You right about 12 gauge wire being on a 20 amp breaker. BUT, If that # 12 wire was feeding an AC condenser I would check the nameplate rating of that equipment. The manufacture dictates the minimum wire ampacity and maximum over current protection. So as an example, the manufacture may say minimum circuit ampacity is 18 amps and maximum breaker size is 30 amps. So, you could run a # 12 wire and install that on a 30 amp breaker. The air conditioner guy may have installed the right size breaker , The manufacture follows all the codes in the NEC to be able to determine that.
You right about 12 gauge wire being on a 20 amp breaker. BUT, If that # 12 wire was feeding an AC condenser I would check the nameplate rating of that equipment. The manufacture dictates the minimum wire ampacity and maximum over current protection. So as an example, the manufacture may say minimum circuit ampacity is 18 amps and maximum breaker size is 30 amps. So, you could run a # 12 wire and install that on a 30 amp breaker. The air conditioner guy may have installed the right size breaker , The manufacture follows all the codes in the NEC to be able to determine that.
I was literally looking for someone to make this comment. 100% correct.
A/c can sometimes be higher value breaker, depending on nameplate and jurisdiction.
GOOD GOOD