How box fill is counted (NEC 314.16(B))
Box fill isn't just a count of wires. The code assigns a volume allowance to each item in the box based on conductor size:
- Conductors: each insulated conductor that enters the box counts once. A conductor that runs through the box without a splice also counts once. A conductor that starts and ends inside the box, such as a pigtail, doesn’t count.
- Equipment grounding conductors: all of them together count as one conductor, at the size of the largest. Since the 2020 NEC, if more than four enter the box, add a quarter allowance for each extra one.
- Internal cable clamps: one or more count as one conductor, at the largest conductor size in the box. Clamps outside the box don’t count.
- Support fittings: each fixture stud or hickey counts as one conductor, at the largest size.
- Devices: each yoke or strap holding a switch or receptacle counts as two conductors, based on the largest conductor connected to that device.
- Small fixture wires: up to four fixture wires smaller than 14 AWG coming from a luminaire canopy into the box can be left out.
Volume per conductor (Table 314.16(B))
| Conductor size | Volume per conductor |
|---|---|
| 18 AWG | 1.50 in³ |
| 16 AWG | 1.75 in³ |
| 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 |
Worked example: a receptacle in a 4" square box
Two 12/2 NM cables (with ground) enter a 4" × 1-1/2" square box with internal clamps, feeding one duplex receptacle.
It fits with 3 in³ to spare. Add a mud ring or extension and its marked volume adds to the box volume (314.16(A)(1)). A 3" × 2" × 2-1/2" device box (12.5 in³) would be overfilled by this combination.
Standard metal box volumes (Table 314.16(A))
| Box (trade size) | Volume |
|---|---|
| 4" × 1-1/4" square | 18.0 in³ |
| 4" × 1-1/2" square | 21.0 in³ |
| 4" × 2-1/8" square | 30.3 in³ |
| 4-11/16" × 1-1/4" square | 25.5 in³ |
| 4-11/16" × 1-1/2" square | 29.5 in³ |
| 4-11/16" × 2-1/8" square | 42.0 in³ |
| 4" × 1-1/2" octagon | 15.5 in³ |
| 4" × 2-1/8" octagon | 21.5 in³ |
| 3" × 2" × 2" device | 10.0 in³ |
| 3" × 2" × 2-1/2" device | 12.5 in³ |
| 3" × 2" × 2-3/4" device | 14.0 in³ |
| 3" × 2" × 3-1/2" device | 18.0 in³ |
Nonmetallic (plastic) boxes and many newer metal boxes have their volume molded or stamped inside. Use that number with the "Other" option. Plastic boxes typically run 18 to 22.5 in³ for a single gang.
Common mistakes
- Counting each ground separately. Up to four grounds count as one allowance in total.
- Forgetting the device counts double. A receptacle or switch takes two conductor allowances.
- Using the device’s wire size for everything. Clamps and fittings use the largest conductor in the box, while each device uses the largest conductor connected to it.
- Ignoring extension rings. A marked plaster ring or extension adds volume. Unmarked ones don't.
Frequently asked questions
How many 12 AWG wires can go in a 4" square box?
A 4" × 1-1/2" square box (21.0 in³) holds nine 12 AWG allowances (21 ÷ 2.25 = 9.3). Remember that grounds, clamps and devices use allowances too, so the real number of wires is lower once a receptacle is installed.
Do wire nuts count in box fill?
No. Wire connectors (twist-on, push-in or lever) don’t count. Neither do pigtails that begin and end inside the box.
Do grounds count in box fill?
Yes, but all of them together count as one conductor, sized by the largest ground. Under the 2020 NEC and later, more than four grounds add a quarter allowance for each additional one.
Does a GFCI receptacle count more than a regular receptacle?
The code counts each device yoke as two conductors regardless of type. However, a single device wider than a standard 2-inch box requires two allowances per gang it occupies, and bulky GFCI and smart devices simply need more physical room, so choosing a deeper box is good practice.
Where do I find the volume of a plastic box?
It’s molded into the box, usually on the inside back or side wall, in cubic inches. Enter it with the “Other” box option.
Last reviewed October 11, 2026. Results are estimates for planning. Always verify against the edition of the NEC adopted in your area, local amendments and the authority having jurisdiction. Found a mistake? Tell us.