New Pump Trips Breaker on Start-Up
A driller installs a 2 hp, 230 V single-phase submersible pump in a 200 ft well with a 60 ft static level and a pumping level of 120 ft at 15 gpm. The control box, pressure switch, and 40-gallon tank are new. On start-up the pump runs for a few seconds, then the 30 A breaker trips. The driller checks that the pump turns freely by hand at the top of the drop pipe before setting it, and the wiring to the control box is correct. The motor winding resistance reads normal. The driller has a clamp meter, a megohmmeter, and spare start capacitors on the truck.
1. List the likely causes of the breaker trip and the order in which to check them.
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The trip occurs after a few seconds of running, not instantly, which points to a current problem under load rather than a dead short. First, clamp the running current on each leg and compare to the motor's nameplate full-load amps; a high reading suggests the pump is working against too much head or is mechanically binding. Second, check the start capacitor and control box - a failed start capacitor can leave the motor drawing locked-rotor current too long. Third, verify the voltage at the motor during run; low voltage raises current. Fourth, use the megohmmeter to check insulation resistance of the motor leads, since a wet splice or damaged cable can leak to ground and trip after warm-up. Check in that order: current, start components, voltage, then insulation.
2. The clamp meter shows current near nameplate but the breaker still trips after about 10 seconds. What does that suggest, and what should the driller do?
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Current near nameplate rules out a locked rotor or a badly overloaded pump, so the breaker itself or the circuit is suspect. A 30 A breaker on a 2 hp, 230 V motor is oversized for the motor's typical full-load current, so nuisance tripping from an undersized or aging breaker is possible, but the driller should first confirm the breaker is the correct type and rating for the circuit and that connections are tight. A loose lug or undersized wire heats up and trips after seconds. The driller should check wire gauge against the run length, tighten all terminations, and measure voltage drop at the motor. If the circuit is sound, replace the breaker with the correctly rated one and re-test, watching current for the first minute.