In addition.......
These automotive type of circuit breakers have different "trip values". Since the entire system was designed and setup to work on 20 Amps, then one would assume, that who ever installed that CB, installed the correct size. So, the possibility exists, that "maybe" a circuit breaker was installed that has a lesser valve then 20A.
There should be markings on the device, either inked or stamped, that would identify it. See what these markings indicate and post those findings here.