Pump Electric Advice

[BigTallGuy]

Since your original circuit has a 15 amp breaker, I can only assume that this circuit is using 14 gauge wire, but you didn't mention how far this circuit run is from the breaker panel to the outlet where the extension cord is plugged in, or if there are other outlets on this same circuit. In all likelihood, you could have a slight amount of voltage drop before you ever get to the extension cord. Like Al said your bigger motor may require a bigger circuit. You might want to consider installing a complete new circuit with the proper rating for your new motor.