I'm finishing a basement and think it's best to start fresh with a new circuit for a room we're building. There is a spare circuit breaker already in the panel (30 amps), so I suppose I could just run the 12 gauge wire from it (no need to change to 20 amp breaker?). Also, how many more 20 amp circuits and breakers can I add to the panel - what's the maximum? Many thanks - G

#12 wire is only rated for 20 amps, so a new 20 amp circuit needs a 20 amp circuit breaker. You risk a fire if you connect a #12 wire to a 30 amp breaker.

The label on the inside of the panel door will list how many circuits your panel can handle, the maximum for any panel is 42 circuits, including each two pole breaker counted as two circuits.

I'm with TK.

Q: When you say room. What kind of room?
Q: Year of home [or kind of service rated panel: Manufact, breakers or fuses, etc?]

Washinton, this post is almost a year old. I would hope the basement is done by now.