I have a 3-gang metal (ca. 1980, three boxes with removable sides) electrical box that currently has 4 NM cables entering it; at least 3 of them are 12/3, the 4th is either 12/3 or 14/3. All four cables have grounds as well.
There are two 3-way switches in the box (a third has been removed and the two cables involved joined with wire nuts).
I'd like to feed a new nearby box with a 12/2 fed from the black and white wires of the two cables that are currently wire-nutted together (and then on to a new light fixture). Would this violate the maximum fill of this box? If so, I can pull that pair of cables from this box, and join them in a new box (this is in a garage, so an awkwardly placed box is not an issue).
Best Answer
What size box is it? It should either have a volume printed on it, or will be a standard size. Without knowing the box volume, we can't give you a definitive answer.
Assuming it's all 12 AWG, it's at least (12 (current carrying conductors) + 1 (grounds) + 4 (device fill)) * 2.25 = 38.25 cu. in. currently.
Adding another two conductor cable with ground would require an additional 4.5 cu. in. For a total of 42.75 cu. in., not counting any clamp fill not mentioned.
As long as the box is larger than this, you'll be okay.
Note that if you put the third device in, you'll have to add another 4.5 cu. in.
See this answer for more detail on box fill calculations.