blows circuit breaker when press start

If you have used this in the past and it worked it is likely a bad breaker, bad microwave or you have added something to the circuit to over-load it. If you will let me know how much electrical experience you have I can maybe suggest some ways to narrow it down.

Jim

Since you tried the microwave on two different circuits with the same result I would suggest that the microwave has blown/burned out a component. Since some parts of the microwave circuitry could hold extremely high voltage even when unplugged, this is definitely not something you want to try to repair at home unless you are an electrical engineer or have experience in this area. Take it to a repair shop or buy a new one.

NOT a bad breaker. The problem is the door monitor switch. If it does not activate when the door is closed it places a dead short across the power lines and trips your circuit breaker.

It is either the switch or a worn trip mechanism. Disassemble and insure that the door is tripping the switch when closed. Some MWs are adjustable, some not. If not, replace the plastic tripper. If so, replace the switch.