Box Fill Calculator (NEC 314.16)

How the volume is counted (NEC 314.16(B))

1. Conductors: each conductor that enters the box and is spliced or terminated, or passes through unbroken, counts once at the volume in Table 314.16(B): 14 AWG 2.00 in³, 12 AWG 2.25, 10 AWG 2.50, 8 AWG 3.00, 6 AWG 5.00. A pigtail or jumper that never leaves the box is not counted.
2. Clamps: one allowance of the largest conductor present if the box has one or more internal cable clamps.
3. Support fittings: one allowance of the largest conductor for each type of luminaire stud or hickey.
4. Devices: two allowances of the largest conductor for each yoke or strap (a switch, receptacle or combination device).
5. Grounds: one allowance based on the largest equipment grounding conductor or bonding jumper in the box.

The total is the minimum volume the box must provide. For metal boxes the volume comes from Table 314.16(A); for nonmetallic boxes it is marked inside. Plaster rings, extension rings and domed covers that are marked with a volume add to it.

Worked example

Three 12-2 NM cables (six 12 AWG conductors plus their grounds) enter a box with internal clamps and feed one receptacle. Conductors: 6 × 2.25 = 13.50 in³. Clamps: 2.25. One yoke: 2 × 2.25 = 4.50. Grounds: 2.25. Minimum volume = 22.50 in³. A 3 × 2 × 3-1/2 in. device box (18.0 in³) is too small; the smallest standard box is a 4-11/16 × 1-1/4 in. square (25.5 in³), and a 4 × 2-1/8 in. square (30.3 in³) also works.