# Voltage leak? Getting 25v over 3ph legs when breaker is *off*.



## jggramlich (Jul 11, 2013)

Howdy all. I just installed a 3ph receptacle on a 220v circuit for a welder we use in our shop. I was checking voltages when I noticed something strange. When the breaker for the 220v-3ph-60A circuit is in the *off* position, I read 25v (ish) between each leg of the 3ph and the conduit. (there is no ground wire)

We've been having some strange breaker tripping issues with our 120v circuits coming out of the same breaker box as the 220-3ph circuit...i.e. the conduit EMT) for all of it is bonded together. The 25v could be coming from anywhere.

Or...I could be an idiot. Thoughts?


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## FastFokker (Sep 18, 2012)

So what's the problem?


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## backstay (Feb 3, 2011)

FastFokker said:


> So what's the problem?


My guess is a digital meter.


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## jggramlich (Jul 11, 2013)

*The problem is tripping 120v breakers*

The "problem" is that I have multiple 120v-20A breakers tripping when used. There isn't an apparent cause. The tools being used on those circuits are 4" and 7" angle grinders. They don't always trip the breakers, but they're tripping 2 or 3 times a day at least.

I've replaced 2 of the 120v breakers with brand new, but to no avail. They keep tripping randomly. The only symptom I've found with the system is that there is 25v sitting on my conduit (EMT).

I'm assuming that there should be 0v potential between ground and a circuit leg that is switched off. Is my assumption correct?


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## Big John (May 23, 2010)

jggramlich said:


> The "problem" is that I have multiple 120v-20A breakers tripping when used. There isn't an apparent cause. The tools being used on those circuits are 4" and 7" angle grinders....


 A 7" grinder can draw 14-15 amps to begin with. If a guy bogs down on it, or there's more than one on the same circuit, there's no surprise it would trip a breaker.


> ...I'm assuming that there should be 0v potential between ground and a circuit leg that is switched off. Is my assumption correct?


 No. The energized circuits in the same conduit are coupled to your de-energized one and you will read voltage present. Kill everything in that pipe and I'm betting the voltage will go away.

Get a current splitter and put an amp clamp on one of those grinders when it's in use. I'll bet money you'll see it go over 20A.


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## backstay (Feb 3, 2011)

jggramlich said:


> The "problem" is that I have multiple 120v-20A breakers tripping when used. There isn't an apparent cause. The tools being used on those circuits are 4" and 7" angle grinders. They don't always trip the breakers, but they're tripping 2 or 3 times a day at least.
> 
> I've replaced 2 of the 120v breakers with brand new, but to no avail. They keep tripping randomly. The only symptom I've found with the system is that there is 25v sitting on my conduit (EMT).
> 
> I'm assuming that there should be 0v potential between ground and a circuit leg that is switched off. Is my assumption correct?


Use a low impedance meter to see if the voltage is really there. Next check to amp draw on the tripping circuits.


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## BBQ (Nov 16, 2010)

jggramlich said:


> The 25v could be coming from anywhere.
> 
> Or...I could be an idiot. Thoughts?


Read this link


www.nema.org/Technical/Documents/Bull_88_reaffirmed_12_15_11.pdf


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## jggramlich (Jul 11, 2013)

*Thanks to all about checking the tools*

The reason I suspect the wiring and not the tools is that this happens on several tools, but, that said, I will be checking the current draw on the tools. The tripping happens almost always on motor start and it's quite possible that the starting current draw of the grinders is the culprit and these circuits really need to be, uh, larger.

Thanks again.


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## jggramlich (Jul 11, 2013)

Big John said:


> A 7" grinder can draw 14-15 amps to begin with. If a guy bogs down on it, or there's more than one on the same circuit, there's no surprise it would trip a breaker. No. The energized circuits in the same conduit are coupled to your de-energized one and you will read voltage present. Kill everything in that pipe and I'm betting the voltage will go away.
> 
> Get a current splitter and put an amp clamp on one of those grinders when it's in use. I'll bet money you'll see it go over 20A.



Just an fyi but there are no other circuits in the conduit that I'm finding the 25v on. If it were just 5v or so I'd blame my meter immediately, but it's too consistent for me to do that right away. The 25v is constant, stable and always there.

And to be clear, this is 25v between what should be a de-energized leg of a 3ph-220v circuit and the conduit...any of my conduit.

But, that said, I did some research on the big grinders who trip the breakers the most and it's awfully close. They say 15A rating...but folks on another forum are saying 2400W motor. If that's true...then I could easily see how these grinders are tripping a 20A breaker.


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