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I've noticed that when I turn on a microwave on one of my circuits, there is a 10-13VAC drop on that circuit and unrelated circuits on the other leg experience a rise of 10-13VAC.

I was trying to diagnose this further to see if it is an issue that I could fix/diagnose on my side, or if this is an issue that can only be fixed by my utility company.

Some stats:

Voltage between both legs is 246.3 VAC

When I apply a load to the right leg (running circular saw), I observe this change - from no load to circular saw load:

  • On the left leg 119.2 VAC goes to 123.1 VAC, and 8.7A goes to 9A
  • On the right leg, 127.4 VAC goes to 123.5 VAC, and 1.9A goes to 7.7A

Does it mean that I have a bad service neutral coming in? Does it mean something else?
Can one confidently say that the issue is "on my side" or "on the service side"? Is there anything I can do to help diagnose or fix this issue?

From Comments:

(All measurements were taken using Fluke 323).

With all breakers ON, grounding wire shows 5.5A.

  • When I applied load on right leg, GEC went down to 2.3A.
  • Applying load on the left leg GEC went from 6.1A to 2.8A. (not sure what caused it to go from 5.5 to 6.1 between my measurements)

Are Certain multiple Circuits leaking current into GEC?

I have 24 breakers right now in my panel. When I turned off breakers 2, 4, 6, 8, 10, 11, 12, which I figured out by trial and error, the grounding wire went to 0A!

Once I identified all the "problem breakers", I turned them off (leaving non-problem breakers on) and tested problematic breakers one at a time.

  • Breakers 2, 6, 8, 10 one at a time GEC was showing between 0.1A and 0.3A for each.
  • Breaker 4 put 0.9A into GEC
  • Breaker 12 put 1.1A
  • Breaker 11 put 5.5A.

Earth ground to Panel ground

With all breakers in the ON position, the voltage measured from Earth-ground to panel-ground using a method mentioned by @Edwin was 4.7 Volts. Method - wrap a conductor around a screwdriver, stick it into the ground 15 feet away from the panel, then measure voltage between panel ground and earth ground.

Single-breaker experiment

I shut off all other breakers, installed a new 20A breaker with a short wire, rigged it up into a circuit connected to right leg hot and neutral bus, and tested it with a circular saw load. No ground (saw plug had no ground either).

Results:

  • Measured load running circular saw: 6A

  • When the saw is on that sole breaker, it also threw 3.9A current onto the grounding conductor.

  • during no load right leg: 123.7 VAC

  • during no load left leg: 123.6 VAC

  • with circular saw load on right leg: 127.3 VAC

  • with circular saw load on left leg: 120.0 VAC

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  • In lost neutral situations I expect to see more voltage swing than that. Since you seem to have some measuring ability, any chance you can put a clamp meter on your Grounding Electrode (bare wire exiting main panel to go clamp ground rod or water pipe). Current moving on that (increasing with neutral imbalance) is a smoking gun.
    – Harper - Reinstate Monica
    Commented Jan 9, 2022 at 1:55
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    Hmm interesting. ... Measuring the grounding wire shows 5.5A. When load is applied on right leg, the measurement goes down to 2.3A. Applying load on the left leg the measurement goes from 6.1A to 2.8A. (It looks like "no load" amperage changed from 5.5 to 6.1 during the measurements, hence the difference in the starting number...). Discrepancies aside, what does that mean? Is grounding conductor considered safe, or is it energized? It seems to me like that's pretty high current going on on the grounding conductor.
    – dennismv
    Commented Jan 9, 2022 at 2:31
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    Current on the Grounding Electrode Conductor should be zero. If not, either your neighbor has a Lost Neutral and is returning neutral via the dirt and your ground rod and your neutral, or you have a Lost Neutral and are returning current through your neighbor's. With current flowing on the GEC, if you placed yourself in series with that current, you'd die. So I would not categorize that as safe.
    – Harper - Reinstate Monica
    Commented Jan 9, 2022 at 3:21
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    @dennismv 4.7 Volts is high. There is no "normal", but 4.7 is not right. I would start at everything on breaker 11. Make sure that all the neutrals and equipment grounding conductors on that circuit are separate and well-connected. I don't have any practical troubleshooting experience with this specific scenario, but 5.5 Amps is a lot of current. The goal is zero. You don't want current flowing on the equipment grounding conductors.
    – Edwin
    Commented Jan 9, 2022 at 8:22
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    Nice question @dennismv! I guess advice going forward is that if some experiences the problems you did, first thing to do is look up at your mast to see if the neutral is busted off!
    – Edwin
    Commented Jan 20, 2022 at 6:10

2 Answers 2

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Call your power company and report an outage NOW.

Given the current on your Grounding Electrode Conductor, and that they are influenced by your breakers, this is certainly a Lost Neutral, almost certainly on the "service drop" between the utility's power pole and your weatherhead. Because that's where it is 95% of the time.

It's so likely that it isn't even worth wasting $150 (or the lengthy time!) needed to bring in an electrician.

This is a 20 minute job for a power company guy with a bucket truck. This work is free in almost all such cases, because the power company is responsible for the entire service drop from pole to weatherhead. If they aren't dealing with recent storm damage, they'll most likely be out in an hour or so on a Sunday even.

This is a full-on power outage. You may think "well it semi-works" but lopsided loading of your phases will result in excessive voltage on one side, potentially damaging equipment which is not rated for 150V or more.

For a full backgrounder on how 120/240V works, see this video. A lost neutral means nothing holds the neutral in the middle.

When the neutral wire is lost, neutral current attempts to path via the neutral-ground bond in your main panel, your Grounding Electrode Conductor, the ground rods... and the various ground rods, GEC and bonds of your neighbors and the supply transformer (which has one too). This also has the effect of electrifying the dirt, which animals intensely dislike, and considerable voltage differentials can show up down metal things like fence lines or dog runs.

240V loads are unaffected. Those are the only loads you can safely run right now.

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  • Thanks. Well, it's 10:30pm on a Sunday night ... I just reported an outage. I wanted to wait till Monday morning just so that there will be daylight for any workers who show up. But I'll see what happens.
    – dennismv
    Commented Jan 10, 2022 at 3:27
  • Also I know that my house might have some haywired or miswired circuits, maybe more than one. So on a quest to drill down if the problem is still truly on my end or not, I've updated my question with a "single breaker" experiment. With a single breaker there is still voltage fluctuation on both legs and also current leaking into the ground wire. So for now, I will stop trying to find an issue on my side and see what utility company says and does.
    – dennismv
    Commented Jan 10, 2022 at 3:30
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    @dennismv A single circuit, no matter how misfired, cannot raise voltage higher than normal on one phase of the panel. Nor can it put current on the GEC unless it is itself grounding to earth outside. Good idea to be following up on any other flaws you uncover. We see that a lot in lost neutrals :)
    – Harper - Reinstate Monica
    Commented Jan 10, 2022 at 5:19
  • It ended up being a 20 minute job with a respectable-looking bearded guy and a ladder, not even a bucket truck. My neutral has been snapped at the mast. I am really REALLY surprised I haven't had other issues. The situation has been going on for at least two weeks. The neutral snapped and was almost touching both hot wires through warn down insulation. I'll see if I can post a picture.
    – dennismv
    Commented Jan 11, 2022 at 10:28
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    @dennismv Yeah, that's pretty much textbook. Weeks of mystery gremlins then "fixed just like that". Very anticlimactic! For us, the spark was when my sweetie says "I'm sorry the toaster is so slow" - wait, toasters don't have speeds.
    – Harper - Reinstate Monica
    Commented Jan 11, 2022 at 20:20
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Your netural is the problem as you have diagnosed. It could be a bad connection in your home possibly where it is bolted to the panel. Measure the voltage at the panel between each phase and neutral. Have somebody cycle the oven. If it drops/rises any appreciable amount there, it is highly likely the neutral feeding your home is bad. If it is stable the problem is in your home. The best solution is get a qualified electrician.

A 240V load will not have any effect on the neutral. The load has to be on one phase or the other but not both. If the load causes one leg to drop and the other to rise by about the same amount (a few volts or more) it is the neutral connection. The electrician will validate your diagnosis that it is the incoming power unless you want to test it at the meter. If you are sure it is a bad neutral feed then call your power supplier.

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  • my oven is on a 240VAC connection, so it will involve both legs. Should the load need to involve one one leg?
    – dennismv
    Commented Jan 9, 2022 at 6:00
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    Note that the first step with a faulty neutral is to contact the utility not an electrician, because neutral failures are often the utility's problem to fix
    – ThreePhaseEel
    Commented Jan 9, 2022 at 8:43

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