@mikey
I assume a 230V pump?
That's what ours is; on a 20 amp breaker, so must be 12ga wire. Nominal 4600 watts at the box. Dunno how much voltage drop by the time it gets down the well, but we've never had a problem in the 4 yrs we've been here.
@mikey Since he replaced 12-2 with 10-2, he had to replace the 20 amp breaker with a 30 amp breaker. Seems like overkill to me, but I obviously don't have the whole picture...