The circuit breaker’s job is to protect wiring in the walls of your house, not the air conditioner and not the air compressor. As long as cross sectional area of the conductor is properly matched to the circuit breaker or fuse, it’s ok. That said, if you want to protect your air compressor, you should install a motor starter protection device calibrated to the nominal current, or at most 105% of the nominal current. This device may be installed either instead of the circuit breaker, or downstream from it. A three phase motor protection device, such as Eaton’s PKZM-0 series device may be wired in a special manner for use in a single phase circuit.To meet code you should run the wire to the compressor that can handle 30 amps. Off the top of my head, I think this would be 10 gauge copper. Depending on the length of the wire run, it might be cheaper to just change out the breaker with a 20 amp. If it is a short run and the wire is just going to be in the air from the box to the compressor, I would feel safe running #12 wire without changing the breaker since the compressor is never going to be pulling anywhere near 15 amps except when starting.

There is a very slight chance the compressor could fail in a way that put 29 amps on the wiring constantly. This would overheat the wire and maybe cause a fire, but this is so unlikely a situation that it would truly be a freak accident like in one of the Final Destination movies.typical car ac repair costThe other possibility of using 12 gauge wire with a 30 amp breaker would be if someone in the future removed the compressor and put in something that did draw near 30 amps continuously like a welder. ac unit replacement pricesThey could see the breaker was 30 amps and assume the wiring could handle this. buy house ac unitA good electrician would catch this because he would check the wire size when he installed a new load.I’m assuming you are planning on hard wiring the compressor, not use something like an oven or dryer receptacle rated for 30 amps.

In that case I would always run the proper wire size. I just looked the proper wire size up and my memory was correct.Written The answers below are somewhat valid. Assuming the wiring from the panel to the point the compressor receives its supply, is #10 it is could be OK. Codes permit Electric motor loads to have a much higher breaker or fuse rating than the nameplate running current amps. Overload protection is provided by the motor running over current protection that is either inside the motor or in a motor control associated with it.The NEC permits inverse time breakers, that you likely have, to be sized as much as 250% over the motor’s nameplate full load current. So if your Compressor Motor has a nameplate full load current of no less than 12 amps, it is ok. That current would mean a minimum compressor motor rating of 2HP. However if not, you will need a smaller breaker, or a breaker located at the point the 30 amp circuit ends and connects to the compressor. It is easier to replace the 30 amp breaker with a 15 or 20 amp rated breaker that is made for the panel, same brand or otherwise listed on the breaker’s package as fitting in that particular panel.