Compressor Trips on Breaker After Ten Minutes
A 3-ton R-410A condensing unit, 208/230 V single phase, nameplate RLA 16.5 A, LRA 88 A, run capacitor 45 µF ±6%, no start components. Complaint: it cools for about 10 minutes, then trips the 40 A breaker; after 20 minutes it restarts and repeats. Line voltage at the disconnect is 233 V, 232 V at the contactor line side, and 204 V at the compressor terminals while running. Amp draw is 19.6 A after two minutes and climbs to 24 A before the trip. The run capacitor measures 26 µF after discharging. Winding resistance is 0.6 Ω common-to-run, 1.0 Ω common-to-start, 1.7 Ω run-to-start, and each terminal to ground reads over 200 MΩ at 500 V. The contactor contacts are pitted and discolored, and one push-on terminal at the compressor is loose and heat-discolored. Outdoor ambient 92°F, condenser clean, subcooling 9°F before the trip.
1. Evaluate the electrical readings. Which values are abnormal, what do they indicate, and how do the capacitor and voltage drop explain the rising amp draw?
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Normal or acceptable: winding resistances are internally consistent (0.6 + 1.0 ≈ 1.7 Ω), and over 200 MΩ to ground at 500 V means no grounded or moisture-contaminated winding. Insulation is not the problem.
Abnormal: amp draw at 19.6 A is about 119% of the 16.5 A RLA and climbs to roughly 145% before the trip - the motor is working far harder than rated. Voltage at the compressor terminals is 204 V versus 232 V at the contactor under load, a 28 V drop, about 12% of system voltage. Branch-circuit voltage drop should be under about 3%; this points to series resistance from the pitted contacts and the loose, heat-discolored terminal. Lower voltage means higher current for the same load, and the bad connection also generates its own heat.
The run capacitor at 26 µF is only about 58% of its 45 µF rating, far outside ±6%. A weak run capacitor reduces running torque, so the motor draws more current and runs hotter. Together, low voltage, high resistance, and low capacitance push current up until the internal overload or breaker opens; the 20-minute recovery is the overload cooling and resetting.
2. What do you repair or replace, in what order, and what must you verify before clearing the call?
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Lock out and tag the disconnect, then verify de-energized. Replace the run capacitor with the exact rated microfarad and voltage (or higher voltage), and check it after installation. Replace the contactor and the heat-damaged terminal and any discolored wire; cut back to clean copper and use the correct crimp terminal, not a squeezed push-on. Torque all connections.
Restore power and verify: line voltage at the compressor terminals within a few volts of the contactor line side while running; amp draw at or below RLA; capacitor value checked under load if your meter supports it; superheat and subcooling stable; no overheating at terminals (check with a clamp meter or thermal probe after 20–30 minutes). Confirm the unit runs a full cycle without tripping and that the breaker is the correct size for the nameplate maximum overcurrent protection. Only then clear the call and document before-and-after readings.
3. What evidence would justify condemning the compressor, and why don't the present readings justify it?
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Condemnation requires evidence the compressor itself has failed: an open winding, a shorted winding (resistance far off the expected relationship), low insulation resistance to ground, a locked rotor with correct voltage and a good capacitor, internal mechanical noise, or a compressor that bypasses internally (low head, high suction, no capacity). A weak internal overload is possible but is diagnosed only after the electrical supply is corrected and the compressor still trips.
Here, the windings are balanced, insulation is excellent, and the compressor starts and runs - it simply draws excessive current. That current is fully explained by 12% voltage drop at the terminals plus a run capacitor at 58% of rating. Replacing the compressor before fixing the contactor, terminal, and capacitor would put a new compressor on the same bad circuit and likely destroy it. The disciplined sequence is to correct the electrical defects, re-measure, and only then consider the compressor.