Box fill, is the box big enough?

Short answer

Every conductor, device yoke, set of clamps and the ground bundle takes a set volume based on wire size: 2.00 cu in for 14 AWG, 2.25 for 12 AWG, 2.50 for 10 AWG. A 3 x 2 x 2-1/2 in device box holds 12.5 cu in. Four 12 AWG wires, one device (2 allowances), clamps and a ground need 18 cu in, so a deeper box is needed.

Worked example

A 3 x 2 x 2-1/2 in device box (12.5 cu in) with two 14 AWG cables (4 conductors = 8.0), one switch (2 x 2.00 = 4.0), clamps (2.00) and the ground bundle (2.00) needs 16.0 cu in, so it is too small by 3.5 cu in. A 4 in square box 2-1/8 in deep (30.3 cu in) would cover it.

Formula, assumptions and sources

Required volume = sum of the allowances in NEC 314.16(B): each conductor entering and ending or passing through the box counts once (a conductor that starts and ends in the box, such as a pigtail, counts zero; an unbroken loop at least twice the 300.14 length counts twice); one allowance for all internal clamps and one per type of stud or hickey, each based on the largest conductor in the box; two allowances per device yoke, based on the largest conductor connected to it; one allowance for the equipment grounding conductors, plus a quarter allowance for each beyond four as in the 2020 NEC. Allowances per conductor: 18 AWG 1.50, 16 AWG 1.75, 14 AWG 2.00, 12 AWG 2.25, 10 AWG 2.50, 8 AWG 3.00, 6 AWG 5.00 cu in. Standard box volumes are from Table 314.16(A). Not modeled: isolated-ground sets, barriers, fixture wires omitted under the dome exception, conduit bodies and boxes marked with a different volume (use the custom field).

Table values here follow the NEC as reproduced in the sources below (editions 2008 to 2020 era); fill percentages, tables and allowances can change between editions, so confirm against the edition your jurisdiction has adopted.

• NFPA 70 (NEC) 2008 text of 314.16 and Tables 314.16(A) and (B), reproduced with NFPA permission by Elliott Electric (retrieved 2026-10)
• Leviton, 2020 NEC change summary on equipment grounding conductor box fill (retrieved 2026-10)

Code and safety notes

This tool follows the U.S. National Electrical Code (NFPA 70). The Canadian Electrical Code (CSA C22.1) and local amendments differ in several tables and rules, so Canadian installations must be checked against the CEC and the authority having jurisdiction.

Planning aid only, not electrical advice. Electrical work can cause fire and shock; the adopted code edition, the equipment listing and a licensed electrician or inspector decide what is allowed.

FAQ

How many 14/2 cables fit in a 12.5 cu in single-gang box?

With internal clamps (2.00), one device (4.00) and the ground bundle (2.00) taking 8 cu in, 4.5 cu in remain. Each 14 AWG conductor is 2.00 cu in, so only one 14/2 cable (4 cu in) fits; a second needs a bigger box.

What volume does a 4 inch square box have?

NEC Table 314.16(A) lists 18.0 cu in for 1-1/4 in deep, 21.0 for 1-1/2 in deep and 30.3 for 2-1/8 in deep.

Do pigtails count in box fill?

No. A conductor that starts and ends inside the box, with no part leaving it, is not counted.

Does a switch or outlet count as one wire?

A device yoke counts as two conductor allowances, sized on the largest conductor connected to it.