![]() If you remember, this box has a 32 cubic-inch capacity, so we are within the code requirement here. Size 14 wires get multiplied by 2 for a total of 28. All we need to do is multiply the number of allowances by the number in the NEC Table below. You’ve been keeping up, right? We have 14 allowances in our example. ![]() However, this is not common, and measuring the exact length would be outside a home inspection’s scope. Each loop of wire that is more than twice the minimum length (6 inches) must be counted as two allowances.Fixture studs, such as used to mount a ceiling light, are counted as one allowance based on the largest conductor in the box.If you have different size ground wires, you use the largest size in your calculations.That is one more allowance in our scenario. Just count them to keep things manageable. There is some debate as to whether the integral clamps of PVC boxes count towards your allowance. All of the internal clamps count as one allowance per the largest conductor. External clamps do not count toward allowances. Each device, such as a switch or outlet, counts as 2 allowances for the size of the larger conductor connected to it. White is 15 yellow is 20, and Orange is 30. This is easy with the color coding of cables today. You need to know the conductor size or ampacity of the conductor. In our four 14-2 cable examples, there will be four ground wires, which count as one conductor and 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. Count the Grounding Wiresīefore 2020, all of the equipment ground conductors in a box counted as a single conductor, so a single allowance. For example, four 14-2 cables have eight current-carrying conductors, which would count as eight allowances. Each current-carrying conductor counts as one allowance. You need to count the number of conductors (number of wires). But first, there are a few rules to know. These also generally have the volume printed on them.īox Fill Calculation Steps This is a lot easier than it sounds. You will also need to account for any rings or extenders installed on the box. For example, the Carlon 2 gang box in the photo below has 32 cubic inches of capacity printed on the nail flap. These boxes have the volume printed on them. You will most likely find the blue Carlon PVC boxes for new construction and new installations. The first step is to find the box fill volume. Obviously, the box should have enough capacity based on that calculation. In other words, when installing a box, you need to do a fill calculation. 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). It states:īoxes and conduit bodies shall be of an approved size to provide free space for all enclosed conductors. The box fill code is described in 314.16 of the national electrical code (NEC). Overfilled boxes have the same potential danger as most electrical defects – arcing, overheating, and fire. These open boxes are often installed by amateurs and are more likely to be overfilled. ![]() But we all know it is common to see open boxes in the attic, basement, and crawlspace. To be clear, I don’t think inspectors should open up every box and outlet and perform box-fill calculations. But home inspectors should constantly be learning and improving their craft. Even opening an outlet box or junction box is not required. ![]() Determining if an electrical box has too many conductors by doing box fill calculations is outside the scope of practice for a home inspector.
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