I'm trying to find what the fill capacity on electrical junction boxes are.

I found this chart

The thing that is stumping me is do you count grounds or not?

For example, the specific instance that has me looking is that I have a 4x4x1 1/4 box, with 4 12/2 cables in it.

If you don't count the ground, I have 8 wires, and I'm fine (the box is stuffed, but allowable)
If you DO count grounds, I have 12 wires, and I have to rip the box out, and make it bigger.

If I do have to make it bigger, can I use an extension ring? How many additional wires would 1 1/2 inch add?
http://www.electrical-supply.net/ima...53151-S(5).jpg

Thanks

You should be fine with what you have. You are at the max for your 4x4x1 1/4.

Thanks. If I had to add a cable, would the extension ring allow me to add one? I need to extend this cicuit, and I'd rather come out of this junction box, rather than a light fixture that is feeding from it.

You can add an extension ring but you may have to unmount the box and push it deeper in the wall. Otherwise the extension will stick into the room. That may not be possible especially if any of your existing wires eneter the box from the rear

As you can see by the chart, a 4x4x1.25 box can have 8 #12 wires. All the grounds are counted as one, so with 4 #12-2 cables each with grounds, you have 9 wires.

Adding the extension ring, which is needed for your 4 cables, would add capacity for 9 additional wires per the chart. With your existing cables, the extension ring will allow for 8 more #12 wires, since one will be needed for the grounds you already have.

This chart and other information can be found in Section 314.16 of NFPA 70: National Electrical Code®

Thank you. So my course of action is clear now. I need the extension ring, so I might as well bring the power out of this junction box.

Thanks again.