Box fill calculation practice

What box fill is checking

Every conductor, clamp, device and ground inside a box takes up room. Section 314.16 turns each of those into a number of "volume allowances," and each allowance is worth a set volume based on conductor size. If the total volume needed is more than the box provides, the box is too small.

Volume allowance per conductor

Values from Table 314.16(B).

The counting rules, in worksheet order

1. Conductors. Each insulated conductor that enters the box and is spliced or terminated counts as 1. A conductor that passes through without a splice also counts as 1. 314.16(B)(1)
2. Internal cable clamps. If the box has one or more internal clamps, add 1 allowance in total, based on the largest conductor. Clamps on the outside of the box don't count. 314.16(B)(2)
3. Support fittings. Each fixture stud or hickey counts as 1. 314.16(B)(3)
4. Devices. Each yoke or strap holding a switch or receptacle counts as 2, based on the largest conductor connected to that device. 314.16(B)(4)
5. Equipment grounding conductors. Up to four grounds together count as 1, based on the largest. Since the 2020 NEC, each ground beyond four adds a quarter allowance. 314.16(B)(5)

Add the allowances, multiply by the volume per conductor, and compare to the box volume.

Worked example

A 4 in square by 2⅛ in metal box (30.3 cu in) with internal cable clamps has two 12/2 NM cables and one 12/3 NM cable entering it. It holds one receptacle. Is the box large enough?

The same cables in a 4 in square by 1½ in box, which is 21.0 cu in, would not fit. That's the type of comparison exam questions love.

Common box fill mistakes

• Counting each ground separately. Up to four grounds count as one allowance in total.
• Counting a device as 1. A device yoke counts as 2.
• Counting the bare ground as an insulated conductor. Handle grounds in their own step so you don't count them twice.
• Missing internal clamps. Read the box description. Metal boxes with internal clamps add one allowance; most plastic device boxes don't have them.
• Counting pigtails. A conductor that starts and ends inside the box, like a pigtail or jumper, isn't counted.

Special cases to know

• Unbroken loops. An unbroken conductor at least 12 inches long that's looped or coiled for a later splice counts as 2.
• Small fixture wires. Under a domed luminaire canopy, up to four fixture wires smaller than #14 plus one ground can be left out of the count.
• Mixed sizes. When conductors of different sizes are in the box, each conductor uses its own size's volume. Clamp, fitting and ground allowances use the largest conductor, and devices use the largest conductor connected to them.
• Plaster rings and extensions. A marked ring or extension adds its marked volume to the box.

Where to find box volumes

Standard metal box volumes are listed in Table 314.16(A). Nonmetallic boxes and other boxes are marked with their volume by the manufacturer. Exam questions usually give you the volume or the box dimensions.