Check the Cable Box Size

Boxes come in different sizes based on the installation need. Rectangular boxes, the most common ones used for single devices, generally are two inches by three inches for residential use. Depth ranges from 15/8 inches to 319/32 inches. The deeper the box, the more wires it can accommodate and the easier it is to tuck in the wires and install a device and still meet code. You’ll really appreciate this when you’re dealing with 12-gauge wire.
A round box with
extendible bars.
How many cables can your box accommodate? Well, the bigger the box the better, but to be more exact …

• Count the number of intended cables for the box. Each hot and neutral conductor counts as one wire each, and all the grounding conductors together count as a single wire.
• Take this total and add one for any cable clamps (if they’re the same type of clamp). If you have two different types, you have to count each as a separate number.
• Take this new total and add two for each device (switch or receptacle).
• If the box contains 14-gauge wire, multiply the total number (of wires, clamps, and devices) by 2 cubic inches. If 12-gauge wire is being used, multiply the total by 2.25 cubic inches.
• The result of this multiplication is the minimum allowable volume of wires, clamps, and devices for that box.

(The volume of a box usually is stamped on the back of the inside of the box.)
Let’s say you have a light switch in a plastic box with two 14-gauge cables coming
into it. (One is the line; one is the load.) This gives you two hot conductors, two neutrals,
two grounding conductors (these count as one wire in our calculations), and
one switch. Therefore …
Two hot conductors: 2
Two neutrals: 2
One grounding conductor: 1
Device: 2
Total: 7
7 2 cubic inches = 14 cubic inch minimum box size (Note: Most plastic boxes do not have any type of clamp.)