AC or Furnace Keeps Tripping the Breaker

By Ray Bennett · Updated 2 October 2026 · 6 min read · General guidance, always check your own manual

AC or Furnace Keeps Tripping the Breaker

Short answer

A breaker does not get tired and it does not trip for no reason, so reset it once and no more. When it goes tells you most of what you need: straight away is a short, on start up is usually the capacitor, and twenty minutes in is heat. Three of the heat causes are free to fix. Never fit a bigger breaker, because the breaker is protecting the wire in your wall, not the machine.

A breaker is not a switch that gets tired with age. It trips because it measured something it did not like, and it is the only thing standing between a fault and the wiring buried in your walls.

So the rule is simple and it is the most useful thing on this page. Reset it once. If it goes again, stop, because the third push is you feeding a fault that has already been found.

Before you start

What to carry: a torch, a garden hose, and a screwdriver. Twenty minutes for the free checks.

Gas or electric? It changes what is on that circuit. A gas furnace draws very little, usually a single small breaker for the blower and the controls, so a gas furnace tripping a breaker is usually the blower motor or a wiring fault. An electric furnace or an air handler with heat strips pulls an enormous amount and sits on several large breakers, and so does the outdoor unit of an air conditioner or heat pump.

Find out which breaker it is. Outdoor unit, indoor unit, or both. That one fact rules out most of this page before you start.

Two hard lines. Never fit a larger breaker to stop the tripping. The breaker is sized to protect the cable running through your walls, not the machine on the end of it. A bigger one does not make the wire thicker, it just stops anything from noticing when that wire is overheating. There is a drawing of exactly this below.

A warm breaker, a buzzing panel, a burning smell or scorch marks. Stop there and call an electrician today. That is a connection arcing inside the panel, and it does not get better on its own.

When it goes tells you what it is

Push it back on once and watch the clock. The timing narrows it down faster than anything else you can do.

Check 1

Reset once, and time it

Turn the thermostat to Off first so nothing is calling for the system, then push the breaker fully off and fully on, and watch.

It trips the instant you push it on, with everything off at the thermostat. A dead short. Either the cable has been damaged, or a motor or compressor winding has failed to earth. Nothing on this page will fix that one, and it needs a meter rather than a guess.

It holds, then goes the moment the outdoor unit tries to start. Starting is the biggest current the system ever pulls, so anything that makes starting harder shows up here. A tired capacitor and a seized fan are the two common ones. The tell is a hum from outside with the fan standing still.

It holds, runs fine, then goes ten to thirty minutes later. Heat. Something is making the system work harder than it should, and the current creeps up until the breaker has had enough. This is the group with free fixes in it.

It goes at random, with the system not running. Then it may be the breaker itself, which does wear out, or something else sharing the circuit.

The free checks, in order

Check 2

Airflow outside, then airflow inside

The outdoor coil. Look at the fins all the way round the outdoor unit. Grass clippings, cottonwood seed, dust and leaves mat into them and the unit can no longer dump its heat, so the compressor runs hot and pulls more current every minute. Switch it off at the breaker, then rinse from the inside outwards with a gentle hose, never a pressure washer.

The outdoor fan. With it running, that fan should be turning steadily. A fan that is slow, stopped or stiff to spin by hand leaves the compressor cooking itself, and that is a common reason a breaker gives up after twenty minutes.

The filter. A blocked filter starves the indoor blower and makes everything downstream work harder. It also freezes coils, and ice on a coil makes the blower struggle too: why a coil ices up.

What else is on that circuit. An outdoor unit is usually on its own. A furnace or an air handler often is not, and a second thing plugged in on the same breaker can be the straw rather than the fault.

Where the panel lives. A breaker in a hot garage or in direct sun trips at a lower current than the same breaker in a cool basement. It is not the cause, but it is why some systems only do this in August.

Never the bigger breaker

The breaker is sized for the cable behind your plaster. Fitting a bigger one moves the weakest point from the panel into the wall.

This is worth being blunt about, because it is common advice and it is wrong.

The breaker exists to trip before the cable feeding the machine gets hot enough to matter. Put a 50 amp breaker on a circuit wired for 30 and the tripping does stop, exactly as promised. What has actually happened is that the wire is now the thing that fails first, inside a wall, where you will not see it.

The same goes for holding the handle, wedging it, or fitting a slow blow anything. If a circuit is genuinely undersized for the equipment, the cure is a properly sized cable, which is an electrician’s job and a permit in most places.

When it is a fuse or a GFCI rather than a breaker

Not everything that cuts the power is a breaker, and the other two mean different things.

Check 3Free to find

Work out which one actually went

A small blade fuse on the control board. Furnaces and air handlers carry a 3 or 5 amp fuse that protects the low voltage side. It blows when a thermostat wire shorts out, and the usual place is where that wire enters the outdoor unit and has rubbed through against the metal. Fitting another fuse without finding the rub is a five minute repair that fails again the same day: where that fuse is and what else it means.

A GFCI outlet. Some outdoor units, and most condensate pumps, are on one. A GFCI trips on a tiny leak to earth rather than on a large current, so damp alone can do it after rain, and a GFCI that trips only in wet weather is usually telling you about water rather than about the machine.

A button on the machine itself. That is a different thing again, with its own rules: where each reset button is and why it trips.

Questions people ask

How many times is it safe to reset it?

Once. A breaker that trips a second time has measured the same fault twice, and nothing has changed in between except that you have sent the current through it again.

It only trips on the hottest days.

That fits the heat group. The system works hardest then, and a coil that is only half blocked copes in June and does not in August. Start with rinsing the outdoor coil.

Can a dirty filter really trip a breaker?

On its own, rarely. What it does is push everything else closer to the edge, which is why it is worth ruling out before paying anybody to look.

The breaker feels warm to touch.

Stop using that circuit and get an electrician out. Warmth means resistance where there should be none, and that is a connection on its way to arcing.

My mini split trips the breaker.

Same three timings apply. Check the isolator box by the outdoor unit too, because a wet or corroded one causes this: the three places a mini split loses power.

Should I just replace the breaker myself?

Not unless you are confident around a live panel, because the wires coming in from the meter stay live with the main off. It is also the cheapest part of the job to pay somebody for.