A/C on shore power trips breaker after 30 minustes

One other thing. I have an extension cord going to a 20 amp wall socket that hooks up to my 30 amp cord using an adapter. It has never been a problem. Especially since the load is less than 20 amps. Still confuses me how my generator puts out the same AC 120 volts to the same circuits and the AC runs as long as needed. What could be the difference between shore power and generator power?