Electrical Box Fill Calculator (NEC 314.16)
Required box volume under NEC 314.16 from your conductors, devices, clamps, fittings and grounds, checked against a standard metal box from Table 314.16(A) or your box's marked volume.
Formula
Required volume = Σ(conductors × Table 314.16(B) allowance) + 1 allowance for clamps + 1 per fitting type + 2 per yoke + 1 for all grounds, each at the largest conductor size present. Allowances: 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.
Worked example
Inputs: 14 AWG conductors = 4; 12 AWG conductors = 0; 10 AWG conductors = 0; 8 AWG conductors = 0; 6 AWG conductors = 0; Devices (yokes/straps: switch, receptacle) = 1; Internal cable clamps = None; Luminaire stud/hickey types present = 0; Equipment grounding conductors = Yes (one or more); Box = 3 × 2 × 2½ in device (12.5 cu in); Marked box volume (for "Other") = 0 cu in.
| Conductor volume cu in | 8 |
|---|---|
| Allowances cu in | 6 |
| Required volume cu in | 14 |
| Box volume cu in | 12.5 |
| Box is large enough | No |
| Spare cu in | -1.5 |
Yokes, clamps and fittings use the largest conductor in the box here; the code bases a yoke on the largest conductor connected to that device and the ground on the largest grounding conductor, which can be smaller. Add plaster rings, extension rings and domed covers marked with a volume to the box volume. Your inspector has the final say.
FAQ
How do you calculate box fill?
Count each conductor that enters and ends or splices in the box once, add two allowances per device yoke, one for all grounds together, one for internal clamps and one per fitting type, then multiply by the Table 314.16(B) volume. Two 14/2 cables (4 conductors) plus a switch and grounds = 4 + 2 + 1 = 7 × 2.00 = 14.0 cu in.
Is a 3 × 2 × 2½ in device box big enough?
Table 314.16(A) gives it 12.5 cu in, so the 14.0 cu in example above does not fit. A 3 × 2 × 2¾ in box (14.0 cu in) does.
How many 12 gauge wires in a box?
Divide the box volume by 2.25 after subtracting the allowances. A 4 × 2⅛ in square box (30.3 cu in) holds 13 #12 conductors with nothing else in it, the same count Table 314.16(A) lists.
Do grounds count?
Yes, but all equipment grounding conductors together count as one allowance (plus one more for an additional isolated-ground set). Conductors that never leave the box, like pigtails, do not count.
What about plastic boxes?
Nonmetallic boxes and boxes not in Table 314.16(A) must be marked with their volume. Choose "Other" and enter that number.
Sources
Related
- Wire Size & Voltage Drop Calculator (AWG, Copper/Aluminum)
- Generator Size Calculator (Running & Starting Watts for Your House)
More Electrical calculators
- Conduit Fill Calculator (NEC Chapter 9, EMT and PVC)
- Wire Size Calculator (Ampacity, 125% Rule and Voltage Drop)
Embed this calculator
Free to embed on your site; please keep the credit link.
Last reviewed 2026-10-30.