Will the wires fit in the box?

How do I calculate box fill?

Count volume allowances from NEC 314.16(B) and compare the total with the box volume. Each insulated wire that enters the box counts once at its size: 2.00 cubic inches for 14 AWG, 2.25 for 12 AWG, 2.50 for 10 AWG. Internal cable clamps add one allowance of the largest wire, each device (a receptacle or switch) adds two, and all the ground wires together add one.

How many 12 AWG wires can go in a box?

Two 12/2 cables with ground feeding a receptacle, with internal clamps: 4 wires × 2.25 + clamps 2.25 + the receptacle 4.50 + grounds 2.25 = 18 cubic inches. That just fits a 3 × 2 × 3½ inch device box (18.0) and fits a 4 × 1½ inch square box (21.0) with room to spare.

Do ground wires count in box fill?

Yes, but all of them together count as one wire of the largest ground size. Pigtails and wire nuts that stay inside the box do not count, and neither does a wire that starts and ends in the box.

How much does a switch or receptacle count?

Each device on a single strap counts as two wires of the largest conductor connected to it. A receptacle on 12 AWG wire counts 2 × 2.25 = 4.5 cubic inches. A device wider than 2 inches that takes more than one gang counts two for each gang.

What about plastic boxes?

Nonmetallic boxes are marked with their volume inside the box — enter it as a custom volume. The same counting rules apply; many plastic boxes have no internal clamps, so set clamps to No when the cable is stapled outside.

How the allowances add up

The required volume is the sum of five kinds of allowance, each taken from Table 314.16(B) at a wire size: the conductors (one allowance per wire at its own size), the clamps (one allowance in total, however many clamps there are), the support fittings such as fixture studs and hickeys (one each), the devices (two each) and the equipment grounds (one in total). The clamp, stud, device and ground allowances use the size of the largest conductor involved, so a single 10 AWG wire in an otherwise 14 AWG box raises several lines at once. The total is then compared with the volume stamped on, or listed for, the box.

Worked example: a receptacle on 14 AWG wire

A cable comes in and a second cable leaves, both 14/2 with ground, feeding one receptacle in a box with internal clamps. Four insulated conductors at 2.00 cubic inches each give 8; the clamps add 2; the receptacle counts twice, 4; the grounds together add 2. The total is 16 cubic inches, so the smallest box on the list that holds it is a 4 × 1¼ in square (18 cubic inches). A 3 × 2 × 2¼ inch device box (10.5) would be far too small for this layout.

Worked example: mixed wire sizes

Now suppose three 14 AWG wires and two 12 AWG wires meet at one receptacle, with clamps and a 12 AWG ground. The conductors give 3 × 2.00 + 2 × 2.25 = 10.5. Clamps, the device and the ground are all sized by the largest wire, 12 AWG (2.25): 2.25 for the clamps, 4.5 for the receptacle and 2.25 for the grounds. The total is 19.5 cubic inches, which calls for a 4 × 1½ in square (21 cubic inches) or larger. Swapping the 12 AWG wires for 14 AWG would have saved 1.5 cubic inches.

Common box fill mistakes

Counting every clamp separately: all internal clamps together are one allowance. Counting pigtails or wire nuts: a conductor that starts and ends inside the box is not counted, and external connectors that mount through a knockout are not counted either. Forgetting that a receptacle or switch counts twice, not once. Sizing the clamp and device allowances from the smallest wire instead of the largest. Reading a box volume for a different depth: the same 3 × 2 inch device box holds 7.5 cubic inches at 1½ inches deep and 18 at 3½.

Sources and related calculators

Sources: NFPA 70, National Electrical Code, Section 314.16 (Number of conductors in outlet, device and junction boxes), Table 314.16(A) for metal box volumes and Table 314.16(B) for the volume allowance per conductor, as summarized in electrical continuing-education notes and Electrical Contractor magazine’s box-fill series. Related: conduit fill calculator, wire size calculator and breaker size calculator.