Can you use a 20 Amp breaker with this unit?

No you cannot. The Amana PTH153G50AXXX requires a breaker with a maximum size of 30A. While it can run at 20A, if it tries starting up on a hot day with a 20A breaker you run the risk of damaging it. 30A is needed to prevent any issues with running the PTAC.

If you have any further questions, please call us and we will be happy to assist you.