Which Box Do I Need?
In your house you will be using several different shapes and sizes of junction, outlet, switch and lighting boxes. There are metal boxes, plastic boxes, and phenolic boxes. All of them come in a range from single gang to six gang. Most also come in 3/0 or 4/0 (3" or 4") round and square junction box styles. Then there are specialized boxes such as those made for paddle fans, range outlet boxes, and boxes designed for use in tight spaces. Here I'll go through the most common types you might use in your house.
Boxes also come in different depths, the deeper the box, the more cubic inches it has. Why does this make a difference? Simply put; more cubic inches mean you can fit more wires into the box. Every wire takes up a certain number of cubic inches in a box, as does every device (switch or outlet). Most of the time if you use good wiring practices and don't run any more cables into a box than you really need to, you won't have to worry about this, but it's worth a short discussion.
For the most part you will probably be using plastic boxes and the manufactures of these have made it easy. All plastic boxes have both the cubic inches of the box, and the maximum number of different wire sizes molded into them, usually inside the box way in the back. Metal and phenolic boxes have just the cubic inches marked on them. Either way this isn't too hard, we just have to do a little math.
If you're using plastic boxes this is pretty simple. Add 1 wire for each hot and each neutral conductor. Now add 1 for ALL the ground wires together (not each one). Now if you are installing devices (outlets or switches) add 2 for each one of them. Now simply look at the list on the box for the correct wire size, and be sure your number is less than that listed.
I have two 14-2 nm cables running into an outlet box. I would count the two hot (black) wires and the two neutral (white wires) and get a total of 4. Now I would add 1 for my grounds, for a total so far of 5. Lastly I would add 2 for the outlet I will be installing in the box, for a grand total of 7. Now I look on the box and see that it says I can install nine #14 wires, so I'm in great shape!
What if my box doesn't have a list?
If your box doesn't have the number of wires listed, but just the cubic inches, we have to do a little more math. Every wire size takes up a pre-determined number of cubic inches. The main ones we'll need to deal with in a house are #14, #12, and #10. The values we use for them are 2.00in, 2.25in and 2.50in, respectively. So now that we know that, let's continue with the example:
Just as above, we come up with a number of 7. We are using #14, so each one is worth 2.00in. Now we multiply our 2.00 cubic inches by seven and get 14 cubic inches total. Now we look at our box and see that it is 18 cubic inches. Once again, we're in great shape!
Is there an easier way?
But of Course there is! I've made this step really simple for you with my box fill calculator.