AC Compressor Trips Breaker

Common Symptoms

• Breaker trips immediately when the AC system calls for cooling
• Breaker trips 5–15 minutes into a cooling cycle
• Outdoor unit hums loudly but won't start — then breaker trips
• AC worked fine until temperatures exceeded 90°F — now trips breaker
• Condenser fan spins but compressor hums and stalls
• Breaker is warm or hot in the electrical panel
• Hard reset works once but breaker trips again on next cooling cycle

Most Likely Causes

1. 1

   Failed Run Capacitor (#1 Cause — Try This First)

   The dual-run capacitor provides the phase-shifted current needed to start and run both the compressor and condenser fan. When the capacitor fails, the compressor draws locked-rotor amperage (LRA) at startup — typically 5–8 times normal running current — which trips the breaker within seconds. Capacitor failure is the #1 cause of compressor breaker trips and costs $15–40 to repair. Always test and replace the capacitor first before any other diagnosis.

2. 2

   Locked Rotor (Mechanically Seized or Shorted Compressor Windings)

   If the compressor motor is mechanically seized or has developed shorted windings, it draws locked-rotor current continuously and trips the breaker immediately every time. A hard-start kit can sometimes provide enough torque to free a mildly seized compressor for a season, but a locked rotor due to shorted windings means compressor replacement or a new system.

3. 3

   Refrigerant Overcharge

   An overcharged refrigerant system creates abnormally high suction and discharge pressures. The compressor must work against excessive pressure on startup, drawing elevated current that can trip the breaker — especially on hot days when pressures are already elevated. Refrigerant overcharge requires a licensed HVAC technician with gauge manifold to diagnose and correct. Never attempt to add or remove refrigerant yourself.

4. 4

   Dirty or Blocked Condenser Coils

   When the condenser coils are packed with dirt, grass clippings, cottonwood seeds, or debris, the outdoor unit cannot reject heat. Head pressure rises dramatically, the compressor works harder, current draw increases, and the breaker trips — usually after 5–15 minutes of operation rather than immediately. Cleaning the condenser coils is free and should always be done before more expensive diagnostics.

5. 5

   Undersized or Aged Breaker

   A breaker that's degraded or was undersized at installation may trip at lower-than-rated current, especially on the hottest days when the compressor's startup current peaks. Verify the breaker's amperage matches the unit's nameplate Maximum Overcurrent Protection (MOCP) rating. An electrician can replace a degraded breaker for $50–100.

6. 6

   Burned or Loose Wiring at Contactor or Compressor Terminals

   High-resistance connections at the contactor, capacitor terminals, or compressor terminal block generate heat under load, eventually causing the wire insulation to fail or the breaker to trip from elevated impedance. Inspect all connections for discoloration, melted insulation, or loose spade connectors.

Quick DIY Checks

1. 1SAFETY CRITICAL — Capacitor discharge before any internal work: After turning off the outdoor unit disconnect and the circuit breaker, the run capacitor inside the unit RETAINS A DANGEROUS CHARGE — typically 370–440V DC — for 5–30 minutes. Never touch capacitor terminals without discharging first. Method: hold a 20,000Ω (20kΩ) resistor with insulated probes (or a commercially available capacitor discharge tool) across each terminal pair — HERM-to-C, then FAN-to-C — for 5 seconds each. Do NOT short terminals with a screwdriver. Only after discharging is it safe to remove the capacitor.
2. 2Record exactly when the breaker trips: Immediately on startup (within 2 seconds) → locked rotor or failed capacitor. After 5–15 minutes → thermal overload from dirty condenser, refrigerant overcharge, or compressor approaching end of life. Only on very hot days → marginal system that's overwhelmed, aging breaker, or refrigerant issue. This timing is crucial diagnostic information.
3. 3Clean the condenser coils — do this first and for free: Turn off the outdoor disconnect. Use a garden hose to rinse the condenser coils from the inside out (spray through the coil fins from inside the unit outward to push debris out). Straighten any bent fins with a fin comb. Remove debris from around the unit within 2 feet. Restore power and test — if the breaker no longer trips after cleaning, dirty coils were the cause.

1. 4Inspect and test the run capacitor: Turn off the outdoor disconnect and circuit breaker. Wait 15 minutes. Open the condenser access panel (2–4 screws on the side). Locate the capacitor — a cylindrical metal can. Discharge as described in Step 1. Visually inspect: a bulging top dome or oil residue indicates failure. Test with a multimeter in capacitance (µF) mode — probe HERM-to-C for the compressor winding value and FAN-to-C for the fan value. A reading more than 6% below the stamped µF rating means the capacitor has failed. Replace with an exact µF and voltage match.
2. 5Install a hard-start kit if the capacitor is good or newly replaced: If the compressor hums and trips the breaker even with a good capacitor, install a Supco SPP6E hard-start kit ($20–35). Wire it in parallel with the capacitor — one lead to HERM, one to C. The hard-start kit provides a large extra burst of starting current for 500–800ms, dramatically reducing locked-rotor current draw. This can allow a marginally seized compressor to start and is an excellent low-cost first attempt before condemning the compressor.
3. 6Check contactor and wiring connections: With power off and capacitor discharged, inspect the contactor (the relay that switches 240V to the compressor). Look through the small inspection window — contacts should appear clean and silver, not pitted or blackened. Check all wire terminals at the capacitor, contactor, and compressor terminal block for burn marks, melted insulation, or loose connections. A bad contactor costs $12–25 to replace.
4. 7Test compressor motor windings (Advanced — requires multimeter in resistance mode): Locate the compressor terminal block (three terminals: C = common, R = run, S = start). With power off and capacitor discharged, disconnect the wires. Set multimeter to resistance mode. Measure C-to-R, C-to-S, and R-to-S. The C-to-R reading plus the C-to-S reading should approximately equal the R-to-S reading — this confirms intact windings. Also test each terminal to the bare metal of the compressor shell (ground) — any reading other than OL (open/infinity) means the windings are shorted to ground and the compressor has failed.
5. 8Call a licensed HVAC technician if: compressor windings test shorted, hard-start kit doesn't help, or you suspect refrigerant overcharge (requires licensed technician with gauges). Present your findings — breaker trip timing, capacitor test results, and winding test results — to get an accurate repair quote rather than a replacement quote.

Repair vs Replace

Recommended Tools & Parts

• Dual Run Capacitor (matched to unit's µF and voltage rating)

  Most common first repair for a compressor tripping the breaker. Read the µF and voltage rating from the label on the existing capacitor. Replace with an exact µF match; voltage rating can be equal or higher.

  $12–$30

  Buy on Amazon →

• Hard-Start Kit (Supco SPP6E or equivalent)

  Adds a start capacitor and PTC relay to reduce compressor locked-rotor inrush current. Wires in parallel with the existing capacitor. Effective on aging compressors that hum and trip the breaker even with a good capacitor.

  $20–$40

  Buy on Amazon →

• Capacitor Discharge Tool

  Safety tool with built-in 20kΩ resistor for safely discharging capacitors inside HVAC units. Mandatory for safe work inside the outdoor condensing unit.

  $10–$20

  Buy on Amazon →

• HVAC Contactor (2-pole, 30A or 40A)

  Replacement contactor for the outdoor condensing unit. Pitted or welded contacts cause voltage drops that can trip the breaker under load. Match the amperage rating and coil voltage to your unit.

  $12–$30

  Buy on Amazon →

• Digital Multimeter with Capacitance Mode

  Required for testing run capacitor µF values, compressor winding resistance, and voltage at the contactor. Essential for all HVAC diagnosis.

  $25–$50

  Buy on Amazon →

• Non-Contact Voltage Tester

  Verify power is off before opening the outdoor unit. An essential safety tool for any electrical work.

  $15–$25

  Buy on Amazon →

Links are Amazon affiliate links (tag: fixitfastai-20). Prices are estimates.

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