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I am trying to understand and solve an electrical issue in our house, which is 75 years old. The issue is variably low voltage on some circuits, between 103v-111v (tested with a Klein outlet tester with voltage readout, also a Sperry clamp meter). Some circuits are reliably 120 (give or take 2v). The voltage on low-voltage circuits is the same at a given time (they are all 107v, or all 111v). The main real-world effect is flickering on some LED lights, occasionally the oven electronics blip out for a second.

Most circuits show variable low voltage, 5 show constant 120. We have a 6-breaker subpanel (10 years old), and two of the constant-120 circuits are on that panel (so is one variable circuit), the other 3 constant and variable circuits are on the main panel (27 breakers). To diagnose the problem, I mapped every receptacle to see what it was connected to (with the aid of a circuit tracer), tripping the breaker to verify the breaker-to-receptacle map. Near the end, I noticed that a previously low circuit was now 120, indeed all circuits read 120! Problem sorta solved, for a few weeks, but the problem has crept back. I did try throwing all of the breakers one at a time, but that didn’t fix it.

There is no obvious pattern to the low-voltage circuits vs. normal voltage. About half of the circuits in the house (including the subpanel) were installed within the past 10 years, and the “good” vs. “bad” circuits are randomly distributed across old vs. new. Almost all of the receptacles were replaced, but one of the reliable 120s is on original wire and an old receptacle. The problem doesn't correlate with length of the circuit run. At this point, I am looking for a ballpark explanation and direction of a remedy, so that I could have an informed discussion with an electrician (the previous guy’s response was a formulaic “replace everything, for a lot of money”). It seems most likely that it is about the breakers, but what?

In tagging the breakers, I saw that I mis-counted (forgot two "never use" circuits). Anyhow, here is a picture of the two panels (main the left, subpanel on the right). The three tandem breakers seem to be "the same" (120 or low voltage). Blue tape marks 120, drywall tape (the other sticky think I have) marks variable. The main panel is GE Load Center TM2020c, it came with the house when we bought it, the subpanel was installed when we moved in a decade ago. The unmarked one are "don't know" because they don't have receptacles on it. On the main panel, 9+11 and 2+4 are 240 for AC, dryer and 6+8 is for the subpanel; 5 is a floor heater. On the subpanel, 4, 6, 7, 8 are dishwasher, track lights etc. I might get more info with the help of my "supervisor" who has better tools and can tell me what not to do.

Breakers

"Resolution". The problem is "outside", beyond the zone that Mr. Sparky is allowed to touch. One leg is 105 (at the moment). The follow question would be appropriate for Politics SE: how do I get City Light to answer the phone and schedule a check of the meter and lines from the street. Given the suggestion of a bad leg, diagnosis time was under 2 minutes.

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    It's not receptacles. It's also probably not wire. Tell us about your main electrical panel - what brand, when was it installed? The problem probably lies there, but the details will help to confirm it.
    – KMJ
    Dec 4, 2023 at 18:48
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    Pictures of your panels showing which circuits have the problems may be very informative. Also can you check 240V receptacles (eg, dryer) when the problem happens? Dec 4, 2023 at 18:54
  • If you turn off the all breakers with under voltage except one, does it return to the same voltage as the "good" outlets? Are the "bad" circuits all on even or all odd rows on the panel? That all suggests a bum leg, which you'll need a sparky to come out and look at.
    – dandavis
    Dec 4, 2023 at 20:45
  • Shut off all breakers serving 240V loads. Do the "drywall" circuits now go dead as a doornail? Dec 4, 2023 at 22:51
  • Tell them you have one of your hot lines out. If that doesn't get through to them, tell them your power is out. Period. They may question it, but they'll get a truck there and figure out. Dec 5, 2023 at 17:53

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Check in the main circuit breaker panel if there is an actual pattern to the "random" voltage drop circuilts: if the circuit breakers whose voltage has dropped are interleaved exactly with the full-voltage circuits:

row 1: 120
row 2: 103
row 3: 120
row 4: 103
row 5: 120
etc...

then you probably have a bad connection on one of the two main power cables going to the panel (neutral is probably good: if it were a bad neutral the 120 would not be 120 when the other side is 103; bad neutrals have one side go higher as the other side goes lower proportionately). It may be on the utility side of the meter or on the house side. Be careful with the main power cables: they are likely unfused and will deliver dangerous currents before the power pole fuse blows.

Edit: here is the pattern showing on the main panel (left side of picture):

low voltage alternate rows

The rows marked red show the low voltage. The three top rows are paired with a 240V double breaker, so you'd not see the low voltage on those breakers on the same rows unless you opened those appliances and measured both power legs. The fourth row shows low voltage completely across. Tandem breakers feed off the same pin in the panel, so they'd have the same voltage for both halves.

This works because panels usually feed main 120V leg A to the first row, 120V leg B to the second row, leg A to the third, leg B fourth, etc.

Without a model number to determine how the right picture's panel is set up, it would be hard to guess the pattern there.

But I'd wager further that it's a bad side on the main power lines to the house.

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    Looks like one hot leg has a bad connection. This could be at the pole and so first call your electrical supplier. If it is in their equipment it will be fixed with no charge to you. They won't charge for checking even if the problem is in your panel. In that case though the repair would be on you. Dec 4, 2023 at 19:33

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