Box Fill Calculation Explained (NEC 314.16)
NEC 314.16 · Box fill · Updated 2026-07-10
Overstuffed boxes overheat, damage insulation, and fail inspection. NEC 314.16 prevents that by assigning every conductor size a volume allowance and requiring the box volume to cover the sum of everything inside.
The rule is simple arithmetic once you know what counts - and what doesn't.
Volume allowance per conductor (NEC Table 314.16(B))
| Conductor size | Volume allowance |
|---|---|
| #18 AWG | 1.50 cu in |
| #16 AWG | 1.75 cu in |
| #14 AWG | 2.00 cu in |
| #12 AWG | 2.25 cu in |
| #10 AWG | 2.50 cu in |
| #8 AWG | 3.00 cu in |
| #6 AWG | 5.00 cu in |
What counts, per NEC 314.16(B)
- Each conductor that originates outside the box and terminates or is spliced inside: 1 allowance. A conductor passing through unbroken: 1 allowance.
- Each device yoke (receptacle, switch): 2 allowances, at the size of the largest conductor connected to it.
- All equipment grounds together: 1 allowance at the largest EGC size. Since NEC 2020, each additional set of EGCs (a second raceway's grounds, or an isolated ground) adds a 1/4 allowance - 314.16(B)(5).
- Internal cable clamps (all of them together): 1 allowance at the largest conductor size.
- Each luminaire stud or hickey: 1 allowance at the largest conductor size.
- Doesn't count: wire nuts, pigtails that never leave the box, locknuts, bushings, and equipment-bonding jumpers that originate and terminate inside.
Worked example: one receptacle, two 12/2 NM cables
| Item | Count | Volume |
|---|---|---|
| Circuit conductors (2 per cable × 2) | 4 × 2.25 | 9.00 cu in |
| Equipment grounds (all together) | 1 × 2.25 | 2.25 cu in |
| Receptacle yoke | 2 × 2.25 | 4.50 cu in |
| Internal clamps (all together) | 1 × 2.25 | 2.25 cu in |
| Total required | 18.00 cu in |
An 18.0 cu in single-gang nail-on passes exactly; the common 16.0 cu in box fails. If the box is metal, its volume is stamped or listed in Table 314.16(A); plastic boxes have it molded inside. Mixed conductor sizes? Each conductor uses its own allowance, and the device counts at the largest size landing on it.
Box fill with the counting rules built in. Tap in the conductors, devices, and clamps - the app applies every 314.16(B) rule for your NEC year, shows PASS or FAIL with the citation, and exports the math as a stamped PDF. Download Conduit Fill & Bending Calc on the App Store.
Frequently asked questions
Do wire nuts count in box fill?
No. Wire connectors, locknuts, and bushings carry no volume allowance under NEC 314.16(B). Pigtails made up entirely inside the box don't count either - only conductors that originate outside the box.
How do ground wires count toward box fill?
All equipment grounding conductors in the box together count as one allowance at the size of the largest EGC. From NEC 2020 on, each additional set of EGCs (for example, from a second cable with an isolated ground) adds a quarter allowance - 314.16(B)(5).
How much does a receptacle count in box fill?
Two volume allowances per yoke, at the size of the largest conductor connected to the device. A receptacle on #12s counts 4.5 cu in; a switch on #14s counts 4.0 cu in.
How many 12/2 cables can enter an 18 cu in box with a device?
Two. Four #12 circuit conductors (9.0) + grounds (2.25) + device (4.5) + internal clamps (2.25) = 18.0 cu in exactly. A third cable adds 4.5 cu in minimum and needs a deeper box or a mud ring.