# Shared neutral, hot leaking to neutral or ground, and...?



## Dennis Alwon (May 9, 2009)

Greg I am confused by your terminology. You talk of knob and tube and then in the next sentence you mention grounds. Do you mean neutrals?...


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## Greg Sparkovich (Sep 15, 2007)

Good point. I haven't actually opened the receptacles yet, I've been told that there is some knob and tube in the house and after making an electrical map these seem like likely candidates since both of these circuit are in leveral rooms.
In the panel everything is grounded romex. I do a lot of work in old houses so I am used to finding romex in the basement and knob and tube in the ceilings and receptacles. I'm kind of assuming that there is probably some old BX in the receptacles , or that there is some wire mesh from plastering that's touching a metal box and a radiator pipe, or something like that and therefore I am getting my ground readings.
I don't want to do this job today, hence my hope to put both circuits on a single AFCI and come back another day.


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## Greg Sparkovich (Sep 15, 2007)

I suspect the only thing I can do is to disconnect both of the neutrals, turn everything on the house, and try and get voltage between one of those two disconnected neutrals and the bus bar. Because at this point I suspect that there is a 3rd circuit shared one of those 2 neutrals. That would also explain the AFCI tripping when all the neutrals and grounds are put together. Then I can turn all the Breakers off one by one and find out if there's a third breaker feeding the shared neutral. :-(

I'm just trying to shortcut the process of checking each location one by one


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## Dennis Alwon (May 9, 2009)

Try and find where the junction is for the multiwire branch circuit if in fact it is one. Split it there. I usually go about halfway thru the circuit and disconnect everything and see if the first half works. If it does then the problem is down the line. Go half again until you find it


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## Fibes (Feb 18, 2010)

Greg Sparkovich said:


> I suspect the only thing I can do is to disconnect both of the neutrals, turn everything on the house,


That probably isn't a good way to trouble shoot this. If there is a multi wire branch circuit involved you may be frying some items which won't go over very well with the owner.


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## Greg Sparkovich (Sep 15, 2007)

I only use things like lights, fans, lamps etc. All computers, stereos TVs etc are unplugged.
I could not get voltage on either neutral. However I found another circuit that obviously has a shared neutral because when I took the neutral off the bus the lights didn't go off. BUT WAIT...! That's not all: then when I separated the neutral and wires the lights went dim. That probably means that the ground is providing a neutral somewhere else in the house.
I'm sure these people hate me by now. :-(


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## telsa (May 22, 2015)

You wouldn't happen to have any three-way light switching in the home ?

If so look for Carter switching// Colorado switching// California 3-way... and any of the other nic names it's picked up.


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## Greg Sparkovich (Sep 15, 2007)

Dennis Alwon said:


> Try and find where the junction is for the multiwire branch circuit if in fact it is one. Split it there. I usually go about halfway thru the circuit and disconnect everything and see if the first half works. If it does then the problem is down the line. Go half again until you find it


Two things: Yes, that is how I generally TS AFCI failures. Divide and conquer.
Also: Dude, you have awesome taste in music!



Fibes said:


> That probably isn't a good way to trouble shoot this. If there is a multi wire branch circuit involved you may be frying some items which won't go over very well with the owner.


I appreciate this, but that's kind of like saying "Don't ever disconnect a neutral -even if the circuit is off- because it might be connected to another voltage feed". Wise advice, but not always practical.
The thing I do to mitigate destruction of property is to try to make the only loads be lightbulbs/non electronics. I have definitely seen a bulb get brighter due to a voltage spike when the issue is a bad neutral. Hot tip: a terrific tool when mapping circuits is a box full of nite-lites -readily available at a $1 store. 

So, this leads me to ask: how would you (quickly) search for a 3rd circuit if not through a load?


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## ELECTRICK2 (Feb 21, 2015)

I was moving stuff around, I got a shock from that wire -after I had the hot, neutral, and ground disconnected. Meaning that there is voltage on a disconnected wire.
Above pasted from Greg's post
Which wire had voltage?


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## ELECTRICK2 (Feb 21, 2015)

Forgot to ask in previous post, how would AFCI help in this case?


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## Greg Sparkovich (Sep 15, 2007)

ALL the wires had voltage -which I could expect from the hot and neutral, but not the ground. 

The plan for the AFCI was/is to add safety. As far as I'm concerned, this shows exactly why AFCIs should be required on old circuits more than new.

So, it's clear to me that this will be a "divide and conquer" job. A typical 'find the AFCI problem' situation. I was hoping that by putting the ground and neutral together on the AFCI, I would "eliminate" the ground to neutral fault between all the circuits (if for example, there was simply continuity through an incandescent lightbulb).

So now I'm stuck with a voltage leak between hot and ground. And that is also confirmed by getting continuity between the disconnected hot and ground.


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## Dennis Alwon (May 9, 2009)

If there is a leak from hot to ground it would blow the breaker

If you got voltage from neutral to ground then you probably have an open neutral


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## MF Dagger (Dec 24, 2007)

What are you using to test? Try an older shaker style wiggy. EDITED... Didn't read far enough, my gut first instinct would be to go look for the BX hacked onto the knob and tube


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## Greg Sparkovich (Sep 15, 2007)

Dennis Alwon said:


> If there is a leak from hot to ground it would blow the breaker
> 
> If you got voltage from neutral to ground then you probably have an open neutral


Yeah, I would say that sounds right, But I have definitely found circumstances where there is a low-amperage "leak" from hot to ground. Specifically in a bathroom where cloth romex was plastered into a tile wall with metal lathe that had gotten wet (there was also a shared neutral in that circuit and I ended up rewiring the 2nd and 3rd floors of a victorian twin).

In essence, this is the same danger of knob and tube that gets wet in an attic and leaks from hot to neutral...but never trips the breaker.




MF Dagger said:


> my gut first instinct would be to go look for the BX hacked onto the knob and tube


Yes; I'm suspicious of that. Possibly the bx is hitting a water pipe or metal mesh used for plaster patching.


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I'm satisfied that I have explored the "shortcuts" to figuring this out and I'm stuck with the arduous task of trial and error exploring the two circuits. I get this kind of stuff a few times a year, but I often second guess myself wondering if there is something smarter that I could trying. Or some cool tool that I should get to help me figure this out faster. 

If I find anything interesting, I'll post...in the end, I will also be looking at just rewiring -since in the long run that might be more valuable to the customer.


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