Electrical Box Fill Calculator

An electrical box fill calculator — also called a junction box fill calculator or electrical box volume calculator — checks that an electrical box has enough free space for everything inside it. Under NEC 314.16, the box volume (from Table 314.16(A) or marked on the box) must never be less than the fill calculation from 314.16(B): conductors, cable clamps, support fittings, devices, equipment grounding conductors, and (since 2023) terminal blocks, each converted to cubic inches with Table 314.16(B).

Take a 4-inch square box, 1-1/2 inches deep (21.00 in3). Put in two 14/2 cables and one switch: 4 circuit conductors x 2.00 = 8.00 in3; 2 ground wires count as one allowance = 2.00 in3; internal cable clamps count as one allowance = 2.00 in3; the switch yoke counts double = 2 x 2.00 = 4.00 in3. Total = 16.00 in3, which is 76.19% of 21.00 in3 — the box passes with 5.00 in3 to spare.

Since the 2020 NEC, all equipment grounding conductors in the box are counted together: the first four count as a single allowance, and each additional ground counts as 0.25 of an allowance, based on the largest ground wire present. Example: five 12 AWG grounds = 1.25 x 2.25 = 2.81 in3. With four or fewer grounds, they simply count as one allowance of the largest ground size.

Under NEC box fill 2023 rules, 314.16(B)(6) added Terminal Block Fill: each terminal block assembly now gets a single volume allowance based on the largest conductor terminated to it. Example: two terminal block assemblies with 10 AWG conductors terminated = 2 x 2.50 = 5.00 in3 added to the fill. The 2020 and earlier editions had no rule for terminal blocks.

Conductors only, no devices or fittings: a 4-inch square box 1-1/2 inches deep holds 21.00 in3, and each 12 AWG conductor needs 2.25 in3, so the maximum is 9 (21.00 / 2.25 = 9.33, rounded down). The deeper 4-inch square box at 2-1/8 inches deep holds 30.30 in3, so it fits 13 (30.30 / 2.25 = 13.47, rounded down). These maximums match Table 314.16(A) exactly. For reference on how many 14 gauge wires in a box: the same 4-inch square box 1-1/2 inches deep fits 10 (21.00 / 2.00 = 10.50, rounded down).

Each device yoke or strap counts as two conductor allowances, based on the largest conductor connected to that device — NEC 314.16(B)(4). Example: a switch wired with 12 AWG counts as 2 x 2.25 = 4.50 in3. A yoke that spans two gangs counts as four allowances: on 14 AWG that is 4 x 2.00 = 8.00 in3.

No. NEC 314.16(B) says raceway and cable fittings, including locknuts and bushings, are not counted, and twist-on wire connectors (wire nuts) get no allowance either. Count the conductors they join, the devices, the clamps, the grounds, and the terminal blocks — not the small fittings themselves.

It still counts as one conductor allowance of its size. The only conductors that count double are looped, unbroken conductors at least twice the minimum free-conductor length — those count as two. Example: one looped 12 AWG conductor counts as 2 x 2.25 = 4.50 in3.

Yes. Each conductor is counted at its own size from Table 314.16(B). Example: three 14 AWG conductors (3 x 2.00 = 6.00 in3) plus two 12 AWG conductors (2 x 2.25 = 4.50 in3) = 10.50 in3 of conductor fill. Cable clamps and support fittings use the largest conductor present anywhere in the box, while each device uses the largest conductor connected to it.

Add up the fill first, then choose the smallest standard box whose volume meets or exceeds it — that is what the reverse sizer mode above does, working as a junction box size calculator NEC 314.16(A) edition. Example: a 16.00 in3 fill needs at least 16.00 in3, and the smallest standard box that qualifies is the 4-inch square box 1-1/4 inches deep at 18.00 in3, leaving 2.00 in3 of spare room.

No. Table 314.16(B) stops at 6 AWG (5.00 in3 per conductor). Boxes enclosing conductors 4 AWG or larger must also comply with NEC 314.28, which sizes boxes from the raceway entries instead of conductor volume allowances. This calculator covers 18 through 6 AWG only.

No. This is a general reference based on the NEC. Your local Authority Having Jurisdiction may adopt a different code edition or add amendments, and real installations involve details this tool does not cover. Always verify the final design with the locally adopted code and a licensed electrician.

A handy box fill calculator is this same calculator set up for a handy (utility) box — the small single-gang surface-mount box. The NEC 314.16(B) counting rules do not change; only the box volume differs, so enter the volume marked on the box as a custom box and count conductors, devices, clamps, grounds, and terminal blocks exactly as usual.

No. A masonry box is a single-gang box made for block or brick walls, and masonry box fill uses the same NEC 314.16(B) allowances as any other box. This calculator includes standard masonry boxes — for example the 3-3/4x2x2-1/2 in box at 14.00 in3 and the 3-3/4x2x3-1/2 in box at 21.00 in3 — or enter any marked volume as a custom box.