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Very recently, about half my house’s power outlets drops in voltage during storms. Each time it’s the same circuits that are losing power, upstairs bed and bath, downstairs router, and a couple other rooms. The basement and living room never lose power. I measured the voltage of the outlet in the bedroom and it was fluctuation 30-10 volts AC and occasionally jumps back up to 120 volts. I’ve checked the breakers, all of them look good. My neighbors are not having issues with their power. What could be causing this issue?

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    My first difficulty in reading you is about "half my houses." This could mean "half of the thousands of homes I own" or it could mean "half of the power system in my house." But I'll take the latter meaning. A couple of things pop to mind. One, because you say 120 V, I think "North American" grid and therefore split-phase and then the two phases on your breaker panel. Which should be balanced. It would then make some sense if there is an explanation why your current return on one phase only is forced through Earth to get back to the power transformer. That ground return might be just enough.
    – jonk
    Commented Dec 12, 2021 at 3:58
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    Do you mean half the lights and half the plugs stop working, or do you mean that the voltage drops to 60V? (And per @jonk, I think you mean "half of the electricity in my house", or "half of my house's electricity"?)
    – TimWescott
    Commented Dec 12, 2021 at 4:00
  • Is there a way to test for a ground fault that occurs on one of the phases? Commented Dec 12, 2021 at 4:14
  • If you look at your breaker panel, does it seem like when there are problems it is with alternating rows? Commented Dec 12, 2021 at 4:27
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    Not an answer to your question but a band-aid to your situation. I have found that power supply units to be beneficial to times of intermittent electrical issues in homes. If you use a UPS for your router, you can keep your internet running even when you have such power fluctuations.
    – LeanMan
    Commented Dec 13, 2021 at 21:11

4 Answers 4

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There are two tests you can do when this happens.

  • Check the voltage of the good circuits. If their voltage is over 120V by an equivalent amount (e.g. 100/140V or 90/150V), that confirms you lost a neutral wire - extremely dangerous. Turn off all 120V loads and immediately call the power company to get it fixed.

  • Go to your service panel and turn off all 240V circuits - water heater, dryer, range, A/C, electric heaters; anything with a double breaker. Now check the "weak" circuits. If they are completely dead now, you lost a hot/live/phase wire - call the power company when convenient.

98% of the time, this is because of a downed pole-house wire, which the power company is responsible to maintain. That flaps in the breeze, slowly metal-fatiguing the wires - and the neutral wire doubles as the physical carrier wire!

What will have happened? The USA is a 240V country. We get 120V because neutral is in the middle. If you lose neutral, it's not in the middle anymore, which is very bad for the appliances on the higher-voltage side!

If you lose a phase, half your 120V loads should just go dead. However, 240V loads are attached to both phase wires. So when they turn on, they connect the dead phase to the live one, powering it back up (through the resistance of the 240V appliance).

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    You say "turn off all 120V loads" but wouldn't it be safer to just turn off the main breakers in this case? Otherwise you could be putting 240V across 120V loads while you are still turning them off. Commented Dec 13, 2021 at 18:50
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    " If you lose neutral, it's not in the middle anymore, which is very bad for the appliances on the higher-voltage side!" even more fun when the same thing happens on a European 400V three phase system...... Commented Dec 13, 2021 at 19:59
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    And the fun will only increase with EV chargers connecting a large piece of metal on rubber feet outside your front door to TN-C-S systems (as separated TT is often too much faff), relying on the 18th edition "underspecified magic" later indents at the charge point to spot a broken PEN. It's really important to get a good charge point like a Zappi [no personal interest] that does this detection well, I think. This is going to become a horrible safety issue in a few years for folk who go for budget options. It's gonna be in the papers at some point.
    – Dan
    Commented Dec 14, 2021 at 13:09
  • @PeterGreen I bet it is. And worse, it requires coordination with neighbors to even identify! We had that happen here (each unit gets a single 120V phase) but fortunately my storeroom was on the other phase, and I could go there and see the high voltage. Commented Dec 14, 2021 at 21:00
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Because it is a storm, this is almost certainly an outside your home problem, which almost always is a utility company issue.

Because it is "half" and "intermittent", this is most likely one of your incoming hot wires (you have 3 incoming wires - 2 hots and a neutral) has a loose connection. When the weather is normal, everything is OK. When the connection either gets wet or battered by wind, the connection breaks.

This is not just an inconvenience, it is extremely dangerous. It can damage equipment (most things can handle power outages just fine, but low voltage can cause a lot of strange problems with modern electronics) and the intermittent nature of the problem could mean sparks, which can lead to fire.

CALL YOUR UTILITY COMPANY EMERGENCY NUMBER. You don't need to wait for a storm. In fact, they'll prefer to send when it isn't a storm, as it is easier and safer to work on the wires and they won't have everyone else calling at the same time. But if you have a storm now, call now. Tell them exactly what you said here: No power to half the house. They should send a truck and the problem will most likely be either where the wires connect to the pole or where the wires connect to the meter, both of which are there responsibility. In the unlikely case that they can't find the problem when it is not a storm, you may need to wait for a storm and call them back.

There is a small possibility the problem is past the meter and your responsibility and not the utility's responsibility. If that turns out to be the case, the utility will not normally charge and you will have to call in an electrician. But unless you live on a big property where the meter is at the edge with a long outside run from the meter to the house, I doubt that will be the case.

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    I agree. I had a similar problem with intermittent short blackouts on half of my outlets during a storm. Called power company and they took it very seriously and rolled a truck immediately. They found a bad connection, replaced it, and I've had no problems since then. Commented Dec 12, 2021 at 19:30
  • Thank you for all your help, I did end up calling the power company and it was a tree branch on the power line. The branch was only on one of the hot wires causing the weird behavior. Commented Dec 14, 2021 at 21:34
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I've had a similar issue occur with one phase drooping at times. After calling out the power company they rapidly diagnosed it as a damaged transformer a block up the street. Once they replaced that transformer there we no more voltage drops on that leg.

Replacement of the transformer only took them a couple hours, this type of failure is apparently prioritized since it can damage loads and upstream systems.

The storm may just be coincidental, call your power company.

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The most recent edit saying that voltage is changing erratically makes a clearer picture, and that is that most likely, the transformer behind your house has a minor leak and the insulation between some of the coils is failing.

Partial loss of voltage to one phase indicates most probably that some of the coils on the secondary side are being shorted out of the circuit by water. This is especially likely if the weather head on your service (the place where wires enter your house) looks intact and there's no sign of water in the breaker box.

It is still somewhat possible that you may be taking water into the system somewhere else. If you have an attic that you can fit in, check for leaks that might be hitting a cable and following the sheathing down to exposed conductors. Check for moisture around outlets and switch boxes. These are good things to just check periodically in heavy rain anyway, as the wiring in a house can have a surprising amount of water in it before anything happens.

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    Even if it's the transformer, it's still a Call the PoCo NOW situation. That's still their responsibility.
    – FreeMan
    Commented Dec 13, 2021 at 17:57
  • @FreeMan, well, it's decidedly less urgent to be sure, but yes, absolutely. The most urgent action to take would be to turn off or unplug any loads that might be damaged by the voltage transients, especially anything that runs a motor, like washers/dryers, ceiling fans, air conditioning and the like. Newer electronics with DC power supplies and things like LED bulbs are probably fine, as they'll shut themselves down when they go to far out of tolerance.
    – user35614
    Commented Dec 14, 2021 at 0:47

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