The Magic Numbers
2. Deciphering the Code
Here’s where those ‘magic numbers’ come in. The NEC assigns a specific cubic inch value to each wire size that enters a box. For example, a 14-gauge wire takes up 2.00 cubic inches, a 12-gauge wire eats up 2.25 cubic inches, a 10-gauge wire needs 2.50 cubic inches, and an 8-gauge wire demands 3.00 cubic inches. These numbers are crucial for our calculation, so keep them handy. Think of them as the “rent” each wire pays to live in the box.
Now, let’s talk about devices. Devices like switches, outlets, and even grounding conductors also take up space. Each counts as one or more conductors, depending on the situation. For example, all the grounding conductors combined only count as one conductor based on the largest wire size used for the grounding conductors. A device (switch or receptacle) counts as two conductors, based on the largest wire connected to it. These are a couple of scenarios where you would account for the device that is in the box.
So, how do we put it all together? First, count the number of each size wire in the box. Multiply that number by the corresponding cubic inch value. Do this for each wire size. Then, add up all those totals, and add the cubic inch values for devices and grounding conductors. If the final number is less than or equal to the box’s cubic inch rating (22 in our case), you’re good to go! If it’s over, you need a bigger box.
Lets say we have a 22 cubic inch box with six 14-gauge wires, one switch, and some grounding wires. Six 14-gauge wires take up 6 2.00 = 12 cubic inches. The switch counts as two 14-gauge wires, which is 2 2.00 = 4 cubic inches. The grounding conductors count as one 14-gauge wire, which is 2.00 cubic inches. So, the total fill is 12 + 4 + 2 = 18 cubic inches. We’re under the 22 cubic inch limit, so we’re golden!