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My house was built in the late 60/early 70s - 3 bed 2 bath. We have a weird situation where our house is partially losing power. If the AC is on, power is great no issues, once the AC reaches the temp we set it at, ac will power down (go idle), which is normal.

Weird issue is that randomly, minutes later or even hours later (if the ac is not triggered on manually or automatically), the power to like 80-90 percent of the house dies. The only power that stays on is the hallway power (thermostat is located in hallway), one bedroom power, and guest restroom power...other than that - all power is out (inside, outside, garage, kitchen, etc.).

We check the breakers and all breakers look good and nothing seems to be popped or out of the ordinary.

The weird thing is the only way to get the rest of the power back on is if we get the ac going by turning the temp down, just so it can trigger on.

Any ideas on this? Any help or guidance would be appreciated. Not sure how to go about troubleshooting this issue.

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    As a side note, I'd recommend you stop using the HVAC to get your other circuits to work -- this could easily damage your HVAC system.
    – Nate S.
    Commented Sep 18, 2020 at 16:42
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    Where is this house located? Country and state. Commented Sep 18, 2020 at 16:50
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    Does the same thing happen if you turn on the electric hot water heater, electric range or electric dryer? Commented Sep 18, 2020 at 17:40
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    Can you post photos of your panel please? Do you have any other 240V appliances (electric hot water heater, electric range/cooktop/oven, electric dryer) in the house? Commented Sep 19, 2020 at 1:06
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    My recommendation is best left to a pro, or the power company. But if you are troubleshooting until they come; what test equipment do you have? Test your voltages, maybe try some outlets in the working room, and some in the parts that die. Are the 'dead' ones really dead? -read 0Volts (less that 1V) or do they read somewhere in between 0 and 120V? Do any read above 120V (somewhere between 120 to 240V)? I was going to ask if the AC actually runs, but you did say it gets down to temp, so that must be yes. This is a regular AC with a compressor, not a swamp cooler, correct? Commented Sep 20, 2020 at 6:18

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I'm going to slightly disagree with the other answers and guess you have an open hot conductor. Either way, the answer is the same: call the power company and report a power outage. The problem is very likely at the pole, so they'll come out and fix it for free.

For more information on why losing one of your hot conductors will cause the symptoms you're seeing here, take a look at Harper's answer to a similar question.

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  • Also, if the ac subpanel has those shot filled gigantic fuses on the hots, check that one hot is not open (pull breaker, remove fuse, check with ohm meter)
    – mark f
    Commented Sep 18, 2020 at 17:01
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    The OP indicated that his a/c is working. Hard to see how it could be with one lost hot leg. Maybe he has a split bus panel and there is a lost hot in the 2-pole breaker or one of the wires leading to the lower bus for the 120 V 1-pole breakers in the lower section. Commented Sep 18, 2020 at 17:27
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    Yup. This thing keeps returning in questions with HVAC, ovens, all kind of equipment running on 240V in the US.
    – Mast
    Commented Sep 19, 2020 at 14:25
  • I agree that there are some contradictions here. I have this idea; a loose hot, but not completely open. This can happen on cars, no battery voltage, crank starter, sparks a connection, then works. Stop, and the connection is lost again. I suspect this situation is occurring: The higher amperage load of the AC is sparking a connection, which can heat up and maintain. Everything works. Then the AC cuts off, the connection cools, and connection is lost (high resistance, not completely open). Could be minutes or hours before it breaks the connect. Just my best guess. All agree GET IT FIXED ASAP! Commented Sep 20, 2020 at 5:58
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A similar issue just happened to me. Air conditioner was on, but not blowing cold air. When I turned it off at the thermostat most of the house power went off too. I read this blog and realized I should call the electrical company. They told me that since it was the AC it’s probably an internal problem and have Electrician or HVAC look at it. I called an electrician and they charge $150 just to come out. Instead of paying that, I called the electrical company back and said, the electrician says it was an “ external intermittent power with a neutral issue.” They came out and found that the wires going to my house had rubbed so badly that there was a short. He actually showed me where it had burned through the steel support wire. Definitely a safety issue! Two hours it was fixed for free and my air conditioning blowing cold air right now.

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Posted by question author:

What [the energy company] said when they came out is that everything is getting power properly, but when he pulled the breaker for the AC out it was all melted and bud bars had some burning on it.

Seems it’s a common issue with Zinsco breakers.

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You have an open or lose neutral or high leg and that it dangerous. Call your power company and report an outage... right now. They will respond quickly. These problems are usually their problem and they will fix it for free. In the event that it's your problem, they might still help you out and fix it if they can.

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This may be a problem with the neutral. The a/c condensing unit does not use the neutral so it will work fine, but the air handler inside does use the neutral.

Depending on where an intermittent or loose connection is in the neutral wiring there could be damaging high voltage on some 120 V circuits and low voltage on other 120 V circuits. At the point of a loose connection you would have damaging and dangerous local heating.

Have you had any lights blow or equipment fail after these outages?

A problem with the neutral is an emergency requiring immediate intervention, very likely professional, unless you are experienced. If the problem is at the pole or between the pole and your meter, then the power company will fix it. But if the problem is in your house wiring you need a professional repair immediately.

Edit Does your house have aluminum wiring? If so, you may have loose connections at the neutral bar in the panel.

What is the brand of your panel?

You can open the door of the panel, but I don't advise you to remove the "dead front" unless you are experienced or can otherwise do it safely. There is real risk of injury or damage with the dead front off.

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