Box Fill Rating Calculations for Home Inspectors

this one box has several electrical defects is filled over capacity

Determining if an electrical box has too many conductors by doing box fill calculations is outside the scope of practice for a home inspector. Even opening an outlet box or junction box is not required. But home inspectors should constantly be learning and improving their craft.

To be clear, I don’t think inspectors should be opening up every box and outlet and performing box fill calculations. But we all know it is common to see open boxes in the attic, basement and crawlspace. These open boxes are most often installed by amateurs and more likely to be overfilled.

Overfilled boxes have the same potential danger as most electrical defects – arcing, overheating, and fire.

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. For new construction, and new installations, you will most likely find the blue Carlon PVC boxes. 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 and that is 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.

carlon boxes have the fill rating capacity 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.

NEC Table 314.16(B)

Box Fill Calculation

This is a lot easier than it sounds. There are a few rules to know.

  1. You need to count the number of conductors (number of wires). Each current carrying conductor counts as one allowance. Four 14-2 cables have 8 current-carrying conductors, so that would count as 8 allowances.
  2. 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, so that counts as one conductor and allowance.
  3. 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, Orange is 30.
  4. 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. You need to count the number of clamps. External clamps do not count towards 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 easy. That is one more allowance in our scenario. Wire nuts do not add to your allowances.
  6. If you have different size ground wires, you use the largest size in your calculations.
  7. Fixture studs, such as used to mount a ceiling light, are counted as one allowance based on the largest conductor present in the box. We will say there are none in our box.
  8. Each loop of wire that is more than twice the the minimum length (6 inches) must be counted as two allowances. This is not common and measuring the exact length would definitely be outside the scope of a home inspection.

Now just 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 32 cubic inch capacity so we are within the code requirement here.

box fill calculation chart

Part Number Hint

The Carlon boxes have the maximum fill area in their part numbers. For example a B232 box is a 2 gang box with 32 cubic inch of capacity. A B112 box is a 1 gang box with 12 cubic inches of capacity.

Other Defects to Inspect for

While you are at it, there are some more things you can look for when inspecting electrical boxes.

  1. All boxes should be secured to the framing and flush with the finish surface such as drywall.
  2. All cables should be clamped to the boxed, and secured within 12 inches of the box.
  3. Generally, every romex connector can only have one cable unless the installation guidelines state differently.

Bottom Line

Box fill calculations are out of the scope of practice for home inspectors, and more in line with the work of electrical inspectors. That being said, improperly installed electrical equipment is a hazard for homeowners. This is an easy skill for home inspectors to learn and master allowing them to provide a better service.