A 12,000 btu air conditioner draws about 12 amps. You should be fine with a 30 amp breaker, although I suspect that the branch circuit feeding the A/C is really 15 or 20 amps. If it were really 30 amps, the A/C circuit would have to be wired with 10 gauge wire. It is probably wired with 12 or 14 gauge.
David