box is large enough for everything inside it. Crowd too many conductors, devices, and clamps into a box and you risk damaged insulation and an inspection failure. The rules live in NEC 314.16: each item is assigned a volume in cubic inches, and the total must not exceed the box's volume. This app counts it all for you.
Volume allowances (Table 314.16(B))
| Conductor size | Volume each |
|---|---|
| 14 AWG | 2.00 in³ |
| 12 AWG | 2.25 in³ |
| 10 AWG | 2.50 in³ |
| 8 AWG | 3.00 in³ |
| 6 AWG | 5.00 in³ |
How items are counted
- Each current-carrying conductor that enters and ends in the box = 1.
- All equipment grounding conductors together = 1 (largest size present).
- One or more internal cable clamps = 1.
- Each device yoke or strap (a receptacle or switch) = 2, based on the largest conductor connected to it.
Worked example
A box with six #12 conductors, internal clamps, one receptacle, and grounds. All sizing is at 2.25 in³ for #12.
Conductors 6 x 2.25 = 13.50, clamps 1 x 2.25 = 2.25, device 2 x 2.25 = 4.50, grounds 1 x 2.25 = 2.25.
Total = 22.5 in³, so the box must be at least 22.5 in³. The app flags a box that is too small and suggests the next size up.
Related calculators
Use it alongside the conduit fill calculator for the raceway and the wire size calculator for the conductors feeding the box.
Box fill FAQ
How are grounding conductors counted?
All equipment grounding conductors in the box count as a single conductor volume, based on the largest grounding conductor present, per NEC 314.16(B)(5).
Why does a receptacle count as two?
Each device yoke or strap is counted as two conductors of the largest size connected to it, per 314.16(B)(4), because the device body displaces space in the box.
Where do the box volumes come from?
Standard box volumes are listed in Table 314.16(A); for non-standard boxes the volume is marked on the box. The app includes the standard metal boxes and lets you enter a custom volume.
Do cable clamps count?
Yes. One or more internal clamps count as a single conductor volume of the largest conductor in the box. External clamps do not count.
Does it work offline?
Yes, it runs 100% offline with no account. Free with rewarded ads; a one-time $2.99 Pro unlock adds PDF export and saved history.
Get Box Fill on your phone
Free with rewarded ads · $2.99 lifetime Pro · iOS & Android. Works 100% offline on the job site.