Box Fill Calculation Examples (NEC 314.16)

Box fill is a counting problem more than a math problem. The multiplication is easy. What costs points is counting the wrong things: every ground wire separately, the wire connectors, the pigtails, or forgetting the device and the clamps. This guide covers the counting rules in NEC 314.16 and works through three examples with real numbers.

The two tables you need

• Table 314.16(A) lists the volume of standard metal boxes in cubic inches, plus the maximum number of conductors of one size for each box. That second part is only useful when every conductor is the same size and there are no devices or clamps.
• Table 314.16(B) lists the volume allowance for each conductor size. This is the one you use for almost every exam problem.

Nonmetallic boxes and many metal boxes are marked with their volume. When a box is marked, use the marked volume. Plaster rings, extension rings and raised covers that are marked with a volume add to the box volume.

The counting rules

Here is what counts, in plain terms. The rules are in 314.16(B)(1) through (B)(5).

Wire connectors (twist-on or push-in splicing devices) are not counted, and neither are cable connectors whose clamping part sits outside the box. Small fixture wires coming from a luminaire canopy can be left out in some cases; 314.16(B)(1) has the exception and its limits. A device wider than a standard 2 in. device gets 2 for each gang width it needs. There is also an extra allowance when isolated grounding conductors are present; read 314.16(B)(5) if a question mentions them.

Worked example 1: three cables and a receptacle

A 4 in. square box, 2-1/8 in. deep, has three 12/2 NM cables with ground entering it, one duplex receptacle, and internal cable clamps. Table 314.16(A) gives 30.3 cu in for this box. Is it big enough?

Now try a 4 in. square box that is 1-1/2 in. deep. Table 314.16(A) lists it at 21.0 cu in. 22.5 is larger than 21.0, so the shallower box is too small, unless you add a marked extension ring or plaster ring that brings the total volume up to at least 22.5 cu in.

Worked example 2: a switch box

A metal device box, 3 x 2 x 3-1/2 in. (18.0 cu in in Table 314.16(A)), has one 14/2 NM cable in and one 14/2 NM cable out, a single-pole switch, and internal clamps.

Worked example 3: mixed conductor sizes

When sizes are mixed, each conductor uses its own allowance, and the clamps, grounds and devices use the largest size that applies to them. A 4-11/16 in. square box, 2-1/8 in. deep (42.0 cu in), contains:

• two 12/2 NM cables with ground (20 A circuit),
• two 14/2 NM cables with ground (15 A circuit),
• one receptacle connected to the 12 AWG conductors,
• internal cable clamps.

If the receptacle had been connected to the 14 AWG conductors instead, its allowance would be 2 × 2.00 = 4.00 cu in, because the device is based on the largest conductor connected to that device, not the largest in the box.

Solving backwards: how many conductors fit?

Some questions give the box and the devices and ask how many conductors you can add. Subtract the fixed allowances first, then divide.

Here you always round down. A box that is 0.47 of a conductor short is still too small.

Common box fill mistakes

• Counting each ground wire separately. All equipment grounding conductors together count as one.
• Counting wire connectors or the pigtails that never leave the box.
• Forgetting the device yoke counts as two, not one.
• Forgetting internal clamps. If the question mentions them, they count once, at the largest conductor size.
• Using the conductor count column of Table 314.16(A) when sizes are mixed or there is a device in the box.

Box fill and conduit fill are both space problems, and they reward the same habit: write each item on its own line before you multiply. If you want more problems laid out like this, get 5 worked problems in your inbox.