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For the past couple of weeks I have been losing power occasionally to half my house. My central AC goes out, half my kitchen, living room and my kids' rooms. There are no tripped breakers though.

I have noticed that when the AC finally turns back on, so does everything else. It will be that for a while, then all of a sudden everything goes off again. Once I get the air conditioning running, everything powers up again. My house is only 13 years old and have never had this problem.

What is the likely cause of the issue, and what can I do to fix it?

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    Breakers aren't universal detector-all's. They only detect overcurrent (too much power being used, e.g.by a faulty appliance). – Harper Jan 2 at 16:50
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    Is there an outside main breaker near the meter? Are the circuits that go dead all on one side of the electric panel? – Kris Jan 2 at 16:52
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    Please turn off the breakers to your air conditioner, electric oven, clothes dryer, and electric heating if any of that is 240V; you can tell because the breakers are double-wide. This condition is dangerous for those appliances. Also, use extreme caution if you are inspecting the location where the power comes into your breaker box; something is very wrong between there and the pole. Hopefully the fault is in the transformer, but you don't know, and until you know, treat every system as though its safety systems are in an unknown state, because they are. – Eric Lippert Jan 2 at 18:48
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    Also, minor usage issue: it is confusing to use "AC" to mean "air conditioning" when you are asking a question about alternating current. :-) – Eric Lippert Jan 2 at 18:50
  • Some crazy answers for a 1 or 2 possible problem. First I would call the power company and ask for an open neutral test because this could be the cause the 2nd would be a bad hot leg but that is less likely unless you have an old rule of 6 service panel with 1 failing breaker. – Ed Beal Jan 3 at 5:06
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If you're in the US, my guess is that you're losing one leg of your incoming power somehow. Power comes into your home as split phase 240 volts - there are two 120V "legs" that are 180 degrees out of phase. Typically one leg goes to each side of the breaker panel. I bet if you trace the circuits that are down, they all go to the same half of the panel. When the air conditioning, which uses both legs, turns on, it bridges the gap and backfeeds power to the other half of your house.

Now, how you're losing a leg of your power is another question entirely. This is likely a utility problem and you should call them ASAP. The utility will check things out and if it is on their side of things then they will fix it for free. If they determine that the problem is in your panel, they will let you know and then you need to call an electrician for a repair. This is an unsafe condition and can cause all sorts of problems, so get it checked out ASAP.

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    @NickT That doesn't make it safe, that only eliminates the possibility of everything that needs that leg running through the A/C. The leg is still broken, and now half your house doesn't work. You still have the problem of what caused the failure in the first place, and depending on what it is, that could be a serious fire- or electrical-hazard, just waiting to happen. – Der Kommissar Jan 2 at 18:48
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    @202_accepted: That is exactly right. The takeaway here should be we have no evidence that anything works correctly right now and that includes safety systems. The attitude that a system operating outside of its design parameters for unknown reasons can be made safe without understanding why it is operating incorrectly is how we lost two space shuttles, and the people running NASA are not idiots. Treat broken systems as broken until they are known to be fixed. – Eric Lippert Jan 2 at 18:54
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    @EricLippert On top of that, as it sounds, half the house is currently running through the A/C, which means that the A/C is doing a whole lot of work outside it's safety-parameters, and that the breaker running the A/C is also potentially faulted. Best case: the line is failed from meter -> house, worst case: well...there's a whole lot in the worst-case. – Der Kommissar Jan 2 at 18:58
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    I had precisely this problem. The utility company first had me shut down that half of my house and all 240 volt breakers. They were going to schedule the work for first thing next morning (since it was dark and after hours). Then, on further checking, they discovered that the problem was in the service neutral as well, shut the service off entirely, and arranged to have it fixed within minutes. It suddenly became much more urgent as they checked the entire line. – David Schwartz Jan 2 at 23:55
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    ASAP is not strong enough. Call the utility IMMEDIATELY. – J... Jan 3 at 17:44
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You are losing a hot from the pole

In North American {or multiphase European} service, When you lose all your 240V* loads and about half of your 120V** circuits, that means you have lost a leg of power. Here is how most North American panels are laid out, if your lost circuits fit the pattern, that confirms it.

That problem may be around the main breaker in your service panel. It may also be inside your meter pan. However it is most likely out at the electric pole or somewhere in between pole and meter. That is the power company's bailiwick and they fix that for free as part of the service.

What's happening is when the 240V air conditioner cycles on, it is unable to run because one of the poles is disconnected. However, a motor trying to start is quite similar to a dead short, and this has the effect of shorting the two poles of power inside your panel. For instance if pole L1 lost power, all the breakers on broken L1 are now getting L2’s power via the air conditioner. This is no good thing!


You can get this diagnosed for free by calling the power company and reporting an outage. They will also treat it as an urgent matter.

Turn off your 240V* loads, this can damage them.

You don't see any bold headlines on this answer because it isn't another problem: same problem but with the neutral wire broken. In that case it would be a true emergency. Your 240V* loads would work, but your two 120V** loads would have either too-high or too-low voltage (the two voltages adding up to 240V* ***). The too-high voltage can melt appliances and start a fire. The advice would be the same, we'd just be screaming it.

* in Europe and Eurostyle service on 5 continents, this number is 400V. In NYC and parts of Central America this number is 208V.

** in Europe etc. this number is 230V.

*** in Europe or NYC, the voltages won't quite add up to the higher number, because of 3-phase weirdness. There, power is supposed to be a triangle with neutral in the middle. This failure causes it to float around.

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    diagnosed for free – Mazura Jan 3 at 1:49
  • I observed on my panel if the service neutral were to break, all the 120 circuits come down immediately. I suppose you could jumper service neutral to ground but that just sounds like a bad idea. – Joshua Jan 3 at 19:08
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    @Joshua because you have no active loads on the other leg of service. If you turned on such a load, the loads on the two legs would then be in series. Jumpering service neutral to ground is redundant, it's already jumpered to ground via the neutral-ground bond. It doesn't work because dirt is a poor conductor and is not able to return useful amounts of current, – Harper Jan 3 at 19:34

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