Quick answer
An AC trips the breaker when it pulls more current than the circuit allows. The usual causes are a weak run capacitor making the compressor strain on start-up, a compressor winding shorted to ground, a seized condenser fan motor, a dirty outdoor coil driving pressures up, or a loose, burnt wire at the contactor. Reset it once. If it trips again, leave it off and book a diagnostic.
If your AC is tripping the breaker, the breaker is almost certainly doing its job: something in the outdoor unit is drawing far more current than the nameplate allows. Every condenser lists a Minimum Circuit Ampacity and a Maximum Overcurrent Protection value on its data plate, and the breaker in your panel is matched to those numbers. When the compressor or fan motor pulls beyond them, the breaker opens to keep the wiring from overheating.
The five faults behind most AC breaker trips
- Failing run capacitor — the most common cause. A capacitor that has drifted out of tolerance cannot give the compressor the phase shift it needs to start, so the motor strains at locked-rotor current until the breaker opens. See what a capacitor costs to replace.
- Grounded or shorted compressor winding — the breaker trips the instant the contactor pulls in, often with a hum. This is confirmed with an insulation test, and it usually means a new compressor or a new unit.
- Seized or slow condenser fan motor — with no air across the outdoor coil, head pressure climbs, the compressor overheats and current spikes.
- Clogged outdoor coil — dryer lint, grass clippings and poplar fluff do the same thing a dead fan does, just more gradually. The breaker tends to trip 10–20 minutes into a cycle on the hottest afternoons.
- Loose or burnt wiring — a pitted contactor, a loose lug at the disconnect or a chafed wire on the compressor terminals can arc to the cabinet.
How the timing of the trip narrows it down
When the breaker trips tells a technician almost as much as the fact that it trips.
- Instantly, every time — suspect a shorted compressor, a shorted fan motor or a direct wiring fault.
- A few seconds after start-up — suspect the run capacitor or a hard-starting compressor.
- After 10–30 minutes of running on a hot, humid afternoon — suspect a dirty coil, a weak fan motor or an overcharged system pushing head pressure too high.
- Only during a brownout or when the whole street is at peak load — low voltage makes motors draw more current; the fault may clear when the grid recovers, but a weak capacitor is often the reason the unit could not ride it out.
Humid July afternoons off Lake Ontario are the worst case: high outdoor temperature and high humidity both raise head pressure, so a marginal system that ran fine in June starts tripping in the first heat wave. A newer home in Alton Village or the Orchard with the condenser tucked into a narrow side yard beside the dryer vent will often have a coil felted with lint after a few years, which is a two-minute discovery and a cheap fix.
Sometimes it is the electrical side, not the AC
A breaker that has tripped many times can weaken and start opening below its rating; that is a panel job for a licensed electrical contractor, not an HVAC repair. Homes built in Ontario from the 1960s through the 1980s sometimes still have a Federal Pioneer Stab-Lok panel, which has a documented history of breakers that either nuisance-trip or fail to trip at all, and some insurers ask about them. Older installs may also use a fused pull-out disconnect at the wall: a blown fuse there looks like a dead unit rather than a tripped breaker, so check both.
Two things never to do: do not swap in a larger breaker to stop the trips, and do not keep resetting it. Each reset sends locked-rotor current through a compressor that is already struggling, and a compressor that could have been saved with a $150–$350 capacitor becomes a much larger repair.
What the repair usually costs
The diagnostic visit is $90–$150 and is credited to the repair when you proceed. From there, most breaker-trip calls end in one of a handful of fixes: a capacitor or contactor at $150–$350, a condenser fan motor at $300–$700, or a coil cleaning and wiring repair inside the same range. A grounded compressor is the expensive outcome, and on a unit past 10 years it usually makes more sense to replace than to repair. Our AC repair cost guide lists the full range.
Quick answers to related questions
- Can I reset the breaker myself? Once. Switch it fully off, then on. If it trips again, leave it off and call — repeated resets damage the compressor.
- Why does the breaker trip but the indoor fan keeps running? The furnace blower is on a separate 120-volt circuit. The 240-volt breaker for the condenser is the one opening.
- Is a tripping AC breaker a fire risk? A breaker doing its job is not, but scorched wiring or a melted contactor is. Leave the unit off until it has been inspected.
If the breaker keeps opening in the middle of a heat wave, ZK Mechanical's line is answered 24/7 and we do not add evening or weekend surcharges for emergency AC service. For homes in Oakville, Burlington and Hamilton, request a visit online or call (647) 801-1252 with the model number from the condenser's data plate.