Possible Shared Neutral - How to Troubleshoot?

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Old 03-31-13, 11:21 PM
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Possible Shared Neutral - How to Troubleshoot?

My house is older and most of the wiring is still knob and tube.

I am in the process of installing an exhaust fan in my 1st floor bathroom. Prior to this project, there was just a single light fixture, a single switch. All are being fed from the same circuit.

When I pulled out the switch, I noticed it had three wires connected:

1 x black wire (the "hot" wire)
1 x black wire (going to the light)
1 x wire connected to the green ground nut (looked suspiciously like a neutral knob and tube wire, not bare copper)

As I said, I was suspicious of the "ground" wire. So, I removed the light fixture as well. This also had three wires:

1 x black wire (coming from the switch)
1 x neutral wire
1 x wire connected to the green ground nut (this also looked suspiciously like a knob and tube neutral wire, not bare copper).

Since this was knob and tube (no ground), something seemed out of place.

I removed the 1-gang box behind the switch and looked into the wall. It was pretty clear that the "ground" wire was connected to some white wire ceramic knob and tube running vertically through the wall. So, likely this "ground" was actually a neutral.

On the light fixture side, I have not yet determined where either the neutral or "ground" go. It's possible they all connect back to the same white knob and tube near the switch, but hard to say without ripping away a lot of drywall.

So, for the short term, I clipped the "grounds" away from both the switch and the light fixture as they were likely not true ground wires. About the same time, the power in my 2nd floor bathroom, directly above the 1st floor bathroom, stopped working (didn't find out for a day or two). Upstairs, I was getting a low voltage reading at the switch. when I pulled off the upstairs light fixture I noticed a similar wiring config:

1 x black wire (hot)
1 x white wire (neutral)
1 x ground wire (also appeared to be "white" knob and tube)

I'm suspecting I've run into some sort of shared neutral issue that I need to deal with, but I don't know enough about home electric to troubleshoot.

Any ideas? My thoughts are that if I can clearly match up hot / neutral pairs at in the wall with with hot / neutral pairs at the service panel, I can make sure that I wire things correctly (without a shared neutral) and fix the power issue in the 2nd floor.

Is this the correct approach? Or should I think about this differently?
 
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Old 03-31-13, 11:43 PM
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Welcome to the forums.

"like a neutral knob and tube wire"

You can't tell by looking which is hot or neutral. K & T wasn't labeled. Both wires are black and are in white porcelain tubes. K & T wiring didn't use a ground. I've been in a house where someone actually fished in a green piece of wire for the ground. That is a good thing.

I've been to others where someone figured that since neutral and ground were connected together at some point that they added a jumper from the ground screw to neutral.....either in the box or just outside. Then they installed three prong receptacles. That's a disaster waiting to happen. That may be what you have.

Any ideas ?
Yes.....work towards removing the knob and tube wiring.

You have a reduced voltage circuit. That could be a loose splice in the wall. The only way to know for sure if it's a neutral or hot problem is to check the hot wire with a meter directly to a cold water ground. If you read full voltage....it's a neutral problem.

If the K & T goes directly to the panel.......they usually brought the hot and neutral wires into the panel thru adjacent knockouts so it should be fairly easy to see leaving pairs. However.....who knows what was done in the walls.
 
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Old 04-01-13, 05:43 AM
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RE: Possible Shared Neutral - How to Troubleshoot?

PJMaxx - first, thank you for the quick and informative reply. Looks like I have a larger project ahead of me (installing romex).

That said, I have one more question. The reason I was assuming the "ground" was actually a neutral was due to wire color. The K&T in my house appears to have both black and white wires.

Here is a picture from my upstairs bathroom:
Name:  wiring-labeled.jpg
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If you can tell from the pic, the "neutral" and so-called "ground" wires appear to be white-ish. Most of the wiring I've seen in the wall boxes tends to be pretty worn, so a lot of the white exterior has crumbled or worn off, but there is usually enough left to tell which wire used to be white...so I was always assuming this was neutral. This has been the case whenever I've had to replace worn receptacles or otherwise look at the wiring in a box.

From what you're saying, it sounds like it's never safe to assume, especially with K&T, correct?
 
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Old 04-01-13, 06:54 AM
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Ground is never white. K&T rarely had a ground (probably never). Properly installed K&T always had a white but the color didn't age well. I slightly disagree with PJ on this point. I have never seen K&T where black was used as neutral. The color may be hard to figure out though but it started life as white.
 
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Old 04-01-13, 07:42 AM
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ray2047 - Thanks for the reply. What you're saying confirms what I've seen to date in my house...worn, but generally recognizable neutral cables that were once white.

Anyway, what you've both confirmed for me is that the former home owner incorrectly (and unsafely) re-purposed a neutral wire as a makeshift "ground".

So, the only way to repair things safely is to run new romex as PJ suggested...a good thing in the long run...not so fun now. LOL, I guess that's the way it goes with older homes.
 
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Old 04-01-13, 11:34 AM
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Remember grounds are not needed for equipment to function (with minor exceptions). Grounds are for added safety. Better no ground then a bootleg ground because that reduces safety. GFCIs can be used without a ground to increase safety.
 
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