Box Fill Rating Calculations for Home Inspectors

Box Fill Code

The box fill code is described in 314.16 of the national electrical code (NEC). It states:

Boxes and conduit bodies shall be of an approved size to provide free space for all enclosed conductors. In no case shall the volume of the box, as calculated in 314.16(A), be less than the fill calculation as calculated in 314.16(B).

In other words, when installing a box, you need to do a fill calculation. Obviously, the box should have enough capacity based on that calculation.

Box Fill Volume

The first step is to find the box fill volume. You will most likely find the blue Carlon PVC boxes for new construction and new installations. These boxes have the volume printed on them. For example, the Carlon 2 gang box in the photo below has 32 cubic inches of capacity printed on the nail flap. You will also need to account for any rings or extenders installed on the box. These also generally have the volume printed on them.

If the box does not have the volume printed on it, you can use the below chart from the NEC to quickly find the box volume. This table can only be used when there are only conductors in the box such as a junction box. When there are other devices in the box, you will need to calculate the volume yourself.

Box Fill Calculation Steps

This is a lot easier than it sounds. But first, there are a few rules to know.

1. Count the Number of Conductors

You need to count the number of conductors (number of wires). Each current-carrying conductor counts as one allowance. For example, four 14-2 cables have eight current-carrying conductors, which would count as eight allowances.

2. Count the Grounding Wires

Before 2020, all of the equipment ground conductors in a box counted as a single conductor, so a single allowance. After 2020, up to four ground wires count as a single conductor, and then every ground wire after that counts as 1/4 an allowance. In our four 14-2 cable examples, there will be four ground wires, which count as one conductor and allowance.

3. Determine Wire Size

You need to know the conductor size or ampacity of the conductor. This is easy with the color coding of cables today. White is 15; yellow is 20, and Orange is 30.

4. Count the Number of Devices

You need to count the number of devices. Each device, such as a switch or outlet, counts as 2 allowances for the size of the larger conductor connected to it. Two outlets would count as 4 allowances.

5. Count the Number of Clamps

You need to count the number of clamps. External clamps do not count toward allowances. All of the internal clamps count as one allowance per the largest conductor. There is some debate as to whether the integral clamps of PVC boxes count towards your allowance. Just count them to keep things manageable. That is one more allowance in our scenario. Wire nuts do not add to your allowances.

6. Start Your Box Fill Calculation

1. If you have different size ground wires, you use the largest size in your calculations.
2. Fixture studs, such as used to mount a ceiling light, are counted as one allowance based on the largest conductor in the box. We will say there are none in our box.
3. Each loop of wire that is more than twice the minimum length (6 inches) must be counted as two allowances. However, this is not common, and measuring the exact length would be outside a home inspection’s scope.

Now multiply

You’ve been keeping up, right? We have 14 allowances in our example. All we need to do is multiply the number of allowances by the number in the NEC Table below. Size 14 wires get multiplied by 2 for a total of 28. If you remember, this box has a 32 cubic-inch capacity, so we are within the code requirement here.