How to calculate box fill under NEC 314.16

What box fill checks

Box fill is not a count of wires. It is a volume check: every conductor, device and fitting inside the box is given a volume allowance in cubic inches, and the total of those allowances must not exceed the volume of the box.

The allowance depends only on conductor size. Larger conductors take more room, so the same box holds fewer 10 AWG conductors than 14 AWG conductors.

The two editions number this table differently but list the same allowances, so the arithmetic below is the same under NEC 2023 and NEC 2026.

What counts: 314.16(B)(1) through (B)(6)

Go through the box and add one line per rule. This is the same order the box fill calculator uses.

• 314.16(B)(1) Conductors: each conductor that comes from outside the box and ends or is spliced in it counts once. Each conductor that passes through the box unbroken counts once. A loop or coil of unbroken conductor at least twice the free length required by 300.16 counts twice. A conductor that never leaves the box is not counted.
• 314.16(B)(2) Clamps: all internal cable clamps together count as one allowance, sized by the largest conductor in the box. Clamps outside the box do not count.
• 314.16(B)(3) Support fittings: each type of fixture stud or hickey counts once, at the largest conductor. Two studs of the same type are one allowance; a stud plus a hickey are two.
• 314.16(B)(4) Devices: each yoke or strap counts as two allowances, sized by the largest conductor connected to the device. A device wider than the usual single gang counts the double allowance for each gang it needs.
• 314.16(B)(5) Equipment grounding conductors: all of them together count as one allowance, sized by the largest ground. Each ground beyond four adds a quarter of an allowance.
• 314.16(B)(6) Terminal blocks: each block counts as one allowance, at the largest conductor connected to it.

Clamps and support fittings are sized by the largest conductor present in the box, and that includes the grounds. If a 12 AWG ground is the largest wire in a box of 14 AWG hots, the clamp is counted at 12 AWG.

The calculator does not model the exception to 314.16(B)(1) that lets certain grounds and small fixture wires from a luminaire canopy be left out. If it applies, leave those conductors out of the inputs yourself.

Find the box volume

Use the volume marked on the box when it has one. That marking is what the inspector reads, and nonmetallic boxes carry their own marking. For standard metal boxes with no marking, Table 314.16(A) gives the minimum volume by box size.

A barrier inside the box takes volume away. When the barrier is not marked with its volume, 314.16(A) reserves 0.50 in³ for a metal barrier and 1.00 in³ for a nonmetallic one. Subtract that from the box volume before you compare.

The calculation

Required volume = (B)(1) conductors + (B)(2) clamps + (B)(3) support fittings
                + (B)(4) devices + (B)(5) grounds + (B)(6) terminal blocks
The box passes when: required volume <= box volume

Example 1: a single-gang device box with 12 AWG

A 3 x 2 x 2-1/2 in device (12.50 in³) holds one receptacle fed by two 12/2 cables with ground: four 12 AWG conductors and two 12 AWG grounds, no internal clamps.

314.16(B)(1)  4 x 12 AWG conductor at 2.25 in³ = 9.00 in³
314.16(B)(4)  Device on 1 gang(s), double allowance per gang at 12 AWG = 4.50 in³
314.16(B)(5)  2 equipment grounding conductor(s) at 12 AWG, one allowance = 2.25 in³
Required: 15.75 in³   Box: 12.50 in³   Too small

The box is too small. The smallest standard device box that holds 15.75 in³ is the 3 x 2 x 3-1/2 in device, 18.00 in³. Note that the receptacle alone takes two conductor allowances, which is why a pass-through device box fills up faster than the wire count suggests.

Example 2: a 4 in square box with several cables

A 4 in square box holds a switch and three cables: two 12/2 and one 14/2, with internal clamps and three grounds. The conductors are four 12 AWG and two 14 AWG, and the switch is connected to 12 AWG. First try the 4 x 1-1/2 in square box (21.00 in³).

314.16(B)(1)  4 x 12 AWG conductor at 2.25 in³ = 9.00 in³
314.16(B)(1)  2 x 14 AWG conductor at 2.00 in³ = 4.00 in³
314.16(B)(2)  Internal cable clamps, one allowance at the largest conductor (12 AWG) = 2.25 in³
314.16(B)(4)  Device on 1 gang(s), double allowance per gang at 12 AWG = 4.50 in³
314.16(B)(5)  3 equipment grounding conductor(s) at 12 AWG, one allowance = 2.25 in³
Required: 22.00 in³   Box: 21.00 in³   Too small

That fails. The smallest standard square box that holds 22.00 in³ is the 4-11/16 x 1-1/4 in square, 25.50 in³, which leaves 3.50 in³ spare.

Common mistakes

• Forgetting that a device counts as two conductors, not one.
• Counting every ground separately instead of one allowance for the group, plus a quarter for each beyond four.
• Sizing the clamp or stud allowance at the conductors you just landed, not the largest conductor in the box, grounds included.
• Counting a conductor that never leaves the box, or missing a pass-through conductor that is not spliced.
• Using a Table 314.16(A) volume when the box is marked with a different volume. Use the marking.
• Mixing sizes: each conductor takes the allowance for its own size, so 14 AWG and 12 AWG conductors in one box are added at different values.

Box fill is separate from conduit fill. For raceway sizing, see how to calculate conduit fill and the conduit fill calculator.