How to calculate box fill under NEC 314.16

The rule in one sentence

NEC 314.16 says a box must be large enough for what is inside it: the total of all volume allowances, figured from Table 314.16(B), may not exceed the volume of the box. Box volume comes from Table 314.16(A) for standard metal boxes, or from the volume the manufacturer marks on the box. Plaster rings, extension rings and domed covers that are marked with their volume can be added to the box volume.

The calculation has four steps: list what is in the box, convert each item into allowances, multiply by the volume for the right wire size, and compare the sum with the box.

Step 1: the volume for each wire size (Table 314.16(B))

Every allowance is expressed as the volume of one conductor of a given size:

Notice the jump from 8 AWG (3.00) to 6 AWG (5.00). A single 6 AWG device or clamp allowance costs more than two 14 AWG conductors.

Step 2: count the allowances (314.16(B)(1) to (B)(5))

1. Conductors (B)(1): each insulated conductor that comes into the box from outside and is spliced or terminated inside counts once, at its own size. A conductor that passes through the box without a splice also counts once. A conductor that starts and ends inside the box, such as a pigtail or a bonding jumper, does not count at all.
2. Clamps (B)(2): if the box has one or more internal cable clamps, add one allowance, sized by the largest conductor in the box. Two clamps or four clamps are still one allowance. Connectors mounted outside the box do not count.
3. Support fittings (B)(3): luminaire studs and hickeys inside the box add one allowance for the type of fitting, sized by the largest conductor in the box.
4. Devices (B)(4): each yoke or strap carrying one or more devices (a receptacle, a switch, a combination device) counts as two allowances, sized by the largest conductor connected to that device. A device too wide for a single 2-inch gang counts two allowances for each gang it needs.
5. Equipment grounding conductors (B)(5): in the 2020 and 2023 editions, up to four grounding conductors or bonding jumpers entering the box count as one allowance, sized by the largest. Each one beyond four adds a quarter allowance.

Fixture wires get a break: up to four fixture wires smaller than 14 AWG, plus their ground, coming from a domed luminaire or a similar canopy into the box are not counted.

Older editions counted grounds differently: under the 2017 NEC all grounds together were one allowance no matter how many, and an isolated-ground conductor added a full allowance of its own. Check which edition your jurisdiction has adopted.

Step 3: worked examples

Example 1, receptacle in the middle of a run. Two 12/2 NM cables enter a plastic device box with no internal clamps and feed one duplex receptacle. Conductors: 4 × 2.25 = 9 cu in. Receptacle: 2 × 2.25 = 4.5. Two grounds: one allowance = 2.25. Total 15.75 cu in. A 3×2×2-3/4" device box (14 cu in) is too small; a 3×2×3-1/2" box (18 cu in) passes.

Example 2, three cables and clamps. Two 12/2 and one 12/3 cable enter a metal box with internal clamps and feed a duplex receptacle on 12 AWG. Conductors: 7 × 2.25 = 15.75. Clamps: 2.25. Receptacle: 4.5. Three grounds: 2.25. Total 24.75 cu in. A 4" square box 1-1/2" deep (21 cu in) fails; 2-1/8" deep (30.3 cu in) passes.

Example 3, a junction box with six cables. Six 12/2 cables are spliced in a box with clamps and no devices: 12 insulated conductors and 6 grounds. Conductors: 12 × 2.25 = 27. Clamps: 2.25. Grounds: six is two more than four, so 1 + 2 × 0.25 = 1.5 allowances = 3.375. Total 32.625 cu in. A 4" square 2-1/8" box (30.3) fails; a 4-11/16" square 2-1/8" box (42) passes.

Example 4, mixed wire sizes. A 10/2 cable and two 14/2 cables meet at a 30 A receptacle wired with the 10 AWG conductors. Conductors: 2 × 2.50 + 4 × 2.00 = 13. The receptacle is sized by the largest conductor connected to it, 10 AWG: 2 × 2.50 = 5. Three grounds, largest 10 AWG: 2.5. Total 20.5 cu in, which fits a 4" square box 1-1/2" deep (21 cu in) with 0.5 cu in to spare.

Step 4: compare with the box, and read the tables backwards

If the total is at or below the box volume, the box complies. If not, use a deeper box, a 4-11/16" box, or add a marked extension ring or raised cover and add its volume.

Table 314.16(A) also lists the maximum number of conductors of one size for each standard box. Those numbers are only valid when the box holds nothing else: no clamps, devices, grounds or fittings. They are simply the box volume divided by the per-conductor volume, rounded down:

As soon as there is a receptacle, a clamp or a ground in the box, subtract their allowances first and divide what is left. The box fill chart on this site lists every standard box this way.

Mistakes that fail inspections

• Counting the device as one conductor. A receptacle or switch is two allowances, at the size of the largest conductor landed on it.
• Counting pigtails. Wires that begin and end inside the box are free; only conductors that come from outside count.
• Sizing the clamp or device allowance by the wrong wire. Clamps use the largest conductor in the box; a device uses the largest conductor connected to it.
• Using the Table 314.16(A) conductor count for a box with a device in it. That count assumes an empty box.
• Ignoring the marked volume of plastic boxes. Nonmetallic boxes are not in Table 314.16(A); use the cubic inches molded into the box.
• Applying the old ground rule. Under the 2020 and later editions, five or more grounds cost more than one allowance.

Questions people ask