You should run 3/4; Schedule 40 PVC with 2#8 copper xhhw and 1#12 copper ground. You cannot run any power cable or extension cords in conduit. You need to use individual conductors. It would be cheaper to use #6AL. Not a big fan of aluninum. Your first pump probably burned up from low voltage. You should also put it on a GFI circuit breaker. If you plan to plug it in or use a WP GFI receptacle.R&R is spot on with his voltage drop calculation.
