Submersible Well Pump Voltage Drop and Thermal Trip
A 1.5 hp, 230 V single-phase submersible well pump is 350 ft from the house panel. The branch circuit is 10 AWG copper UF cable, protected by a 20 A two-pole breaker, with a pressure switch at the wellhead. Nameplate current is 8.0 A. With the pump off, voltage is 242 V at the panel and 238 V at the pressure switch. With the pump running, voltage is 236 V at the panel and 198 V at the pressure switch; running current is 8.4 A. The pump delivers normal pressure for 20 to 30 minutes, then the internal thermal overload trips. It resets after cooling. The homeowner has already replaced the pressure switch and the control box capacitor. A contractor recommends replacing the 10 AWG cable with 8 AWG.
1. Calculate the expected voltage drop at 8.4 A and compare it with the measured drop. What does the difference indicate?
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For 10 AWG copper, use about 1.24 ohms per 1,000 ft at 75°C. The circuit length is 350 ft one way, so the round-trip conductor length is 700 ft. Resistance = 1.24 x 700 / 1,000 = 0.868 ohms. Expected drop at 8.4 A = 8.4 x 0.868 = 7.3 V, or about 3.1% of 236 V. Even using a lower resistance at 20°C would give only about 6 V.
The measured drop is 236 - 198 = 38 V. That implies an effective series resistance of 38 / 8.4 = 4.5 ohms, roughly five times the expected cable resistance. Normal voltage drop cannot explain this. The pump current is near nameplate, so the problem is most likely a high-resistance series connection or a damaged conductor: a corroded splice, a burned pressure-switch terminal, a damaged UF cable, or a poor connection at the pump. A failing pump winding would usually show abnormal current or a much shorter run time, so the voltage-drop evidence points first to the circuit.
2. Outline a step-by-step troubleshooting plan, including where to take measurements, to locate the problem without replacing the cable blindly.
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With the pump running, measure voltage simultaneously at the panel and at the pressure switch. Confirm the 38 V drop and measure each ungrounded conductor to ground as well as line-to-line; a bad splice may affect only one leg. If the drop occurs between the panel and the pressure switch, isolate the cable from the pump and check for a high-resistance splice or damaged insulation. Inspect and re-terminate the pressure-switch connections and any wellhead splices. De-energize, lock out, and megger the cable to ground; compare conductor resistance end to end. Check the pump winding resistance and insulation resistance if the pump can be isolated. Verify the control box and capacitor are correct for the motor, even though the capacitor was replaced. Measure pump current and compare it with the nameplate, and check flow and pressure to rule out a worn pump or stuck check valve. The goal is to locate the 4.5 ohm resistance before replacing cable.
3. The contractor proposes replacing the cable with 8 AWG. How would you respond, and what would you tell the homeowner about the likely cause?
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The proposed cable replacement is not justified by the voltage-drop calculation. The existing 10 AWG cable should drop only about 7 V at 8.4 A, which is roughly 3% and is acceptable for a 230 V pump circuit. Upsizing to 8 AWG would reduce the drop to about 4.6 V, a small improvement that cannot correct a 38 V drop. The measured drop indicates a localized high-resistance fault, such as a corroded splice, burned connection, or damaged cable, not normal conductor sizing.
I would tell the homeowner that replacing the cable might accidentally fix the problem if the damage is inside the cable, but it would be guesswork and could leave the real defect in place. The correct approach is to isolate the cable and pump, perform insulation-resistance and continuity tests, inspect all splices and terminations, and repair the high-resistance point. If the cable tests good and the connections are clean, then check the pump motor, control box, and run capacitor. Only after diagnosis should cable replacement be considered, and 8 AWG may be a reasonable upgrade if the cable is being replaced anyway, but it is not the primary fix.