Electrical box fill calculator

A box has to be big enough for the wires, splices and devices in it, with free space for heat and so the insulation is not crushed when the device is pushed back. NEC 314.16 sets the rule: add up an allowance for everything in the box, and the box's volume has to be at least that total.

Each allowance is a number of cubic inches from Table 314.16(B), based on the wire size:

Boxes holding 4 AWG or larger wire also have to meet the pull and junction box sizing rules in 314.28, which go by the conduit size rather than cubic inches.

The five allowances in 314.16(B), added together:

1. Conductors. Each wire that comes into the box and ends or is spliced there counts once. A wire that passes straight through without a splice also counts once. A loop of unbroken wire at least twice the free length 300.14 requires counts twice. A wire that never leaves the box, such as a pigtail, counts zero.
2. Clamps. One or more internal cable clamps together count once, at the largest wire in the box. A connector clamped outside the box counts nothing.
3. Support fittings. Luminaire studs or hickeys count once for each type, at the largest wire.
4. Devices. Each yoke or strap holding a switch, receptacle or other device counts twice, at the largest wire connected to it. A device wider than a single 2 in. gang counts twice for each gang it needs.
5. Grounds. All the equipment grounding conductors together count once, at the largest ground. An extra isolated ground set adds one more.

Locknuts, bushings and wire connectors take no allowance. Grounds and pigtails are where most hand counts go wrong: count the grounds once, not one for each wire, and do not count pigtails at all.

Metal boxes in Table 314.16(A) use the table's volume unless they are marked larger. Plastic boxes and other sizes are marked with their volume inside by the maker, and you use that number.

The maximum counts in the table only apply when all the wires are the same size and there are no devices, clamps, fittings or grounds. As soon as any of those are in the box, work it out the long way, which is what the calculator does.

A plaster ring, extension ring, raised cover or domed cover adds its marked volume. A metal extension ring the same size as a box in the table adds that box's volume.

A receptacle fed through. Two 12/2 NM cables into a 3 × 2 × 3-1/2 metal device box with cable clamps, one receptacle.

• 4 insulated wires × 2.25 = 9.00 cu in
• Clamps: 1 × 2.25 = 2.25
• Receptacle: 2 × 2.25 = 4.50
• Grounds, all together: 1 × 2.25 = 2.25
• Total 18.00 cu in against a box of 18.0. It fits, exactly.

Add a third cable and it is 22.50, so the box is too small. A 4 × 2-1/8 square box (30.3 cu in) with a single-device raised cover would do it.

Two switches, mixed wire. A 4 × 1-1/2 square box with clamps, two 14 AWG and two 12 AWG insulated wires, two switches on their own yokes and grounds. The largest wire is 12 AWG, so every allowance after the wires uses 2.25:

• Wires: 2 × 2.00 + 2 × 2.25 = 8.50 cu in
• Clamps 2.25, switches 2 × 2 × 2.25 = 9.00, grounds 2.25
• Total 22.00 cu in against 21.0. Overfilled by 1 cu in; a 4-11/16 × 1-1/4 square box (25.5) fits.

A plastic box. A single-gang plastic box marked 20.3 cu in, with a raised cover marked 4 cu in, three 14/2 cables, one switch, no clamps: 6 × 2.00 + 2 × 2.00 + 2.00 = 18.00 cu in against 24.3. It fits with room to spare.

Worked the long way for a metal box with internal clamps and grounds in every cable. Take off the clamp allowance for a box without clamps, such as most single-gang plastic boxes.

The middle-of-run receptacle is the one to watch: in 12 AWG it fills an 18 cu in box exactly, so one more cable or a bulky GFCI means a bigger box. Shallow 2 in. and 2-1/4 in. device boxes only suit a single cable and a device.

• Go deeper. A 3-1/2 in. deep device box holds 18 cu in against 10 in a 2 in. deep one.
• Go square. A 4 in. square box with a raised device cover gives far more room than a device box, and the cover's marked volume adds to it.
• Add an extension ring on an old work box you can't replace.
• Move a splice. If cables only pass through to feed something else, splice them in a junction box elsewhere.
• Watch the device. A GFCI, dimmer or smart switch is much bulkier than a plain device. The NEC counts them the same, but they need the room in practice, so size up when you use them.

When you know the boxes for the job, build the estimate in Koira with boxes, covers, devices and labor as their own lines. For the raceway between boxes, use the conduit fill calculator.