Box fill: is the box big enough?
Count the conductors by size, the devices and the grounds. See the volume needed, line by line, and whether the box you picked from Table 314.16(A) holds it.
Count 1 for each conductor that enters and is spliced or ends in the box, and 1 for each that passes through unbroken (2 if it's looped with at least twice the free length 300.14 requires). Pigtails that stay inside the box count 0.
| Item | Count | in³ each | in³ |
|---|
No allowance is needed for locknuts, bushings or wire connectors. A device wider than a single 2-in. gang takes 2 allowances for each gang it needs, so count each gang as a yoke. Nonmetallic boxes and boxes not in Table 314.16(A) use the volume marked on them. An estimate under NEC 2023 314.16: local amendments and the inspector (AHJ) govern.
Under NEC 314.16, each conductor counts its size's volume: 2.00 in³ for 14 AWG, 2.25 for 12 AWG, 2.50 for 10 AWG. Add one allowance for all internal clamps, two for each device yoke, and one for up to four grounds plus a quarter for each extra. Four 12 AWG wires, clamps, a receptacle and grounds need 18.0 in³.
How to use the box fill calculator
Pick the box
Choose a metal box from Table 314.16(A) or enter the volume marked on the box, plus any marked rings.
Count what goes in
Conductors by size, device yokes, grounding conductors, clamps and fixture studs.
Check the total
Volume needed against the box volume, with each allowance shown.
What the box fill calculator does
Per-conductor volumes
18 AWG 1.50 in³ to 6 AWG 5.00 in³, from the 314.16(B) volume table.
All the allowances
Clamps, fixture studs and hickeys, device yokes and equipment grounding conductors.
Current EGC rule
One allowance for up to four grounding conductors and a quarter for each additional (since the 2020 NEC).
Common boxes
Every metal box in Table 314.16(A), from 3 × 2 device boxes to 4-11/16 squares and FS/FD boxes.
Bigger box hint
When it doesn't fit, names the smallest listed box that would.
- Basis
- NFPA 70 (NEC 2023) 314.16(A) and 314.16(B)
- Conductor volumes
- 18 AWG 1.50, 16 AWG 1.75, 14 AWG 2.00, 12 AWG 2.25, 10 AWG 2.50, 8 AWG 3.00, 6 AWG 5.00 in³
- Allowances
- Clamps 1, each fitting type 1, each yoke 2, EGCs 1 + 1/4 per EGC over four
- Box volumes
- Table 314.16(A) metal boxes, or the volume marked on the box and rings
- Not included
- Terminal blocks, boxes for 4 AWG and larger (314.28), barriers dividing a box
Box Fill Calculator FAQ
How do you calculate box fill?
Count each conductor that enters and is spliced or terminated in the box, and each unbroken conductor passing through, at its size's volume. Add one allowance for internal clamps, one per type of fixture stud or hickey, two per device yoke and the grounding conductor allowance. Compare the total with the box's volume.
How many 12 AWG wires can go in a 4 x 1-1/2 square box?
With nothing else in the box, 21.0 in³ ÷ 2.25 in³ = 9 conductors. Each device, clamp and ground allowance takes space from that, so a receptacle (2 allowances), clamps (1) and grounds (1) leave room for 5 current-carrying 12 AWG wires.
Do ground wires count in box fill?
Yes. Up to four equipment grounding conductors or bonding jumpers entering the box count as one allowance at the size of the largest, and each one over four adds a quarter allowance. Pigtails that start and end inside the box don't count.
Do wire nuts count in box fill?
No. 314.16(B) gives no allowance for small fittings such as locknuts, bushings and wire connectors.
What about plastic boxes?
Nonmetallic boxes are marked with their volume in cubic inches; enter that number. Add the marked volume of any plaster ring, extension ring or raised cover fastened to the box.
Which edition does this follow?
The 2023 NEC. The quarter allowance for each grounding conductor over four came in with the 2020 edition. Your area may enforce another edition with amendments, and the inspector (AHJ) decides.
Where the numbers come from
- NFPA 70, National Electrical Code (2023): 314.16, Number of Conductors in Outlet, Device, and Junction Boxes, and Table 314.16(A)
- NFPA: NEC-P08 First Draft report, 2026 cycle (314.16(B) text and the per-conductor volume table)
- EC&M Code Quiz: box volume calculations and EGCs (the 2020 change to 314.16(B)(5))
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