NEC box fill calculator
Box fill is the minimum volume a box needs for what's inside it. Add a volume allowance from Table 314.16(B)(1) for every conductor, device, clamp set, fitting type, ground group and terminal block, then compare the total with the box volume1. A 4 in square box 2-1/8 in deep holds 30.3 in³; six 12 AWG conductors, one receptacle and two grounds need 20.25 in³, so it fits.
Fits, with 10.05 in³ to spare.
| Item | Counts as | in³ |
|---|---|---|
| Conductors | 6 × 12 AWG | 13.50 |
| Device yokes (2 each) | 2 × 12 AWG | 4.50 |
| Equipment grounds (up to 4 count as 1) | 1 × 12 AWG | 2.25 |
Smallest standard boxes that fit: 4 x 1-1/2 square (21 in³), 3-3/4 x 2 x 3-1/2 masonry box or gang (21 in³), 4 x 2-1/8 round or octagonal (21.5 in³).
Volume allowance per conductor
How each item counts in the 2023 NEC
| Item | Counts as | Section1 |
|---|---|---|
| Conductor from outside the box, spliced, terminated or passing through | 1 each, at its own size | 314.16(B)(1) |
| Unbroken loop or coil at least twice the minimum length in 300.14 (12 in or more) | 2, at its size | 314.16(B)(1) |
| Conductor no part of which leaves the box (pigtail, jumper) | 0 | 314.16(B)(1) |
| Grounds and up to four fixture wires smaller than 14 AWG from a domed luminaire canopy | May be left out | 314.16(B)(1) Exception |
| Internal cable clamps, any number | 1, at the largest conductor in the box | 314.16(B)(2) |
| Support fittings (luminaire studs, hickeys) | 1 per type, at the largest conductor | 314.16(B)(3) |
| Device on a yoke or strap | 2, at the largest conductor connected to it; 2 per gang if wider than a 2 in device box | 314.16(B)(4) |
| Equipment grounding conductors | Up to 4 count as 1, at the largest ground; each additional one counts 1/4 | 314.16(B)(5) |
| Terminal block assembly | 1 each, at the largest conductor terminated to it | 314.16(B)(6), new in 2023 |
Two changes catch people working from older references: since the 2023 edition, terminal blocks get their own allowance, and the grounding conductor rule no longer mentions equipment bonding jumpers.1
Standard metal box volumes
| Box, trade size in inches | in³ | Max 12 AWG | Max 14 AWG |
|---|---|---|---|
| 4 x 1-1/4 round or octagonal | 12.5 | 5 | 6 |
| 4 x 1-1/2 round or octagonal | 15.5 | 6 | 7 |
| 4 x 2-1/8 round or octagonal | 21.5 | 9 | 10 |
| 4 x 1-1/4 square | 18.0 | 8 | 9 |
| 4 x 1-1/2 square | 21.0 | 9 | 10 |
| 4 x 2-1/8 square | 30.3 | 13 | 15 |
| 4-11/16 x 1-1/4 square | 25.5 | 11 | 12 |
| 4-11/16 x 1-1/2 square | 29.5 | 13 | 14 |
| 4-11/16 x 2-1/8 square | 42.0 | 18 | 21 |
| 3 x 2 x 1-1/2 device | 7.5 | 3 | 3 |
| 3 x 2 x 2 device | 10.0 | 4 | 5 |
| 3 x 2 x 2-1/4 device | 10.5 | 4 | 5 |
| 3 x 2 x 2-1/2 device | 12.5 | 5 | 6 |
| 3 x 2 x 2-3/4 device | 14.0 | 6 | 7 |
| 3 x 2 x 3-1/2 device | 18.0 | 8 | 9 |
| 4 x 2-1/8 x 1-1/2 device | 10.3 | 4 | 5 |
| 4 x 2-1/8 x 1-7/8 device | 13.0 | 5 | 6 |
| 4 x 2-1/8 x 2-1/8 device | 14.5 | 6 | 7 |
| 3-3/4 x 2 x 2-1/2 masonry box or gang | 14.0 | 6 | 7 |
| 3-3/4 x 2 x 3-1/2 masonry box or gang | 21.0 | 9 | 10 |
| FS single cover/gang | 13.5 | 6 | 6 |
| FD single cover/gang | 18.0 | 8 | 9 |
| FS multiple cover/gang | 18.0 | 8 | 9 |
| FD multiple cover/gang | 24.0 | 10 | 12 |
The maximum columns divide the volume by the allowance with nothing else in the box. Nonmetallic boxes and boxes not in the table are marked with their volume by the manufacturer; use the marked number, and add the marked volume of any plaster ring, extension ring or raised cover.
Box fill in the estimate
Box fill shows up in a bid at multi-device locations, where two or three devices on 12 AWG push a standard box into a deeper box or a 4-11/16 in box with a ring. Those boxes cost more and take longer to set, so they're worth catching at takeoff rather than on site. Counting how many of those locations a job has starts with the device count; that's what AmpCount does from the plans, and the electrical takeoff guide covers the rest of the process.
On paper, give the oversized boxes their own row on the takeoff sheet instead of folding them into the device count, so the box, the ring and the extra labor each get priced. Those rows then carry into the electrical estimate calculator with a labor unit that reflects the bigger box.
Box fill is the last of three related checks. When a box is fed by a raceway, the same conductors have to fit the pipe first, which is what the conduit fill calculator checks. If a long run was upsized after a voltage drop check, run the box again at the new wire size: going from 12 to 10 AWG raises the allowance from 2.25 to 2.50 in³ per conductor. For where these details sit in the whole bid, see the guide to electrical estimating.
More calculators and guides
Sources
- NFPA 70 revision cycle, Code-Making Panel 8 public input responses (quotes the 2023 text of 314.16)
- NFPA 70, National Electrical Code, 2023 edition (read online at nfpa.org)
NFPA 70 and National Electrical Code are registered trademarks of the National Fire Protection Association. AmpCount is not affiliated with NFPA.