Re: Main Breaker Tripping
I think the answer is simple enough. In both cases, you have a short circuit somewhere downstream (i.e., within the pump house, and on the 20 amp branch circuit). Now comes the hard part: finding it. On the 20 amp branch circuit, finding the fault point should be easier. On the sub-panel, you appear to have a "ghost problem," one that comes and goes as though by its own whim.
It should not be a surprise that an upstream breaker trips, instead of a downstream breaker, when there is a fault. Very few facilities have complete coordination of all of their breakers. The fault occurs, and the 20 amp breaker trips. You try to reset the 20 amp breaker, and this time the fault takes out the panel feed breaker. No surprise.
Charles E. Beck, P.E., Seattle
Comments based on 2014 NEC unless otherwise noted.