How do you calculate box fill?

Add up volume allowances from NEC 314.16(B): one allowance for each insulated conductor that enters the box, two for each device yoke (based on the largest conductor connected to it), one for all internal cable clamps, one for each type of support fitting, and one for all equipment grounding conductors. Multiply by the Table 314.16(B) volume for the conductor size and compare the total with the box volume.

How many 12 AWG wires can go in a 4 inch square box?

A 4 × 1-1/2 in square box is 21.0 in³, and each 12 AWG conductor needs 2.25 in³, so it holds 9 allowances. That is the conductor count only if there are no devices, clamps or grounds; each of those uses allowances too.

Do ground wires count in box fill?

Yes, but all equipment grounding conductors together count as a single allowance based on the largest one. An additional isolated ground set permitted by 250.146(D) adds one more allowance.

Do pigtails and wire nuts count in box fill?

A conductor that never leaves the box, such as a pigtail, is not counted (314.16(B)(1)). Wire connectors such as twist-on connectors are not counted either.

Does a receptacle count as two wires?

Yes. Each yoke or strap holding one or more devices counts as two allowances, based on the largest conductor connected to that device. A device too wide for a single 2 in device box counts two allowances for each gang it needs.

Do I count the volume of a mud ring or extension ring?

Yes, if it is marked with its volume or is a standard extension ring with a volume from Table 314.16(A). Add it to the box volume (314.16(A)).

Box Fill Calculator: NEC 314.16 (Free, Instant)

Check whether your conductors, devices, clamps and grounds fit in an outlet, device or junction box under NEC 314.16. Pick a standard metal box or enter the volume marked on a plastic box.

How box fill is calculated

NEC 314.16 limits how many conductors and devices go into a box so there's room to make up splices without damaging insulation or trapping heat. You count volume allowances under 314.16(B), multiply each by the volume for its wire size, and compare the total with the box volume.

Item in the box	Allowances	Based on
Each insulated conductor that enters and is spliced, terminated or passes through	1 each	That conductor's size
Each unbroken loop or coil 12 in (300 mm) or longer	2 each	That conductor's size
Internal cable clamps (one or more)	1 total	Largest conductor in the box
Fixture studs or hickeys	1 per type	Largest conductor in the box
Each yoke or strap with one or more devices	2 each	Largest conductor connected to the device
Equipment grounding conductors (one or more)	1 total, +1 for an isolated ground set	Largest grounding conductor
Conductors that never leave the box (pigtails)	0	Not counted

Required volume = Σ (allowances × Table 314.16(B) volume) ≤ box volume + marked ring volume

Volume per conductor (NEC Table 314.16(B))

Conductor size	18 AWG	16 AWG	14 AWG	12 AWG	10 AWG	8 AWG	6 AWG
Free space per conductor	1.50 in³	1.75 in³	2.00 in³	2.25 in³	2.50 in³	3.00 in³	5.00 in³

Up to four fixture wires smaller than 14 AWG from a domed luminaire canopy, and a grounding conductor coming from such a canopy, can be left out of the count (314.16(B)(1) exception).

Worked examples

Switch fed by two 14/2 NM cables

Four insulated 14 AWG conductors = 4 × 2.00 = 8.00 in³. One switch = 2 × 2.00 = 4.00. Grounds = 2.00. Internal clamps = 2.00. Total 16.0 in³. That fits a 3 × 2 × 3-1/2 in device box (18.0 in³) but not a 3 × 2 × 2-1/2 in box (12.5 in³).

Receptacle with 12 AWG in a 4 in square box

Two 12/2 cables: 4 × 2.25 = 9.00 in³, plus the receptacle 4.50, grounds 2.25 and clamps 2.25 for 18.0 in³. A 4 × 1-1/2 in square box (21.0 in³) passes. Add a third 12/2 cable and it climbs to 22.5 in³, so step up to a 4 × 2-1/8 in square box (30.3 in³) or add a marked extension ring.

Ceiling light box with a fixture stud

Two 14/2 cables in a 4 × 1-1/2 in octagonal box (15.5 in³): conductors 8.00, grounds 2.00, fixture stud 2.00, clamps 2.00 for 14.0 in³. It fits. The luminaire's own fixture wires are not counted when they meet the exception above.

Maximum conductors in standard metal boxes

Box volumes are from NEC Table 314.16(A). Counts are box volume divided by the Table 314.16(B) allowance, for conductors of one size with nothing else in the box. Deduct allowances for devices, clamps, studs and grounds.

Box	Volume	14 AWG	12 AWG	10 AWG	8 AWG	6 AWG
4 × 1-1/4 in round/octagonal	12.5 in³	6	5	5	4	2
4 × 1-1/2 in round/octagonal	15.5 in³	7	6	6	5	3
4 × 2-1/8 in round/octagonal	21.5 in³	10	9	8	7	4
4 × 1-1/4 in square	18 in³	9	8	7	6	3
4 × 1-1/2 in square	21 in³	10	9	8	7	4
4 × 2-1/8 in square	30.3 in³	15	13	12	10	6
4-11/16 × 1-1/4 in square	25.5 in³	12	11	10	8	5
4-11/16 × 1-1/2 in square	29.5 in³	14	13	11	9	5
4-11/16 × 2-1/8 in square	42 in³	21	18	16	14	8
3 × 2 × 1-1/2 in device	7.5 in³	3	3	3	2	1
3 × 2 × 2 in device	10 in³	5	4	4	3	2
3 × 2 × 2-1/4 in device	10.5 in³	5	4	4	3	2
3 × 2 × 2-1/2 in device	12.5 in³	6	5	5	4	2
3 × 2 × 2-3/4 in device	14 in³	7	6	5	4	2
3 × 2 × 3-1/2 in device	18 in³	9	8	7	6	3

Tips from the field

Plastic boxes are marked. Nonmetallic boxes and many metal boxes not listed in Table 314.16(A) carry their volume in cubic inches; use that number (314.16(A)(2)).
Rings add volume. A plaster ring, mud ring or extension ring marked with its volume adds to the box. This is often the easiest fix for a box that's one allowance short.
Large conductors use different rules. Boxes and conduit bodies with 4 AWG and larger conductors are sized under the pull and junction box rules in 314.28, not 314.16.
GFCI and AFCI devices are deep. The code count doesn't change, but a deeper box makes them far easier to fold in.
Working in Canada? Box fill falls under CEC Rule 12-3034 and its tables, which use a different method, so check those for CEC work.

Sizing the wire first? Use the wire size calculator or the wire size for amps chart. Pulling the same conductors in a raceway? Check the conduit fill calculator.

Box Fill Calculator