How many 12 AWG wires fit in a 4 in. square box?

A 4 in. square box 1-1/2 in. deep holds 21.0 cu in, and 12 AWG takes 2.25 cu in each, so nine conductors with nothing else inside. Add one receptacle, which counts as two allowances, and one ground allowance, and six 12 AWG wires are the real limit.

Do ground wires count in box fill?

Yes, but all of them together count as a single allowance, taken at the size of the largest ground in the box. This is the one place the NEC lets a wire count for less than one, and it is also where Canada differs hardest: the CEC does not count bare bonding conductors at all.

Does the cable clamp count?

An internal clamp costs one allowance at the largest conductor in the box. A clamp mounted outside the box costs nothing, which is why plastic boxes with external clamps often fit a wire more than a metal box of the same volume.

What if my box is not on this chart?

Boxes that are not standard sizes are required to be marked with their volume in cubic inches, usually stamped inside the back. Use that number instead of the table and divide it by the allowance for your wire size, or type it straight into the calculator.

Box Fill Chart: NEC Table 314.16

Looking for the Canadian box fill chart? CEC Table 22 is here.

Last updated: September 25, 2026

How many wires fit in a box

Every standard box in NEC Table 314.16(A), and the largest number of same-size insulated conductors it holds when there is nothing else inside. A device, a ground or an internal clamp each take space of their own, so read the counting rules below the chart before you trust a number.

Maximum conductors by box size, NEC Table 314.16(A)

Box	18 AWG	16 AWG	14 AWG	12 AWG	10 AWG	8 AWG	6 AWG
Device box 3 × 2 × 1-1/2"	5	4	3	3	3	2	1
Device box 3 × 2 × 2"	6	5	5	4	4	3	2
Device box 3 × 2 × 2-1/4"	7	6	5	4	4	3	2
Device box 3 × 2 × 2-1/2"	8	7	6	5	5	4	2
Device box 3 × 2 × 2-3/4"	9	8	7	6	5	4	2
Device box 3 × 2 × 3-1/2"	12	10	9	8	7	6	3
4 in. round or octagon × 1-1/4"	8	7	6	5	5	4	2
4 in. round or octagon × 1-1/2"	10	8	7	6	6	5	3
4 in. round or octagon × 2-1/8"	14	12	10	9	8	7	4
4 in. square × 1-1/4"	12	10	9	8	7	6	3
4 in. square × 1-1/2"	14	12	10	9	8	7	4
4 in. square × 2-1/8"	20	17	15	13	12	10	6
4-11/16 in. square × 1-1/4"	17	14	12	11	10	8	5
4-11/16 in. square × 1-1/2"	19	16	14	13	11	9	5
4-11/16 in. square × 2-1/8"	28	24	21	18	16	14	8

Volume allowance per conductor, NEC Table 314.16(B)

Wire size	cu in
18 AWG	1.5
16 AWG	1.75
14 AWG	2
12 AWG	2.25
10 AWG	2.5
8 AWG	3
6 AWG	5

Every count is the box volume from Table 314.16(A) divided by the conductor allowance from Table 314.16(B), then truncated, because a part of an allowance is never another conductor. Both tables are sealed and fingerprinted in the calculator that shares this page. One honest caveat: our conduit chart is checked cell by cell against a licensed copy of Annex C, and no printed NEC is held here to check these counts the same way, so the arithmetic is the code's own, but we have no printed table to check it against. How we verify.

What else takes up room in the box

Each strap-mounted device, meaning a receptacle or a switch, counts as TWO allowances, and both are taken at the largest conductor in the box rather than the wire landing on that device.

All the equipment grounding conductors together count as ONE allowance, sized by the largest ground present. Two grounds or six, it stays one.

Internal cable clamps count as one allowance at the largest conductor. Clamps outside the box, the cable sheath and the mounting screws count as nothing at all.

So the chart is a ceiling for an empty box, not a permission slip. Put a receptacle and a ground in that 4 in. square and the 12 AWG count falls from nine to six. The calculator does the whole sum with your actual contents: Box Fill calculator

Canada does this differently

The CEC works in millilitres and its allowances run about one step smaller: 14 AWG is 24.6 mL where the NEC allows 2.0 cu in, near 32.8 mL. Canada also counts wire connectors, one allowance per PAIR, ignores bare bonding conductors completely and gives internal clamps no allowance whatsoever. Numbers converted from one code to the other will be wrong in both directions.

Box Fill calculator · Conduit fill chart · Wire ampacity chart

Common questions

How many 12 AWG wires fit in a 4 in. square box?: A 4 in. square box 1-1/2 in. deep holds 21.0 cu in, and 12 AWG takes 2.25 cu in each, so nine conductors with nothing else inside. Add one receptacle, which counts as two allowances, and one ground allowance, and six 12 AWG wires are the real limit.
Do ground wires count in box fill?: Yes, but all of them together count as a single allowance, taken at the size of the largest ground in the box. This is the one place the NEC lets a wire count for less than one, and it is also where Canada differs hardest: the CEC does not count bare bonding conductors at all.
Does the cable clamp count?: An internal clamp costs one allowance at the largest conductor in the box. A clamp mounted outside the box costs nothing, which is why plastic boxes with external clamps often fit a wire more than a metal box of the same volume.
What if my box is not on this chart?: Boxes that are not standard sizes are required to be marked with their volume in cubic inches, usually stamped inside the back. Use that number instead of the table and divide it by the allowance for your wire size, or type it straight into the calculator.