Power goes on/off with A/C after unit replaced

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Old 10-13-18, 09:10 PM
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Power goes on/off with A/C after unit replaced

A few months ago the HVAC system was replaced and ever since then there has been an intermittent issue with power loss to certain portions of the house which seem to be triggered by the AC turning on or off. It is getting a little cooler here now and last night the power was off completely in that part of the house until this morning. Turned the Tstat down to get the AC to kick on and the power immediately came back on.

On a whim changed the thermostat and it didn't exhibit the same behavior, I could turn the AC/heat on and off without any other part of the house going off. What did cause it to go off however was the old AAs I put in the new Tstat died after about 8 hours. The power again went off and has stayed off.

The AC folks claim it is all wired correctly. Another electrician did a fairly shoddy job of "checking" things and didn't really tell us anything we didn't already know.

Any ideas?
 
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Old 10-13-18, 09:36 PM
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Lots of possible causes.

Does the air conditioning work (cool the house) propertly?

Are the lights and things that fail all on specifi c branch circuits the entireties of which go dead??

Are you getting strange voltages (not approximately 120 hot to neutral and not approximatley 240 hot to hot?

Do you ahve subpanels? Are the problems affecting only things in a particular subpanel?

Do you ahve problems with other 240 volt appliances such as an all electric water heater?

Maybe one of the two hot legs that together bring in 240 volts is dead, either just going to a subpanel or maybe for the whole house. Then turning on a 240 volt appliance can supply power although not always adequate power to an otherwise dead section of wiring. In so doing, that appliance itself will usually not work to its full capacity.

There may be loose connections somewhere.

With so many things to check, it is not unusual for a different person such as an electrician who comes in for the first time to hit th e same things yo already checked and, since the whole process is hit or miss and time consuming, he will of course charge you for everything he does try.
 
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Old 10-14-18, 07:43 AM
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Thanks for the answer.

AC has been working fine.

It does appear what's going on/off are on multiple breakers. There are nearby things however on their own breaker like the refrigerator which do not go off when this happens.

I did read a couple threads of people having similar issues, mainly with 220v electric dryers, and the issue being the hot leg from the power company not being good for a variety of reasons. No subpanels, just the one main panel.

It makes sense but yesterday after changing the thermostat and turning the breaker back on everything was on even with the AC not running so it's a bit baffling. All I know is it was off for some time and as soon as I set the old stat low enough to trigger the AC, it immediately came back on.

The old HVAC system was ancient and it's possible the folks who installed the new one may have "fixed" something accidentally where power was being fed back from the AC 220v.

What are some good points to test from? The outlets that turn off, the main panel?
 
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Old 10-14-18, 10:02 AM
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I can say for sure the ac thermostat has nothing to do with you losing power to other circuits. You have a problem in the main panel. You need to check the breaker connections of the circuits losing power. Look for corrosion on bus stabs where breaker connect. Pull breakers and reinstall. What brand panel do you have and how old?
 
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Old 10-14-18, 10:51 AM
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What are some good points to test from?

I would start at the main panel and check voltage across the two incoming lines, should be 240 volts. Next, check across the load side of the main breaker the same way, should be 240 volts. Then check each hot leg to neutral and then to ground; should be 120 volts. By this time I think you will have found the problem, but if not, continue checking voltages along the circuits that go out.
 
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Old 10-26-18, 07:21 PM
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Sorry haven't replied to this but just had a chance to check out the panel with a meter. So not sure if it's my meter or not but the main circuits read 124v when the probes were connected to both, the main breakers registered 124v as well. The breakers which do not work when the AC is off only register 2-3v. When the AC is on those breakers register around 118v, however if I measure the main lines against each other during that time they only read 8.8v. As soon as the AC is turned off they go back to 124v.

I didn't have a chance to measure the HVAC circuit which is down by the unit since it's outside and was dark.
 
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Old 10-26-18, 07:27 PM
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You need to check across the main breaker. You need to see 240vAC on the two wires that feed the main breaker. Make sure your meter is capable of measuring 240vAC. Also a good idea to wear safety glasses when testing in a panel.
 
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Old 10-26-18, 08:17 PM
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It's an Etekcity MSR-U1000, which according to their documentation should read AC loads up to 1000v. That was the odd thing with the test leads on the two main wires it never read more than 124v. What I also don't understand is why when the AC was on and the problem circuit breakers went to 118v, the voltage across those seemed to drop to almost nothing. Nothing in the house turned off during that time, the things on the problem circuit just turned on.
 
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Old 10-27-18, 02:35 PM
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That was the odd thing with the test leads on the two main wires it never read more than 124v.

Was that ACROSS the two main wires feeding the panel or from one main wire to neutral and then the other main wire to neutral. It sounds as if you have a [problem in your main panel and when the A-C comes on 120 volts is backfeeding from the other leg to heat up some dead circuits.
 
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Old 10-27-18, 02:40 PM
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Very strange. It appears you have lost one leg of the 240v service but if that was the case... the A/C should not even run.
 
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Old 10-27-18, 02:56 PM
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It appears you have lost one leg of the 240v service but if that was the case... the A/C should not even run.

Exactly what I was thinking so that's why I believe there is a problem of some kind in the main panel. For example, maybe it's a split bus panel and there are some bad breakers involved.
 
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Old 10-27-18, 09:03 PM
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It was across the main wires, they both tested 120v each one to neutral as well from what I recall. I'm going to test it again tomorrow, and take some pictures/video of the behavior I can show to the people on here and to an electrician. I'm just trying to do a diagnosis to cut down on time/money, I plan on having someone more qualified actually fix the issue since it's beyond my skill level. The A/C people said they just hooked it up the way they hook any A/C up to 240v, but it 100% started happening when they did the work. It's possible something with the ancient old system was wired incorrectly that was causing things to actually work.

House was build in 1979 and it is the original panel.
 
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Old 10-27-18, 09:26 PM
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Pictures are good and we can comment on them....... How-to-insert-pictures
 
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Old 10-28-18, 06:06 PM
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Didn't get a chance to do the pictures/video but a bit more info. The dryer heating element stopped working sometime last week. There are some common issues with those and the dryer is 10 years old. The thermostats in the unit tested good so I figured it was the heating element, ordered a new element, but it may be related to the other issues and not the element at all.

The neutral wire on the dryer was burnt and brittle on the dryer side starting about 2 inches up where it connected to the plug wire. It didn't look it happened recently. I replaced that section of wire and decided to measure the voltage across it. The voltage across the two hot leads was 8v, which was similar to the story above. That's actually the differential between the voltages on the two wires. Is that a meter issue? One is reading hot to neutral at 124v, the other was at 116v. Also determined the circuits not working are on the same leg, and they also started to turn on when the dryer was run. Anyways, calling an electrician tomorrow morning because something I believe is fubar either in the panel or from the power company, and it not something I can fix.
 
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Old 10-29-18, 11:53 AM
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Yes..... you have confirmed that you are missing one of the service legs. It could be a power company issue or between the meter and panel or at the panel itself.
 
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Old 10-31-18, 10:07 AM
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The dryer heating element stopped working sometime last week.

That would be when you lost a leg at your service. The elements are the only thing on an electric dryer that requires 240 volts, the dryer basically runs on 120 volts.
 
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Old 10-31-18, 03:30 PM
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Just to close the loop on this. Verified the one leg had no voltage on it. Power company came out today and it turned out one of the lugs inside the meter was burned/bad. Could be a number of reasons for that but looked like it had been arcing for some time. They replaced the lug and all is good now.

Thanks for the help.
 
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Old 10-31-18, 03:32 PM
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You are very lucky. Most power companies don't repair the meter pan.
That job would fall to an electrician.
 
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Old 10-31-18, 03:34 PM
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The overall issue has been going on for months, but there may have still been some voltage and it finally got to the point where it completely crapped out.
 
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Old 10-31-18, 03:39 PM
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The meter was locked by the power company, this was where the leg attached from the transformer to the meter outside the house. They had the parts to fix it in their truck, but they said if they didn't other guys would come out and fix it later.
 
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Old 10-31-18, 05:13 PM
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Here..... the power company would come shut down power and unlock the box. They would return to re-power usually after an inspection. Thankfully the largest poco in my area doesn't lock the meter..... just seal it.
 
 

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