When I run the microwave oven in my rented apartment, the incandescent lights nearby glow much brighter.

What sort of mis-wiring could cause this?

EDIT: Suspecting a bad neutral connection, I measured the voltage between hot and neutral at a nearby electrical outlet. The voltage was 113 VAC. When running the microwave, that same voltage goes up to 129 VAC.

The voltage at that outlet measured between hot and ground is the same as between hot and neutral (i.e. it also increases when the microwave is running).

On the same circuit as the microwave, the voltage is 126 VAC without the microwave and 108 with it running.

I assume this means that the neutral and/or ground connections to the house may be faulty?

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    Same circuit or a separate circuit? Also, if you can get a voltage meter to see how much of a change we are talking about, that could help. – BMitch May 15 '11 at 23:55

The problem was a bad neutral connection from the transformer on a pole.

I called the power company--they had trucks out to replace the power line within 30 minutes.

Now the power variation is very small. Without the microwave running, I measure 117.5 volts on the lamp circuit and 118.2 volts on the microwave circuit. With the microwave running, the voltages go to 118.2 and 114.0 VAC.

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  • I stand partially corrected, the theory was right but my lack of concern wasn't. Be sure to accept your answer so the question gets properly closed out. – BMitch May 17 '11 at 20:29

Have you confirmed that the lights and microwave are wired correctly? live-live, neutral-neutral, ground-ground?

You could test the receptacle with one of these:

Recep tester


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  • I did this. All of the outlets are "correct". – nibot May 17 '11 at 4:14

First, some background: Inside of a breaker panel for your home are three connections, two for each phase of the power coming in, and one for a neutral/ground. Each phase is ~120v, which when added together (they are opposite phases) add together to ~240v. For the ~120v breakers, you connect one phase and the neutral which shouldn't have much current. Note, this is all approximate, 110-120v is pretty standard.

So based on your measurements, here's my theory: while the neutral should be close to 0v, when you turn on a high wattage device (the microwave is in that list) you're pushing a lot in and out of that ground, and out of phase with devices that are on half of the circuits in the home. I'm guessing that rather than going to ground, it's easier for that extra power to go to those other circuits where you see a net power gain, and for the same reason you see a small loss on the same circuit.

Edit: Thinking about this a bit more and I can come up with one problem, a broken ground on the neutral bar. You can check the ohms between the ground in an outlet and something else that should be grounded (metal stake hammered into the earth, or try a plumbing fixture like your sink). Then do the same with the neutral plug (check for voltage from neutral to the ground in the outlet first in case you are testing the hot by accident). If you see a solid connection with the ground/neutral, no voltage is passing through them, and the outlet tester didn't indicate any issues, I personally wouldn't worry.

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    Actually, I would deal with it. Running lights regularly at a higher voltage will burn them out more quickly. This is also true of things that are motorized. – user558 May 17 '11 at 17:00
  • My main fear was that they were going >200V. A deviation of 10V on 120V supply doesn't seem that far out of normal. What would you do to fix this? – BMitch May 17 '11 at 17:18
  • Do you know of any publication listing the nominal tolerances for voltage swings on neutral? My concern would be that it might be a bad connection that may get worse in the future. – nibot May 17 '11 at 17:28
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    In any case, I reported it to the power company. – nibot May 17 '11 at 17:28
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    The voltage swing being related to the neutral is my theory, so I'm more than happy to hear other ideas for the cause. I've seen suggestions that a 10% variation from the target (120V) is within tolerance for the electric company, so that gives you 108-132V. That also seems to be the case for electronics I have around the house, but I'm not sure of any publications that list this. – BMitch May 17 '11 at 18:35

More headaches are caused by floating/open, high resistance/loose, corroded,bad connections anywhere along the neutral in any 120VAC phase to neutral system! Neutals are current carrying conductors in single phase 120VAC systems which need to be properly grounded and have a clean low impedance connection all the way back to their source of power;.. the utility Transformer! The Utility Transformer neutral connection as well needs to be properly grounded!

The same headaches are encountered in a 12VDC automotive electrical system when the common negative is impaired in any way all the way back to the Vehicle Battery Negative terminal!

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  • How does this answer the question? – Mast Sep 28 at 19:56

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