Can I use a junction box to splice together wires that are on a different circuit?

Question

I'm doing some work inside a wall that has some 12/2 NM cable running through where I need to put something. The cable doesn't have any additional slack for me to route it around the obstruction. I initially contemplated using a wire splice kit to add some slack, but am worried about the longevity of the device, so I want to do it right and splice it in a junction box.

However (and I'm sure this is common) I don't really want an ugly cover plate in the middle of the finished wall. Fortunately, there's an existing electrical box very close by that's used for a switched light on a different circuit.

My question is this: can I use the existing electrical box to splice cables that are on a different circuit? Or does each box have to only contain cables on the same circuit?

Answer 1

You can have multiple circuits in the same box so long as you do not overload the volume of the box with devices and wires.

See this answer by @Tester101 that discusses box volume.