Menu

Reviewed · Climate: NOAA hourly records, 2015-2024 · How we calculate

Box Fill Calculator: What Size Electrical Box Do I Need?

Count the insulated wires entering the box by size, the grounds, devices and clamps. You get the cubic inches the box must hold and the smallest common box that does.

Inputs

wires

Insulated wires entering the box; black and white each count.

wires
wires
wires
devices

Your box

Updates as you type
16cu in

cubic inches of box for this fill, counted by NEC 314.16. The smallest common box that holds it: a 3 × 2 × 3-1/2 in device box (18 cu in). Check the volume stamped in your box.

Box fill count
4 × 14 AWG8 cu in
2 grounds (14 AWG)2 cu in
Internal cable clamps2 cu in
1 device (2 each)4 cu in
Show the math
Largest wire 14 AWG: 16 cu in in all

Estimate only. Read the cubic inches stamped in your box, and follow the code edition your town uses; have a licensed electrician check work you're unsure of.

The short answer

A single receptacle with four 14 AWG wires, two grounds and internal clamps needs 16 cubic inches by NEC 314.16, so a 3 × 2 × 3-1/2 in device box (18 cu in) or bigger. Two receptacles with six 12 AWG wires and grounds need 24.75 cu in.

Volume allowances and box sizes

Cubic inches for each conductor (NEC Table 314.16(B)), and the volume of some common metal boxes (Table 314.16(A)).

Volume allowance per conductor by wire size
WireEach counts
18 AWG1.50 cu in
16 AWG1.75 cu in
14 AWG2.00 cu in
12 AWG2.25 cu in
10 AWG2.50 cu in
8 AWG3.00 cu in
6 AWG5.00 cu in
Volume of common metal boxes
BoxVolume
3 × 2 × 2-1/2 in device box12.5 cu in
4 × 1-1/2 in octagon15.5 cu in
3 × 2 × 3-1/2 in device box18 cu in
4 × 1-1/2 in square21 cu in
4 × 2-1/8 in square30.3 cu in

How it works

The formula

fill = each wire entering × its allowance + grounds + clamps + studs + hickeys + 2 × each device
grounds use the largest ground's allowance; clamps, studs, hickeys and devices the largest wire's

Assumptions

  • Allowances, counting rules and box volumes from NEC 314.16 (2017 edition). The 2020 change for grounds (one allowance for up to four, a quarter more for each beyond) is as reported by electrical trade sources; check the edition your town uses.
  • Devices, clamps and studs use the largest wire present, a simplification: the code uses the largest wire connected to each device.
  • Nonmetallic and other boxes are marked with their volume; use that number.

Worked example: one receptacle

  1. 4 × 14 AWG: 8 cu in
  2. 2 grounds (14 AWG): 2 cu in
  3. Internal cable clamps: 2 cu in
  4. 1 device (2 each): 4 cu in
  5. 16 cu in → 3 × 2 × 3-1/2 in device box

Too big or too small

  • Overfilled: heat and damaged insulation. Crammed wires get nicked when the device is pushed in and run hotter.
  • Extension rings add volume. A ring's stamped volume adds to the box's.
  • Off when you work: turn off the breaker and test the wires before opening a box.

Questions people ask

How do you calculate box fill?

Add up allowances: each insulated wire entering the box counts once at its size (2.00 cu in for 14 AWG, 2.25 for 12, 2.50 for 10), all the ground wires together count as one of the largest ground, internal clamps count one, and each switch or receptacle counts two. Compare the total with the volume stamped in the box.

How many wires can go in a box?

It depends on the box and the wire. A 4 × 2-1/8 in square holds 30.3 cu in: fifteen 14 AWG wires with nothing else, or fewer once devices and grounds count. A receptacle with four 14 AWG wires, grounds and clamps needs 16 cu in.

Do ground wires count in box fill?

Yes, but not one each. Under the 2017 NEC all equipment grounds together count as one wire of the largest ground. From the 2020 NEC, one allowance covers up to four grounds, plus a quarter allowance for each beyond four.

What about pigtails and wire nuts?

A wire that starts and ends inside the box, like a pigtail, does not count, and wire connectors don't either. Only wires coming in from outside the box count.