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Discussion Starter · #1 ·
While checking out something else for a client this was brought to my attention.

There is a microwave on one half of a multiwire branch circuit. On a lamp on the other half, there is a 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 meter. House stays consistent at about 122V at the time of testing. Both circuits are on opposite legs.

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 the shared neutral loose.

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


Is there even an issue here??
 

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A slight blip is normal. When a heavy load starts on one leg it is normal for a slight voltage increase on the other leg. The longer the run the more the increase. Even at 250 feet a space heater will cause about a 2 or 3 volt rise on the other leg. A motor load upon start up will probably add a few a bit more from the inrush.

However if you getting anything over than a few volts you have a loose connection beginning to show up.
 

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Discussion Starter · #3 · (Edited)
That's what I was thinking I would see but what is confusing me,

1. there is no rise/fall effect. Both circuits have a small 2-3 volt decrease with the microwave running. Unless my meter is deceiving me.

2. the miniscule voltage I am getting neutral to ground.

thanks for input....
 

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That's what I was thinking I would see but what is confusing me,

1. there is no rise/fall effect. Both circuits have a small 2-3 volt decrease with the microwave running. Unless my meter is deceiving me.

2. the miniscule voltage I am getting neutral to ground.

thanks for input....
There is voltage drop (VD)

And there should be neutral to ground voltage as part of the VD
 

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That's what I was thinking I would see but what is confusing me,

1. there is no rise/fall effect. Both circuits have a small 2-3 volt decrease with the microwave running. Unless my meter is deceiving me.

2. the miniscule voltage I am getting neutral to ground.

thanks for input....
The dip could be from the service as a whole, but either way a small rise or dip is not an issue.

A neutral conductor whether part of a MWBC or not under load will always be slightly above ground in voltage since the wire's impedance causes a voltage drop across it. That voltage drop appears as an elevated neutral to ground, which is normal. however in residential if its over that you have a loose joint.
 

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Discussion Starter · #6 · (Edited)
However if you getting anything over than a few volts you have a loose connection beginning to show up.
What would be an acceptable tolerance before having to hunt down a loose joint?


That voltage drop appears as an elevated neutral to ground, which is normal. however in residential if its over that you have a loose joint.
Over what threshold?

BTW thank you for your thorough explanation and for taking your time with answering me.
 

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Is the service 120-240 or 120-208?

Does it make a difference?
 

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I also tested another circuit on the opposite leg of the microwave (not part of the MWBC) and there was no voltage difference whatsoever.
That's the puzzling part. Like you, I take that to mean the service is not related to this phenomenon.
Is there even an issue here??
Maybe yes maybe no. Like you, I'm curious as to why it's doing this. It does not make sense to me. I would also expect to see the voltage on the light go up slightly or stay the same, not drop a few volts.
 
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Discussion Starter · #9 · (Edited)
It bothered me enough to check again. And I realize I am the idiot.

In my haste, I wrongfully assumed the microwave and the lamp were on opposite legs. They were on the same circuit (although outlet placement surely wouldn't have suggest that).

So Im getting 2V voltage drop on the microwave circuit under load and on the opposite leg of the MWBC I get 1 volt or less rise. Just like your suggesting.

So embarrassed as I am, for future reference, I am still interested in know what those voltage tolerances are before I should be looking for an intermittent connection. And about the neutral to ground being "over" in residential?

(ps this is 120/240)
 

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What would be an acceptable tolerance before having to hunt down a loose joint?




Over what threshold?

BTW thank you for your thorough explanation and for taking your time with answering me.
No problem:)

With one leg under significant load close to the breakers ampacity I would say about 1 to 2 volts max on a 1500 watt load and about 100 feet of run. Another volt for another 100 or 200 feet max. The leg with the load will dip about 4 volts on average. Of course these are rough rules of thumb I go by. If in doubt, always test neutral to ground with only one leg loaded. Make sure the ground is a good one (ie your not working on an old house with romex tapped off of ungrounded knob and tube). Neutral to ground will read about 2 or 3 volts under a 12 amp load.

The reason for the dips and rises in all 3 scenarios is from the conductor's impedance. If a joint is getting loose you will have values above those, since the impedance will be much higher. If your getting readings say of 10 volts neutral to ground or voltage rises above 125 volts on a 120 volt service then without a doubt you have a weak connection. Keep in mind that if both legs are evenly loaded on a MWBC you will see no voltage imbalance and neutral to ground will actually measure close to zero. You could even disconnect the neutral without problem. Hence always test each leg under a heavy load like a space heater while the other only has a 60 watt bulb in it or similar.
 
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