When an air conditioner trips its breaker, the circuit has carried more current than it is built for, or it has seen a short. Usually the fault is in the outdoor unit: a weak capacitor, a compressor that struggles to start or has a shorted winding, a failing fan motor, a worn contactor, loose wiring, or a dirty coil that makes everything run hot. Less often the breaker is worn or the circuit was never sized for the unit. One reset is fine. Resetting again and again is how a small repair turns into a compressor replacement.
What the timing tells you
The breaker protects the wiring from overheating. When it trips is the first clue:
- Instantly, as the AC starts or as you push the breaker back on: a short, such as a failed motor or compressor winding or a wire with broken-down insulation.
- After running a while, often late on a hot afternoon: an overload. Dirty coils, a weak capacitor, a dragging fan motor or a tired compressor pull more current than normal until the breaker heats up and trips.
Most central systems have two breakers: one for the outdoor unit and one for the indoor furnace or air handler. Note which one tripped: it shows where to start.
Why the breaker trips
Failing capacitor. It gives the compressor and outdoor fan motor the boost they need to start. As it weakens, the motors take longer to start and draw more current while they try. One of the most frequent causes, and one of the simplest fixes.
Compressor hard start. An aging compressor can struggle to get moving, especially on a hot day. You may hear the unit groan or hum for a few seconds before the breaker goes. A new capacitor or a hard-start kit sometimes helps; a worn-out compressor does not recover.
Shorted or grounded compressor. A shorted winding trips the breaker instantly, every time. It usually means a new compressor, and on an older system it opens the repair-or-replace conversation.
Outdoor fan motor. Worn bearings make the motor drag and draw extra current. When the fan slows or stops, the coil cannot shed heat, and the compressor’s pressure and current climb too.
Contactor. This switch powers the compressor and fan when the thermostat calls for cooling. Pitted, burned contacts overheat and can chatter, giving the compressor a rapid string of starts.
Loose or damaged wiring. A loose connection at the breaker, disconnect or terminals creates heat. Sun-cracked insulation, corrosion or a wire chewed by rodents can short to the cabinet.
Undersized circuit or worn breaker. Sometimes the circuit was never sized for the unit, for example after a larger AC replaced a smaller one. The nameplate lists what the unit needs. A bigger breaker is never the fix: the breaker is there to protect the wire.
Dirty coils and high head pressure. The outdoor coil sheds the heat taken out of your house. Coated with dust, grass clippings or leaves, it cannot, so the pressure on the hot side (the head pressure) rises and the compressor draws more current. This is the classic “only trips on hot afternoons” cause.
Low refrigerant. A leak shows up first as weak cooling, but it also strains the compressor. Most home compressors are cooled by the refrigerant returning to them, so a low charge makes them run hot, which wears the windings over time. Adding refrigerant without fixing the leak only buys time, and refrigerant work is for an EPA-certified technician.
When the compressor trips but the breaker does not
If the outdoor fan keeps running while the compressor goes quiet, then restarts later, the compressor is usually overheating and its built-in overload is switching it off. The causes are the same: dirty coil, failing fan, weak capacitor, low charge. A compressor that trips often needs the same diagnosis as a tripping breaker.
Symptom, likely cause, what it takes
| What you notice | Likely cause | What it usually takes |
|---|---|---|
| Trips the instant the AC starts or you reset it | Short in the compressor, a motor or the wiring | Electrical testing; wiring repair, motor or compressor |
| Unit groans or hums a few seconds, then trips | Weak capacitor or hard-starting compressor | New capacitor, hard-start kit or compressor work |
| Trips after running a while on hot afternoons | Dirty outdoor coil, failing fan motor | Coil cleaning, fan motor or capacitor |
| Compressor hums, outdoor fan does not spin | Fan motor or its capacitor | Capacitor or fan motor replacement |
| Chattering outdoors, then a trip | Worn contactor or control fault | Contactor, control wiring check |
| Compressor stops, fan keeps running | Compressor overheating | Leak search, coil cleaning, capacitor test |
| Indoor breaker trips, not the outdoor one | Blower motor or its capacitor | Blower testing |
| Burning smell or scorch marks | Overheated wiring or terminals | Leave it off, call now |
What you can safely do
- Set the thermostat to Off, so the AC does not start the moment power returns.
- Wait about 30 minutes for a hot compressor to cool.
- Reset the breaker once. Push it firmly all the way to Off, then back to On. A tripped breaker often rests in the middle and can look like it is on.
- Check the filter. A clogged filter rarely explains a trip alone, but it strains the system.
- Clear the outdoor unit of leaves, weeds and stored items so air moves freely. Do not open any panel on the unit or the disconnect box.
- Turn cooling back on and stay close for a few minutes. Listen for humming without the fan turning.
Never hold or tape a breaker on, never swap in a larger one, and do not touch capacitors: they can hold a dangerous charge with the power off.
When to stop and call
- The breaker trips a second time, or instantly.
- You smell burning or hot plastic, or see scorch marks.
- The unit hums but the fan or compressor does not start. Switch the thermostat off: a motor that cannot start overheats quickly.
- Lights dim sharply each time the AC starts.
Leave the breaker off until a technician has looked. Repeated resets risk overheated wiring and fire, damage the compressor with every hard start, and let a small problem grow.
What the technician checks
The technician measures the compressor and fan current against the nameplate, tests the capacitor and the compressor windings, inspects the contactor and wiring, checks the coil and refrigerant pressures, and confirms the breaker and wire suit the unit. You get a flat price after the diagnosis, before any work. Book AC repair, or on a hot evening use our 24/7 emergency line.
A note for Hayward homes
Near the bay many summer days are mild, so a weak capacitor or dirty coil can hide for weeks, until a heat wave keeps the system running all afternoon and the breaker starts tripping. Units boxed in by fences and shrubs run hotter still. A spring maintenance visit cleans the coil and tests the capacitor and contactor first. If your system also turns on and off in short bursts, read about short cycling; new humming or buzzing is covered in strange HVAC noises.