My faithful RCBS Pro-Melt has been unused for the past four years. The lighted switch failed after 42 years (date of manufacture was stamped inside the case) and a new exact replacement was installed so, YEA! Time to give it a test run. After about three minutes of heating the circuit breaker tripped. This was on a standard trip breaker. After the furnace cooled enough to handle it I took it to another area, still on the same load center but on a combination arc fault/ground fault breaker that I expected to be less tolerant of the mechanical thermostat but it didn't seem to be a problem. This time the breaker tripped after about 18 minutes. The furnace only draws 800 watts according to the data plate and there was very little else on the first circuit. There was nothing else on the second circuit. A 20 amp breaker should be good for 2,340 watts at 117VAC. I saw that Eaton makes breakers for the same load center that are intended for a large tungsten lamp loads which made me wonder if their standard breakers might not like heating loads. In my mind there's not much difference in a lead furnace and big incandescent lights.
The shop and electrical system is less than two years old. I know even new breakers can be weak but I've used this furnace at four other homes with no issues. When it was in storage it was kept in a climate controlled area, never humid/damp or hot. If there was a dead short I would have expected the breaker to trip instantly but that wasn't the case.
Ideas?