Unless you know what the name plate on the motor says, you really gotta go with what's in there. Any idea what gauge wires are in the ground, and from the well house down to the pump?
Better than my mom's house when it caught fire a few years ago....old cartridge fuses, the basement lighting circuit (8 fixtures) was on a 100 amp slow-blow motor starting fuse. 
Burned the insulation off the wire all the way from the bad fixture thru the switch and back to the panel, inside the ceiling and wall, until the wire itself finally melted. Fuse never did blow...it was happy as a clam carrying that load. Like a giant toaster.
