What you'll see

The outdoor unit keeps tripping its breaker. The most important clue is when it trips, and you get that from the customer and from a careful test: instantly the moment the breaker is reset (a dead short), the moment the compressor tries to start (locked rotor / start fault), or after running for a while (an overamp/overheat condition or a tired breaker). Each timing points at a different failure. Don't just keep resetting it — a breaker that trips is reporting a real fault, and repeatedly resetting into a hard short or grounded compressor is dangerous.

Treat the breaker as a witness, not a nuisance.

Walk it in order

  1. Get the timing. Ask the customer and then verify safely. Three buckets: trips instantly on reset (short to ground / dead short), trips on start attempt (locked rotor, shorted start components), or trips after minutes/hours of running (overamp, overheating, weak breaker, high head). The timing is most of the diagnosis.
  1. Power off, inspect, and ohm before re-energizing. With the disconnect pulled, open the unit. Look for burnt wires, melted insulation, charred contactor, rodent damage, water intrusion, and signs of a compressor terminal blowout. Smell for a burnt-winding odor.
  1. Trips instantly → look for a ground or dead short. With power off, check the compressor terminals to ground (chassis) for continuity — a grounded compressor (winding shorted to the shell) trips instantly and is a replace. Also check the contactor, wiring, and the fan motor for shorts to ground. A pinched line-voltage wire or a flooded contactor can dead-short too. A megohmmeter (insulation tester) catches a winding that's leaking to ground before it becomes a dead short.
  1. Trips on start → locked rotor or start fault. If it tries to start, hums, and trips, the compressor may be drawing locked-rotor amps and never spinning (mechanically seized, or a dead run cap / failed hard-start so it can't get over the hump). Test the capacitor, check inrush with a clamp meter, and decide between a cap/hard-start rescue and a genuinely locked compressor (see the humming-condenser article).
  1. Trips after running → overamp or overheat. Clamp the amps while it runs (if it'll stay on long enough). A compressor pulling well over RLA, or a fan motor overamping, heats the wiring and trips the breaker on overload. Causes: high head pressure (dirty condenser, dead/slow fan, overcharge, recirculation), low line voltage forcing high current, a weakening compressor, or a failing motor. Fix the root cause of the high amps.
  1. Rule out the breaker and connections. A weak, oversized-for-its-cycles, or heat-fatigued breaker can trip below its rating, and a loose lug at the breaker or disconnect overheats and trips. Check torque and temperature at the connections. But never upsize a breaker to stop tripping — match it to the equipment's nameplate (MCA/MOCP); a bigger breaker hides a fault and risks a fire.
  1. Check wire condition and rodent damage. Chewed insulation or a wire rubbed through against sheet metal causes intermittent shorts that trip on vibration. Inspect the full whip and internal harness.

What "normal" looks like

  • Compressor windings to ground: open (no continuity); a megohmmeter reads high insulation resistance (commonly well into the megohms — a low/declining reading is a failing winding).
  • Running amps: at or below nameplate RLA; brief LRA spike at start that drops within a second or two.
  • Breaker / MOCP: sized to the unit's nameplate maximum overcurrent protection — never larger to mask trips.
  • Connections: tight, correct torque, no heat discoloration at lugs.
  • Capacitor: within ~±6% of printed µF.

Common faults & what they mean

  • Grounded compressor: trips instantly, winding shorted to shell, megger confirms. Compressor replacement.
  • Dead short in wiring / contactor / fan: instant trip, often visible burn damage or water intrusion. Repair the short.
  • Locked rotor / failed start: trips on start attempt, hums. Test cap/hard-start; confirm locked vs rescuable.
  • Overamping from high head: trips after running. Dirty condenser, dead fan, overcharge, recirculation. Fix the head pressure.
  • Low line voltage: high current, overheating, after-run trips. Check supply voltage under load.
  • Weak/fatigued breaker or loose lug: trips below rating or from a hot connection. Replace the breaker / retorque — don't upsize.
  • Rodent / chafed wiring: intermittent shorts, trips on vibration. Inspect and repair the harness.

Tech tips & gotchas

  • Note when it trips before you do anything else. Instant vs on-start vs after-running splits this call three ways. The timing is the diagnosis.
  • Never upsize the breaker to stop nuisance trips. The breaker protects the wiring. A bigger breaker on a faulting unit is a fire waiting to happen — match the nameplate MOCP.
  • Megger the compressor before condemning it on an instant trip. A reading that's dropping toward ground predicts a grounded compressor; a hard zero confirms it.
  • Don't keep resetting into a dead short. Repeated resets into a grounded compressor or shorted wiring can arc and start a fire. Diagnose with power off first.
  • High head is a sneaky breaker-tripper. A dirty condenser or dead fan raises amps until the overload (or breaker) gives up. Always check the condenser and fan on after-running trips.
  • Check connections for heat. A loose, discolored lug at the disconnect trips a breaker and isn't the breaker's fault. Retorque to spec.

Safety / code notes

  • Lock out and verify zero voltage before opening the unit; treat a tripping circuit as faulted and dangerous.
  • Breaker/fuse sizing must follow the unit's nameplate MCA (minimum circuit ampacity) and MOCP (maximum overcurrent protection) per NEC §440 — never exceed MOCP to stop trips.
  • The outdoor disconnect must be present and within sight of the unit per NEC §440.
  • Discharge capacitors before handling, and recover refrigerant per EPA 608 (40 CFR Part 82, Subpart F) if a repair opens the sealed system.