Back fed neutral

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zcanyonboltz

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What are people referring to when they say there is a back fed neutral somewhere or the problem is a back fed neutral, I was troubleshooting today and problem still not found, my boss at the end of the day when leaving said I likely have a back fed neutral or lost neutral somewhere. Thanks
 
I would think he is referring to a neutral that is from another circuit and feeding the one in question.

A lost neutral in my opinion is a whole different thing. It is just that. A neutral or grounded wire not completing the circuit.
 
Sierrasparky said:
I would think he is referring to a neutral that is from another circuit and feeding the one in question.

A lost neutral in my opinion is a whole different thing. It is just that. A neutral or grounded wire not completing the circuit.
Click to expand...

Yeah, a back fed neutral and lost neutral are 2 different things.
 
Sierrasparky said:
I would think he is referring to a neutral that is from another circuit and feeding the one in question.

A lost neutral in my opinion is a whole different thing. It is just that. A neutral or grounded wire not completing the circuit.
Click to expand...

He said it was likely one of these two issues or both. I understand what a lost neutral is but I'm not sure what he meant by backfed neutral, then it was time to go home.
 
jumper said:
What were you trouble shooting and why? What did you find?
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Sorry if this is too long but this is one of the weirdest things I've found and some of the first 3 phase I've worked on.
I'm working in a 160 unit dorm building built in the 1960s with 120 208 Y panels run in EMT. An outlet in a dorm room is only getting 60 volts when I check hot to neutral screw with my volt meter. I took the receptacle off and got 90 volts when I went hot to neutral wire, there is no grounding conductor on the circuit, the outlet with 90 volts is the end of a circuit with just one hot and one neutral in the box.

I know the box on the adjacent wall feeds the outlet with the low 90 volts, this box has three neutrals in it and two hots, I know one hot and one neutral go the box with the low 90 volts by doing a continuity test and voltage test. Now the weird part- In the box with the three neutral conductors I have 125 volts between hot and neutral on two of the neutrals and 206 volts between hot and neutral with one of the neutrals. When all three neutrals are tied together, which is how it was when I opened it all three neutrals were on the receptacle, I have 125 volts between all neutrals and hot.

I tied the two neutrals with 125 volts together and and left the one with 206 volts alone and still had 90 volts at the low voltage 90 volt outlet, so I tied the neutral with 206 volts to the neutral going to the low voltage 90 volt outlet and got 169 volts and my plug tester was reading there was a ground there even though the receptacle was off the box.

One other thing I have 125 volts between neutral and ground when I go from two of the neutrals to the box, and between the two hot conductors I have 60 volts and I know one of the hots goes to the dead end outlet with just 90 volts. When I put the receptacle back on the box voltage drops to 60 again.

After about two hours I ran out of time, closed it up, talked to boss for a few minutes and went home ... now I keep thinking about what it could be. Thanks for reading.
 
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You said the system was in EMT so what voltages do you have to the conduit from both the hot and the neutral?

You have to be careful opening neutrals if there is any possibility of a multi-wire circuit. The 208 could be the series parallel circuit voltage when you opened the neutral of a multi-wire circuit. In many cases opening the neutral will let the magic smoke out of electronic equipment.

Without more information, I would say you have an open neutral on the circuit you were troubleshooting, but there are other possible causes, including an open hot or tripped breaker. If there are no loads on the hot and it is in the conduit with other energized conductors, a high impedance meter can show that kind of voltage on a open hot.
 
zcanyonboltz said:
Sorry if this is too long but this is one of the weirdest things I've found and some of the first 3 phase I've worked on.
I'm working in a 160 unit dorm building built in the 1960s with 120 208 Y panels run in EMT. An outlet in a dorm room is only getting 60 volts when I check hot to neutral screw with my volt meter. I took the receptacle off and got 90 volts when I went hot to neutral wire, there is no grounding conductor on the circuit, the outlet with 90 volts is the end of a circuit with just one hot and one neutral in the box.

I know the box on the adjacent wall feeds the outlet with the low 90 volts, this box has three neutrals in it and two hots, I know one hot and one neutral go the box with the low 90 volts by doing a continuity test and voltage test. Now the weird part- In the box with the three neutral conductors I have 125 volts between hot and neutral on two of the neutrals and 206 volts between hot and neutral with one of the neutrals. When all three neutrals are tied together, which is how it was when I opened it all three neutrals were on the receptacle, I have 125 volts between all neutrals and hot.

I tied the two neutrals with 125 volts together and and left the one with 206 volts alone and still had 90 volts at the low voltage 90 volt outlet, so I tied the neutral with 206 volts to the neutral going to the low voltage 90 volt outlet and got 169 volts and my plug tester was reading there was a ground there even though the receptacle was off the box.

One other thing I have 125 volts between neutral and ground when I go from two of the neutrals to the box, and between the two hot conductors I have 60 volts and I know one of the hots goes to the dead end outlet with just 90 volts. When I put the receptacle back on the box voltage drops to 60 again.

After about two hours I ran out of time, closed it up, talked to boss for a few minutes and went home ... now I keep thinking about what it could be. Thanks for reading.
Click to expand...
If I am understanding correctly - you have three two wire circuits consisting of A-N, B-N, C-N.

You apparently have lost continuity back to the source on one of those neutral conductors, when you jump them together you do get a "back feed" to the one that is lost and things will work, though not as intended.

The reason you see 208 volts from one of the neutral conductors to one of the "hot conductors" is because there is (lets say) A phase supplying a load and you are testing the open side of the neutral from that load to either B or C phase. If you test from that neutral to either ground or a "good" neutral you will read ~120 volts.
 
kwired said:
If I am understanding correctly - you have three two wire circuits consisting of A-N, B-N, C-N.

You apparently have lost continuity back to the source on one of those neutral conductors, when you jump them together you do get a "back feed" to the one that is lost and things will work, though not as intended.
Click to expand...

This is pretty much what my boss was saying, but then how do I have 120 volts on the neutral that goes to the dead end outlet when all three neutrals are separated, this what is making the least sense.
I have voltage on this neutral when I test it to hot in the box with the hot and all three neutrals.
 
zcanyonboltz said:
Sorry if this is too long but this is one of the weirdest things I've found and some of the first 3 phase I've worked on.
I'm working in a 160 unit dorm building built in the 1960s with 120 208 Y panels run in EMT. An outlet in a dorm room is only getting 60 volts when I check hot to neutral screw with my volt meter. I took the receptacle off and got 90 volts when I went hot to neutral wire, there is no grounding conductor on the circuit, the outlet with 90 volts is the end of a circuit with just one hot and one neutral in the box.
Click to expand...

I can't see how removing the receptacle from the box could change L-N voltage. What is the voltage from Hot-Gnd and N-Gnd?

zcanyonboltz said:
I know the box on the adjacent wall feeds the outlet with the low 90 volts, this box has three neutrals in it and two hots, I know one hot and one neutral go the box with the low 90 volts by doing a continuity test and voltage test. Now the weird part- In the box with the three neutral conductors I have 125 volts between hot and neutral on two of the neutrals and 206 volts between hot and neutral with one of the neutrals. When all three neutrals are tied together, which is how it was when I opened it all three neutrals were on the receptacle, I have 125 volts between all neutrals and hot.
Click to expand...

Are you *sure* there's not another receptacle between this box and the problem receptacle?

zcanyonboltz said:
I tied the two neutrals with 125 volts together and and left the one with 206 volts alone and still had 90 volts at the low voltage 90 volt outlet, so I tied the neutral with 206 volts to the neutral going to the low voltage 90 volt outlet and got 169 volts and my plug tester was reading there was a ground there even though the receptacle was off the box.
Click to expand...

Separating neutrals can produce undesired results. Voltages to ground would be helpful.

zcanyonboltz said:
One other thing I have 125 volts between neutral and ground when I go from two of the neutrals to the box, and between the two hot conductors I have 60 volts and I know one of the hots goes to the dead end outlet with just 90 volts. When I put the receptacle back on the box voltage drops to 60 again.

After about two hours I ran out of time, closed it up, talked to boss for a few minutes and went home ... now I keep thinking about what it could be. Thanks for reading.
Click to expand...

I really don't get the part about the hot to neutral voltage changing when you attach the receptacle to the box. That leads me to think you are seeing phantom voltage. A high impedance meter may not be accurate. I would see what the voltage across the receptacle is with a resistive load on the receptacle, such as an incandescent lamp.
 
zcanyonboltz said:
This is pretty much what my boss was saying, but then how do I have 120 volts on the neutral that goes to the dead end outlet when all three neutrals are separated, this what is making the least sense.
I have voltage on this neutral when I test it to hot in the box with the hot and all three neutrals.
Click to expand...
If you open a neutral that is connected to a "live load" you will have full applied voltage from the opened side to the source - as well as to ground (assuming the source has the neutral grounded). Maybe someone can post an image to show what is happening here - I don't have time to look for one/create my own at the moment.
 
Thanks for all the replies and info, I found the problem. There was another receptacle before the low voltage 90 volt outlet behind a fixed picture. I noticed when I went into another unit and saw a receptacle 54" above the floor in the same spot. Opened this receptacle up and found one hot conductor had melted insulation and blackened/burned the receptacle. Copper wire very loosely attached to the receptacle neutral screws. One of the neutrals had broken off the screw and was barely making contact.
 
zcanyonboltz said:
Thanks for all the replies and info, I found the problem. There was another receptacle before the low voltage 90 volt outlet behind a fixed picture. I noticed when I went into another unit and saw a receptacle 54" above the floor in the same spot. Opened this receptacle up and found one hot conductor had melted insulation and blackened/burned the receptacle. Copper wire very loosely attached to the receptacle neutral screws. One of the neutrals had broken off the screw and was barely making contact.
Click to expand...

Good job!!!:thumbsup: Have a cookie.:)
 
Troubleshooting, half of it is reading your meter, half of it is looking for the clue that leads to the obvious problem.

More than half the time, actually about 99.99999% of the time, finding the problem is way harder than the solution.

100% of the time, don't sell yourself short just because you are good at finding the problem.
 
I would be almost positive you have an open neutral, check neutral to ground, or if you have a no-contact voltage checker (chicken stick), check neutral for line voltage. If it's hot, you have a neutral thats not making it all the way to the buss.
 
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