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How to find/remedy a shared neutral


100yroldhouse's Avatar
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11-13-17, 07:18 PM   #1 (permalink)  
How to find/remedy a shared neutral

Hello. My first posting here. I hope I explain properly.

I have an old house, with 1 circuit still 2-wire (knob and tube). For the time being (ie. until I can afford to rewire the house), I have put in a gfci breaker for that circuit.

Unfortunately, now every time I use the lights on a separate circuit, the gfci breaker trips. I have had 3 electricians I know, who can't do the work due to licensing/insurance reasons (they're all commercial and I need residential), tell me this is caused my a shared/common neutral.

My budget is crap, and I currently am living with a dead (gfci) circuit. I have 2 questions:
1. How do I find the shared neutral?
2. How do I fix it once I find it?

If more info is needed to answer, please ask. Thanks for any help.

 
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Tolyn Ironhand's Avatar
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11-13-17, 07:28 PM   #2 (permalink)  
The shared neutral would originate in the panel. If it is a shared neutral you should see 3 wires (2 hots and 1 neutral) leaving the panel.

I am a little concerned you have circuit breakers but still have a knob and tube circuit. Really you should have 2 knob and tube circuits on a multiwire.


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Last edited by Tolyn Ironhand; 11-14-17 at 07:13 PM.
 
100yroldhouse's Avatar
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11-13-17, 07:44 PM   #3 (permalink)  
Unfortunately "should" seems to be a pipe dream for this house. I have priced out rewiring at $3500 plus.

I am saving for now, and my property insurance provider is aware of the problem.

In the meantime, I have looked at the source. I think this is the case of previous homeowner diy renos (based on precedent, believe me), and tapping into a junction further up the chain. I will triple-check, though, as I would earnestly appreciate an "easy" fix.

If I am right, how would I find such a beast?

 
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11-13-17, 08:08 PM   #4 (permalink)  
I have never seen a shared neutral on knob and tube.

 
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11-13-17, 08:14 PM   #5 (permalink)  
I think what happened is that something was tapped off an incorrect neutral.
It wouldn't be noticed until the GFI was installed.

The chances of finding it are slim to none.


~ Pete ~

 
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11-13-17, 08:34 PM   #6 (permalink)  
Let's pretend for a moment, that the improbable/impossible can actually happen and focus on possible solutions, please. This house has been nothing but one impossibity after another, and this is the next.

If it helps, pretend I didn't tell you abput the 2-wire feed and suggested instead a mwbc. Does that help? How do I find it? How do I remedy it?

 
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11-13-17, 09:19 PM   #7 (permalink)  
When you have K & T wiring..... the two wires are just run thru insulators in the wall. There is really no way of identifying the circuits.

Is the light that causes the GFI to trip on the same circuit ?
Be careful now because if you turn off the GFI it would appear to kill that light when all it's doing is opening the neutral to that light. You can temporarily put a regular circuit breaker back in to see if that light is on a different circuit.

If it is indeed on a different circuit...... the problem is that the neutral to the light was connected to the wrong circuit.


~ Pete ~

 
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11-14-17, 06:05 AM   #8 (permalink)  
Can you at least find all of the lights and receptacles on a different circuit that trip the ground fault circuit interrupter breaker? Are all these other lights and receptacles on just one circuit? To double check your answer, can you then flip off just one other breaker and those items are now dead and everything else including the GFCI protected items work correctly?

With just one other breaker's worth of items affected, you will have identified the other half of a multiwire branch circuit. Correctly the voltage from this hot to the GFCI hot must be about 240 volts. You have two choices, (1) remove the GFCI breaker nd put back a regular breaker, or (2) installing a 2 pole GFCI breaker to take care of both circuits.

If you have or suspect a shared neutral, do not unhook that neutral when any of the hot lines using that circuit are still live.

A shared neutral can exist in knob and tube wiring. The neutral runs through the middle of the house, say all the way from side to side. The hot for one leg runs along the front wall. The hot for the other leg runs along the back wall.

More confusing is hot and neutral knob and tube for one branch circuit running along the front of the house and a second knob and tube circuit in back has hot only and sharing the neutral up front. You need to find out whether you have something like this because use of a back receptacle will trip a ground fault circuit interrupter protecting the front circuit.

In modern circuits the hot and neutral must accompany one another. In the case of a multiwire branch circuit both hots and the neutral must be together.

When a new light or receptacle etc. is added, when hot is taken from a branch circuit, the neutral of the same circuit must be used.

Nothing may be added to a knob and tube circuit, a grandfathered circuit, or any other non-compliant circuit although a portion of a non-compliant circuit may be decommissioned and a modern and compliant circuit extended to re-energize an orphaned portion of a non-compliant circuit pending upgrading of the latter.


Last edited by AllanJ; 11-14-17 at 06:43 AM.
 
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11-14-17, 06:09 AM   #9 (permalink)  
The gfci circuit breaker trips if any light on the other circuit is turned on. Haven't tried the receptacles.

The gfci circuit runs the entirety of my upstairs (half-story). The second circuit is for the kitchen, bathroom and main floor bedroom. All of the receptacles upstairs are ungrounded, so ESA inspector told me the gfci breaker temp shortcut.

I might do what the one electrician suggested and put the regular breaker back in, and put gfci receptacles at every box instead, as I can't find the beginning of the circuit to just put one there.

 
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11-14-17, 06:31 AM   #10 (permalink)  
I too have been trouble shooting some incorrectly shared neutrals in recent weeks in an older house. Those were meant to be MWBCs but things were messed up by DIYers who followed the original work, so breakers were moved around so they are no longer adjacent and handle tied or even on opposite legs, and one circuit had the neutral conductor mis-connected. For example, 3B/5A is one MWBC and 5B/7A is another. However somewhere downstream the neutral for 5A and 5B got pigtailed together. I had a real hard time finding it. Had to open all the junction and switch boxes and trace the whole thing from end to end. Then at times disconnect wiring from the devices at each box and do continuity test using a combination of tone probe and pulling and tugging.

As far as "finding the beginning of the circuit and put one there", you may find it as a byproduct of your circuit trace and trouble shooting. But even if you found it, in my case I had to deal with three circuits, the first may be a switch box too small to accommodate a GFCI receptacle, or a ceiling box with a light fixture which makes it not practical. What I did was I created the first box for all three circuits. I mounted a 3 gang box right next to the panel, put in three GFCI receptacles. Then I made each of the circuit to run to that box and back. I won't really be using those receptacles.

 
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11-14-17, 07:18 PM   #11 (permalink)  
Unfortunately "should" seems to be a pipe dream for this house.
By "should" I mean a multiwire circuit is two hots on opposite legs with a shared neutral and therefore two circuits (2 fuses or circuit breakers). Before the requirement for a common disconnect for a multiwire circuit, it was common for a multiwire to be on two single pole breakers. If yours is not as I describe, then you don't have a multiwire circuit, you have a miswired circuit. If you do have two hots and one neutral you could just protect them with a two pole GFCI breaker.


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11-14-17, 10:11 PM   #12 (permalink)  
Why GFCI at all? Put in 2-prong receptacles like was originally installed.

 
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11-15-17, 07:19 AM   #13 (permalink)  
A GFCI unit provides near perfect protection from electrocution with or without an equipment grounding conductor.

A shared neutral may not go through a GFCI unit and come out the other end (the load terminals) unless both hots sharing that neutral also go through and come out the other end (the GFCI being a 2 pole 120/240 volt model).

A single pole GFCI may be installed on a multiwire branch circuit where the GFCI load terminals are left empty or a 2 wire 120 volt independent subcircuit comes off of those load terminals to feed additional receptacles and lights.. This subcircuit is treated as if it came out of a panel. No part of this subcircuit may be connected to any other existing hot or neutral wire in the house.

Many metal outlet boxes in older homes are too small to hold a GFCI receptacle.

 
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11-15-17, 08:37 AM   #14 (permalink)  
A GFCI unit provides near perfect protection from electrocution with or without an equipment grounding conductor.
Do you work for a GFCI manufacturer? Put your finger in a light socket and see how the GFCI protects you. GFCI only works if you have an EGC or other path back to the electrical source. So works in the kitchen/bathroom if your pipes are grounded.

 
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11-15-17, 04:20 PM   #15 (permalink)  
You can get a shock from a circuit protected by a GFCI but the amount of current that can flow through you in the event of accidental or casual contact with live wires on the GfCI protected circuit before the GFCI trips should not be enough to kill you.

 
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11-15-17, 09:54 PM   #16 (permalink)  
Allan.... you are incorrect. If you touch the white wire and the black wire coming from GFI protection..... you will be killed. The GFI only senses imbalance to an actual ground. In this case there is no actual ground.

Now.... let's get back on topic.


~ Pete ~

 
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