By Blissom · Updated 2026-11-04
Which box for a switch or receptacle
Each common single-gang job in NM cable, with every conductor the same size, the device counted at that size and the grounds as one allowance. The smallest metal 3" × 2" box that passes is shown.
| Job | #14 needs | #14 + clamps | Smallest metal box, #14 + clamps | #12 needs | #12 + clamps | Smallest metal box, #12 + clamps |
|---|---|---|---|---|---|---|
| Switch, one 2-wire cable (switch loop) | 10.00 | 12.00 | 3 × 2 × 2-1/2 | 11.25 | 13.50 | 3 × 2 × 2-3/4 |
| Switch, power in and out to the light (two 2-wire cables) | 14.00 | 16.00 | 3 × 2 × 3-1/2 | 15.75 | 18.00 | 3 × 2 × 3-1/2 |
| 3-way switch, one 2-wire and one 3-wire cable | 16.00 | 18.00 | 3 × 2 × 3-1/2 | 18.00 | 20.25 | none: use a 4" square |
| Receptacle, end of run (one 2-wire cable) | 10.00 | 12.00 | 3 × 2 × 2-1/2 | 11.25 | 13.50 | 3 × 2 × 2-3/4 |
| Receptacle, middle of run (two 2-wire cables) | 14.00 | 16.00 | 3 × 2 × 3-1/2 | 15.75 | 18.00 | 3 × 2 × 3-1/2 |
| Receptacle with three 2-wire cables | 18.00 | 20.00 | none: use a 4" square | 20.25 | 22.50 | none: use a 4" square |
Volumes in in³. Neutrals spliced in the box count, one per neutral conductor entering. A plastic box with no internal clamps uses the "needs" column against its marked volume. Verify against the NEC edition your inspector enforces.
Metal device and handy boxes (Table 314.16(A))
"Empty" is the box divided by the wire allowance. "With device" takes off two allowances for the yoke and one for the grounds. "+ clamps" takes off one more.
| Box | in³ | #14 empty | #14 with device | #14 + clamps | #12 empty | #12 with device | #12 + clamps |
|---|---|---|---|---|---|---|---|
| 3 × 2 × 2 device | 10.0 | 5 | 2 | 1 | 4 | 1 | 0 |
| 3 × 2 × 2-1/4 device | 10.5 | 5 | 2 | 1 | 4 | 1 | 0 |
| 3 × 2 × 2-1/2 device | 12.5 | 6 | 3 | 2 | 5 | 2 | 1 |
| 3 × 2 × 2-3/4 device | 14.0 | 7 | 4 | 3 | 6 | 3 | 2 |
| 3 × 2 × 3-1/2 device | 18.0 | 9 | 6 | 5 | 8 | 5 | 4 |
| 4 × 2-1/8 × 1-1/2 device | 10.3 | 5 | 2 | 1 | 4 | 1 | 0 |
| 4 × 2-1/8 × 1-7/8 device | 13.0 | 6 | 3 | 2 | 5 | 2 | 1 |
| 4 × 2-1/8 × 2-1/8 device | 14.5 | 7 | 4 | 3 | 6 | 3 | 2 |
The 4" × 2-1/8" boxes are the surface "handy" or utility boxes. Ganged metal device boxes add the volume of each section.
Plastic boxes: the marked volume governs
Nonmetallic boxes are not in Table 314.16(A). Their volume is the number molded or printed inside the box, and that is the number to use. For example, if the box is marked:
| Marked volume | #14 empty | #14 with device | #12 empty | #12 with device |
|---|---|---|---|---|
| 14 in³ | 7 | 4 | 6 | 3 |
| 18 in³ | 9 | 6 | 8 | 5 |
| 20.3 in³ | 10 | 7 | 9 | 6 |
| 22.5 in³ | 11 | 8 | 10 | 7 |
Old-work boxes, new-work nail-on boxes and boxes of the same outside size can carry different markings. Check the box in hand, not the one on the last job.
What counts and what does not (314.16(B))
| In the box | Counts as |
|---|---|
| Each insulated conductor entering from a cable and ending or spliced | 1 at its size |
| Switch or receptacle on one yoke | 2 at the largest conductor connected to it |
| All ground wires, up to four | 1 at the largest ground |
| Each ground beyond four | 1/4 |
| Internal cable clamps, any number | 1 at the largest conductor |
| Pigtails, jumpers, wire nuts, push-in connectors | 0 |
| Conductor passing through unbroken | 1 |
Worked examples
1. Switch with power in and out, metal box with clamps
Two 14/2 cables into a 3" × 2" × 2-1/2" metal box (12.5 in³) with internal clamps, a single-pole switch, and the two neutrals spliced in the back:
- 4 × 14 AWG conductors: 8.00 in³
- Internal clamps (1 × 14 AWG): 2.00 in³
- 1 device yoke (2 × 14 AWG each): 4.00 in³
- Equipment grounding conductors (1 × 14 AWG total): 2.00 in³
- Total 16.00 in³: too full by 3.50 in³.
The 3-1/2" deep box (18 in³) takes it with 2.00 in³ to spare. The two neutrals count even though they only splice to each other: each one came in from outside the box.
2. Receptacle in a plastic box marked 18 in³
Two 12/2 cables, a receptacle, no internal clamps:
- 4 × 12 AWG conductors: 9.00 in³
- 1 device yoke (2 × 12 AWG each): 4.50 in³
- Equipment grounding conductors (1 × 12 AWG total): 2.25 in³
- Total 15.75 in³ of 18: fits.
3. Three 12/2 cables at a receptacle, box marked 20.3 in³
Feed plus two branches, with the receptacle pigtailed so the downstream outlets do not depend on its terminals:
- 6 × 12 AWG conductors: 13.50 in³
- 1 device yoke (2 × 12 AWG each): 4.50 in³
- Equipment grounding conductors (1 × 12 AWG total): 2.25 in³
- Total 20.25 in³ of 20.3: fits with 0.05 in³ left.
The pigtails cost nothing. A box with internal clamps would add 2.25 in³ and fail at 22.50. Check your own box in the box fill calculator.
Troubleshooting a crowded box
- The device will not go back in. Usually the box was over fill before anyone noticed. Recount with the device and clamps; a deeper box or a 4" square with a single-gang ring is the fix.
- Inspector counts one more than you. Recount the clamps and the neutrals that were spliced and pushed to the back; both are easy to miss.
- #12 counted as #14. Each #12 needs 0.25 in³ more than a #14, and the device 0.50 in³ more, so a box that just passed at #14 fails at #12.
- Three cables at one device. From the job table: with internal clamps, no 3" × 2" metal box holds three 2-wire cables and a device, at #14 or #12. Use a 4" square with a ring or a large marked plastic box.
- Box volume taken from the outside size. Use the table for metal boxes and the marking for plastic, never a measurement.
Sources: NEC 2023: 314.16(A), Table 314.16(A), 314.16(B)(1)–(5) and Table 314.16(B)(1); Volume marked on nonmetallic boxes, plaster rings and extension rings. Method.
Questions electricians ask
How many 12 gauge wires can go in a single gang box?
It depends on the box. An empty 3" × 2" × 2-1/2" metal box (12.5 in³) holds 5; with a receptacle and grounds, 2. A plastic box marked 20.3 in³ holds 6 #12 with a device and grounds.
How many 14 gauge wires in a single gang box?
With a switch and grounds: 2 in a 3 × 2 × 2 (10 in³), 2 in a 3 × 2 × 2-1/4 (10.5 in³), 3 in a 3 × 2 × 2-1/2 (12.5 in³), 4 in a 3 × 2 × 2-3/4 (14 in³), 6 in a 3 × 2 × 3-1/2 (18 in³). Subtract one more if the box has internal clamps.
Does a switch count in box fill?
Yes. Each yoke or strap counts as two conductors of the largest wire connected to that device (314.16(B)(4)). A device too wide for a single 2" device box counts two for each gang it needs.
Do pigtails count in box fill?
No. A conductor that starts and ends in the box, such as a pigtail or jumper, does not count. The wire nuts do not count either.
Do plastic boxes have internal clamps?
Some nonmetallic boxes have built-in cable clamps and some rely on stapling the cable outside the box. If the box has internal clamps, count one allowance for all of them, at the largest conductor.
Box fill in your pocket
- NEC box fill and conduit fill, smallest box that works
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