Electrical Box Fill Calculator

Enter the number of conductors by gauge, devices such as switches and receptacles, whether grounding conductors and an internal cable clamp are present and the box volume in cubic inches marked in the box. Under the National Electrical Code, each conductor counts by volume, 2 cubic inches for 14 gauge, 2.25 for 12 gauge and 2.5 for 10 gauge, each device counts as two conductors of the largest size, all grounding conductors together count as one and internal clamps count as one. Count conductors that originate in the box.

Conductors that pass through uncut may count differently from those spliced or terminated, and fixtures and fittings add volume. Rules differ for different wire gauges mixed in a box. Box volume is stamped inside the box or given in the catalog.

Work should follow the adopted code and be done by a qualified person.

Required box volume (cu in) from #14 AWG conductors

#14 AWG conductors	4
#12 AWG conductors	0
Devices (switches/receptacles)	1
Grounding conductors present	Yes
Internal cable clamp present	Yes
Box volume	18
Required box volume (cu in)	16
Percent of entered box volume used	88.9%

Common error: Counting only the conductors. Devices, clamps and grounds all take volume, and leaving them out puts too much in the box.

Electrical Box Fill: formula

Required volume = Σ conductor allowances + devices×2×largest allowance + ground×largest + clamp×largest (NEC 314.16(B))

NFPA 70: National Electrical Code

Electrical Box Fill: changing #14 AWG conductors

Required box volume (cu in): assumed #14 AWG conductors scenarios.
#14 AWG conductors	Required box volume (cu in)	Percent of entered box volume used
2	12	66.7%
3	14	77.8%
4 (example)	16	88.9%
5	18	100.0%
6	20	111.1%

Electrical Box Fill: compare input ranges

Electrical Box Fill Calculator: Required box volume (cu in) with each input moved on its own, every other input held at the worked-example value.
Input	Tested from	To	Required box volume (cu in): low input	Required box volume (cu in): high input	Swing
Devices (switches/receptacles)	0	2	12	20	8 (50%)
#14 AWG conductors	3	5	14	18	4 (25%)
#12 AWG conductors	0	1	16	19.3	3.3 (20%)
Grounding conductors present	Yes	No	16	14	2 (13%)
Internal cable clamp present	Yes	No	16	14	2 (13%)
Box volume	16	20	16	16	0

Electrical Box Fill: individual input comparisons

Electrical Box Fill Calculator: change one input at a time; other values remain at the stated example.
Input	Value	Required box volume (cu in)	Percent of entered box volume used
#14 AWG conductors	2	12	66.7%
	3	14	77.8%
	4	16	88.9%
	5	18	100.0%
	6	20	111.1%
#12 AWG conductors	0	16	88.9%
	0	16	88.9%
	0	16	88.9%
	1	19.3	106.9%
	2	21.5	119.4%
Devices (switches/receptacles)	0	12	66.7%
	0	12	66.7%
	1	16	88.9%
	2	20	111.1%
	3	24	133.3%
Grounding conductors present	Yes	16	88.9%
Grounding conductors present	No	14	77.8%
Internal cable clamp present	Yes	16	88.9%
Internal cable clamp present	No	14	77.8%
Box volume	14	16	114.3%
	16	16	100.0%
	18	16	88.9%
	20	16	80.0%
	22	16	72.7%

Required box volume (cu in): #14 AWG conductors against Devices (switches/receptacles)

Electrical Box Fill: Required box volume (cu in) at combined #14 AWG conductors and Devices (switches/receptacles) settings.
#14 AWG conductors / Devices (switches/receptacles)	0	1	3
2	8	12	20
3	10	14	22
4	12	16	24
5	14	18	26
6	16	20	28

Electrical Box Fill: values and definitions

Worked-example inputs and the results they produce for the Electrical Box Fill Calculator.
Quantity and definition	Value used
#14 AWG conductors — Count of 14 AWG conductors entering the box.	4
#12 AWG conductors — Count of 12 AWG conductors entering the box.	0
Devices (switches/receptacles) — Number of switches or receptacles mounted in the box; each counts as two conductors.	1
Grounding conductors present — Whether any grounding conductors are present (they count together as one conductor).	Yes
Internal cable clamp present — Whether the box has an internal cable clamp (it counts as one conductor).	Yes
Box volume — Rated cubic-inch volume of the box.	18
Required box volume (cu in)	16
Percent of entered box volume used	88.9%

Devices (switches/receptacles): half and double

Electrical Box Fill scale comparison; assumed inputs.
Devices (switches/receptacles)	Required box volume (cu in)
0.5	14
1	16
2	20

A box has a fixed volume, and the code counts how much room each conductor, device and clamp takes. In the example, four 14 gauge conductors at 2 cubic inches each give 8, a device counts as two allowances, or 4, and the ground and the internal clamp add 2 each, for 16 cubic inches required. An 18 cubic inch box is 88.9 percent used.

Crowding wires into a small box overheats them and makes them hard to install safely.

Read the volume marked inside the box and use the allowance for the largest conductor size present. Add for extra devices, fittings and any additional grounds.

See conduit capacity on the conduit fill calculator, wire ratings on the wire ampacity page and voltage loss on the voltage drop calculator. The cubic feet and rectangular prism pages relate to volumes, and the watts to amps calculator sizes circuits.

What if the box is too full?

Use a larger or deeper box, or an extension ring where allowed, since overfilled boxes are hazardous and violate code.

Electrical Box Fill Calculator