Dedicated circuit. Replaced 15A breaker w/20A. Replaced outlet with HD 20A. Still trips. Ran HD extension cord to a different 15A outlet. Works perfectly. I am out of guesses.

First, you have created a dangerous code violation if you just changed the breaker and receptacle. A 20 amp circuit must use a #12 AWG copper cable or a #10 AWG aluminum cable. Not a #14 AWG copper cable or a

After this error is correct I would next verify that the circuit is actually a dedicated circuit. Sounds to like it is not. Next, make sure the connections are tight. Loose connections create heat which creates fire.

Most microwaves that I have installed required a 20 amp circuit as does the counter top appliance circuits in the kitchen.

Using 20 amps on a #14 AWG copper cable is a FIRE waiting to happen.

I am aware of that. I know code, I built the house. It is a dedicated circuit. The Westinghouse microwave is 3 yrs old and works perfectly with the extension cord plugged into a wall outlet. The owners manual states that it needs a 15A circuit.

Wires on new outlet are wrapped on the screws, no push ins. Connections are very tight. Double checked that.
Still don't know why it trips a 20A breaker on the dedicated circuit but not on the other 15A circuit. Gremlins?

Please do not use the "Comment" section to return information. Please use the "Answer this question" block.

Sorry, but I do not know what level of experience and knowledge of code you have. And you did not let us know.

There is no logical reason why a microwave that requires a 15 amp circuit, trips the circuit. That said, if you verified the correct operation of the circuit, then the problem may be the microwave.

How did you verify the correct operation of the circuit? Did you use a Multi meter or a Amperage tester?

As you already know, a microwave is not to be run an extension cord.

I still come back to the circuit in question, especially since you say that the microwave works on a multi use 15 amp line.

Does it trip if you try say a toaster there?

The extension cord in question is a 10' 30A which I normally use for my 5HP compressor. I think it is quite adequate.
I plugged the microwave back into its dedicated circuit a bit ago and it is working properly so I put the 15A breaker back in. If it pops again, I'll try the toaster oven on that outlet.
This is just weird. I also suspected the unit but it functioned perfectly on the other outlet.
I guess I'll have to take a wait and see stance here. I'm telling you, there is a gremlin here :) If all else fails, I'll drop a #12 to the basement and put it on a new 20A circuit on that panel.
Thank you for the response. I'll keep you posted.

Since we are into the weird world, did you check to see that there are no high amperage cables adjacent to the microwave cable and inducing a signal into the cable?

Interesting. But the main line comes up from the basement and the MW circuit line runs up through the ceiling and over 20' to the unit which is an over-stove. It does cross over the mains inside the panel.

I don't believe in ghosts. But I may change my mind. Today, the element went out in the dishwasher. My wife just informed me that the dryer element burned out. And the water heater is tripping its internal breaker daily.
I'm moving...

Before calling Ghost Busters, I would be calling in a qualified service electrician to troubleshoot your system, to include the utility feed.

Could be a loose connection in the neutral, or something similar with the live feeds, to be causing the recent rash of damaged appliances and tripping breakers.

I think it was explained when you said "I built the house"

I would check all the incoming main line connections, since it is all over many different devices. It may be in the panel ,the meterbase or the actual feeder drops at the mast. Usually you have to call your utility provider to ck the mast and the meterbase since they don't allow you to break their tamper tag.Good luck.