Compressor Trips the Breaker: Motor or Circuit?
A 3-ton single-phase scroll condensing unit, 208/230 V, nameplate RLA 16.7 A, LRA 96 A, MCA 21.7 A, MOP 35 A. It is fed by a 40 A breaker through what appears to be 10 AWG copper, roughly 60 ft from the panel. On a call the owner says the breaker "pops after about ten seconds" whenever the unit tries to start. With the contactor held in by the thermostat, line voltage at the contactor is 232 V; while the compressor attempts to start, voltage falls to 198 V and the breaker trips. Winding resistance C–R = 0.6 Ω, C–S = 1.1 Ω, R–S = 1.7 Ω; resistance to ground is greater than 100 MΩ. The dual run capacitor measures 44.5 µF on the compressor section (45 µF nominal) and 5.0 µF on the fan section.
1. Evaluate the electrical integrity of the compressor using the winding and insulation readings.
Read the worked answer
- Balance check: for a single-phase PSC/CSIR compressor, R–S should approximately equal C–R + C–S. Here 0.6 + 1.1 = 1.7 Ω, matching the measured R–S exactly. Windings are balanced.
- Insulation check: greater than 100 MΩ to ground indicates no ground fault in the motor (a reading in the low megohms or below would be suspect; a dead short reads near zero).
- Capacitor: 44.5 µF against 45 µF nominal is within the typical ±6% tolerance (about ±2.7 µF), so the run capacitor is not the obvious culprit, though it should still be checked under load with a proper meter and the terminals inspected for corrosion.
Conclusion so far: the compressor motor is electrically sound on a bench basis. That shifts suspicion away from a shorted or open winding and toward a locked rotor or a supply-side voltage problem.
2. The voltage drop during the start attempt is the key reading. Quantify what it implies and explain why a locked-rotor diagnosis alone does not explain it.
Read the worked answer
- Drop = 232 − 198 = 34 V while attempting to start.
- If the compressor were drawing near LRA (96 A), the implied circuit impedance is Z = V ÷ I = 34 ÷ 96 ≈ 0.35 Ω.
- 10 AWG copper is about 1.24 Ω per 1,000 ft at 75°C. A 60 ft run is 120 ft round trip: 0.12 × 1.24 ≈ 0.15 Ω, which would produce only about 14 V of drop at 96 A.
- So roughly 0.20 Ω of the circuit is unaccounted for - an order-of-magnitude-of-concern amount for a 60 ft branch circuit. That points to a high-resistance connection or an undersized/incorrect conductor: loose breaker lug, burned disconnect or contactor line-side terminal, corroded split-bolt or wire nut in a junction box, or a smaller gauge than assumed.
A locked rotor explains high current; it does not explain 34 V of drop on a 60 ft circuit. Both can be true at once, and the voltage problem will cause the locked-rotor condition or make it worse.
3. Outline your diagnostic and corrective steps in order, and state why installing a hard-start kit at this point would be a mistake.
Read the worked answer
- Verify the actual starting current with a clamp meter (peak/min-max) to see whether it approaches LRA; do not infer it from the breaker trip.
- Check the overcurrent protection against the nameplate: MOP is 35 A and the panel has a 40 A breaker - that is a code/listing mismatch that should be corrected regardless of the fault, and it also changes the trip curve you are diagnosing.
- Measure voltage at each point under load: panel lugs, disconnect line and load, contactor line and load, compressor terminals. The point where voltage drops sharply is the bad connection. Perform a voltage-drop test across each connection rather than relying on visual inspection.
- Confirm conductor size against the nameplate MCA and the actual length; replace or upsize if the 0.35 Ω circuit impedance cannot be explained by a single bad termination.
- Repair the connection, tighten/replace terminations, correct the breaker to the listed maximum, then re-check starting voltage.
- Only then evaluate the compressor - if it still locks out at correct voltage, check for a mechanical cause (seized bearings, flooded start, high head from a non-condensable or a fan failure) before condemning it.
A hard-start kit raises starting torque, but it masks a bad connection or low voltage, drives even more current through the marginal circuit, and will leave the customer with a repeat failure. Fix the supply first; add a start device only if the manufacturer calls for one on the replacement compressor.