MWBC- Voltage Readings


Old 01-31-14, 05:49 PM
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MWBC- Voltage Readings

I have microwave on one half of a multiwire branch circuit. On a lamp on the other half, I thought I noticed a very miniscule blip in the light frequency when the microwave would shut off. Cant say if it dimmed nor if it brightened... just a very slight split second 'difference'. I did NOT see any difference in the light when the microwave first turns on either.

So I tested with a low impendence multimeter. House stays consistent at about 122V a the time of testing. Both circuits are on opposite legs of the panel.

While the microwave was running the other circuit was measuring 118-119V Neutral to Hot and Hot to Ground. When the microwave turns off the other circuit then bounces back to 122V NtoH and HtoG. So there is like 2V of sag (perhaps enough to see the blip in the light when it bounces back) with the micro running or not.

I was also getting about 1 volt or so Neutral to Ground with the Microwave running on the other circuit and when the microwave shuts off I would get just a small .10 Neutral to ground which I assume is induced.

The circuit that the microwave is on with it running also sags down about 2 volts. So I am not seeing one circuit going high and one circuit going low as I would assume would be the case if I had a loose neutral.

I also tested another circuit on the opposite leg of the microwave (not part of the MWBC) and there was no voltage difference whatsoever with the microwave running or not.

Do I have an issue here?
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Old 01-31-14, 07:30 PM
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Microwaves can be a big load for a 120 volt circuit, many use 1500 watts or more. I don't think that load should be enough to bring the voltage down an appreciable amount. As long as all connections throughout the MWBC and in the panel are tight, I wouldn't worry about it.
Old 01-31-14, 07:37 PM
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A 2 volt drop and a slight flicker in the opposite circuit is perfectly normal when using a microwave.
Old 02-01-14, 05:38 AM
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thx joe and pj.

have a nice day gentlemen.

Last edited by bigboypete; 02-01-14 at 07:37 AM.

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