New Pump Tripping Breaker In House

[Poconos]

The 1.5 HP pump sounds like extreme overkill for the water volume you have. You didn't state the rating of the old motor and I'm guessing the new one is just pulling too much current and tripping the breaker. Something to remember is that if you have an undersized extension cord you could be taking a large voltage drop and that could be part of the problem. One question, during the 15 seconds before the breaker trip, does the pump come up to speed and stay there until the trip? I'm asking because if it isn't coming up to speed it could be staying on the starting winding that pulls a great deal of current, thus tripping the breaker. Too much of a voltage drop can cause this problem. Grounding should be through the extension cord but shouldn't cause a trip. You already are aware you have to wire things right so we won't go into that issue.
Al