Three-phase motor tripping on current unbalance
A 5 HP, 230 V, three-phase submersible is set at 240 ft in a 6-inch well. The drop cable is 240 ft of 12/3 with ground copper in the well; the run from the panel to the wellhead is 60 ft of 10/3. Nameplate reads 14.0 A at 230 V, service factor 1.15. Control is a magnetic across-the-line starter with a 30 A time-delay breaker, which the previous installer set at 250% of full-load amps as he read it. The owner reports the breaker trips on hot afternoons. With a clamp meter you read 14.1, 13.9, and 11.6 A on the three legs, and line-to-line voltage at the starter reads 241, 240, and 233 V while running. The pump has run four seasons and the well is otherwise fine.
1. Calculate the current and voltage unbalance and decide what to do with the pump right now.
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Average current is (14.1 + 13.9 + 11.6) / 3 = 13.2 A. The largest deviation from average is 1.6 A, so current unbalance is 1.6 / 13.2 = 12.1 percent. Average voltage is 238 V, and the largest deviation is 5 V, so voltage unbalance is 5 / 238 = 2.1 percent.
Both numbers fail. Submersible motors are generally limited to about 5 percent current unbalance, and voltage unbalance should stay near 1 percent; a rough industry rule is that each 1 percent of voltage unbalance can produce 6 to 10 percent of current unbalance. So 2.1 percent voltage unbalance is capable of explaining the 12 percent you measured, and the motor is running hot. Shut the pump down rather than let it keep tripping and resetting. Then investigate: check all terminations and splices, megger the drop cable and motor, compare winding resistances, and read voltage at the service entrance with other large loads off to see whether the unbalance originates at the utility or inside the customer's wiring. Do not simply reset the breaker.
2. Check the drop cable for voltage drop at nameplate current. Is 12 AWG acceptable over 240 ft, and what would you install?
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For three-phase, voltage drop is 1.732 x I x R x length. Using 12 AWG copper at about 1.93 ohms per 1,000 ft and 14.0 A over 240 ft: 1.732 x 14.0 x 1.93 x 0.240 = about 11.2 V, or 4.7 percent of 240 V. That exceeds the usual 3 percent target for a motor branch circuit and is added to whatever the 60 ft feeder loses.
With 10 AWG at about 1.21 ohms per 1,000 ft, the drop is about 7.0 V, or 2.9 percent, which is marginal once feeder loss is included. With 8 AWG at about 0.764 ohms per 1,000 ft, the drop is about 4.4 V, or roughly 1.9 percent, and stays near 2 percent even at the 1.15 service factor current of 16.1 A. Recommend 8 AWG submersible-rated drop cable and check the feeder size at the same time. Conductor sizing is not just about ampacity: on a deep-set pump the voltage drop is a real heat and starting-torque issue.
3. Evaluate the 30 A breaker and state the correct short-circuit protection and overload settings.
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The previous installer did not apply the 250 percent figure correctly. For a three-phase motor branch circuit, an inverse-time breaker may be sized up to 250 percent of full-load current: 2.50 x 14.0 = 35 A. The installed 30 A breaker is only 214 percent, which is why it nuisance-trips on hot afternoons when the motor draws slightly more and the breaker is inside a warm enclosure. A 35 A breaker is the next standard size and is permitted.
Keep in mind the division of labor in motor protection: the breaker provides short-circuit and ground-fault protection, while running overload protection is provided separately in the starter. Set the overload relay per the manufacturer's table at nameplate current, adjusted for the 1.15 service factor if the table allows. Confirm terminations are rated for the breaker's ampacity and that the feeder conductors are protected as motor branch-circuit conductors, then recheck the unbalance issue, because a larger breaker will mask the symptom, not fix the cause.