Borrowed Neutral

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augie47

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I'm having to "second guess" the term, but, if we are using a neutral designated for a different circuit then you stand the possibility of overloading the neutral and setting up a "backfeed" on the neutral from your circuit. 210.4 was added in '08 to prevent such conditions.
 

dbuckley

Senior Member
The evil is that the unwary can get shocked.

I got it in my own home changing a wall light fitting. The breakers that served the area were off, and meters confirmed no volts present.

I should add I'm in 240V-land, and so am careful to avoid getting in the way of 240V.

Anyway, I undid the old fitting, and got belted off the neutral because the electrician who wired the house a few decades back was an incompetent idiot and put my life in danger. I pigtailed the neutral so at least someone else won't get the same medicine.
 

qcroanoke

Sometimes I don't know if I'm the boxer or the bag
Location
Roanoke, VA.
Occupation
Engineering
I'm having to "second guess" the term, but, if we are using a neutral designated for a different circuit then you stand the possibility of overloading the neutral and setting up a "backfeed" on the neutral from your circuit. 210.4 was added in '08 to prevent such conditions.
Got one in my home, they tend to get your attention.
 

mull982

Senior Member
Can someone please explain to me what a borrowed neutral is I cant seem to figure out what we are talking about here.
 

Dennis Alwon

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Chapel Hill, NC
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Electrical Contractor
Can someone please explain to me what a borrowed neutral is I cant seem to figure out what we are talking about here.
I believe it is using a neutral from a different circuit. This is not only illegal, unless it is part of a mwbc, but cause unwanted emf's.
 

mull982

Senior Member
I believe it is using a neutral from a different circuit. This is not only illegal, unless it is part of a mwbc, but cause unwanted emf's.
I'm getting confused with the terminology here.

What is the difference between a borrowed neutral and one that is part of a mwbc? It is my understanding that a mwbc has multiple "hot" circuits sharing one common neutral. Is this correct?
 

Dennis Alwon

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Chapel Hill, NC
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Electrical Contractor
I'm getting confused with the terminology here.

What is the difference between a borrowed neutral and one that is part of a mwbc? It is my understanding that a mwbc has multiple "hot" circuits sharing one common neutral. Is this correct?
It would be like taking a neutral from a circuit that has nothing to do with the circuit you are using. Think of 2- two wire circuits and let's say there is a switch on one circuit that doesn't have a neutral in it- just a hot and return. Now you want to add a receptacle in that switch but the closest ad easiest place is to steal a neutral from that other circuit.
 

mull982

Senior Member
It would be like taking a neutral from a circuit that has nothing to do with the circuit you are using. Think of 2- two wire circuits and let's say there is a switch on one circuit that doesn't have a neutral in it- just a hot and return. Now you want to add a receptacle in that switch but the closest ad easiest place is to steal a neutral from that other circuit.
I thought this is esentially what a mwbc was. Two circuits that share a common neutral. I guess I am wrong about this. Can you explain a mwbc then?
 

mivey

Senior Member
I thought this is esentially what a mwbc was. Two circuits that share a common neutral. I guess I am wrong about this. Can you explain a mwbc then?
Two issues:
1) The borrowed neutral may be on a circuit that is on the same ungrounded leg in the panel so you could have double amps on the neutral.

2) The borrowed neutral is not run along with the ungrounded conductor, creating emfs and inductive heating hazards.
 

jumper

Senior Member
2008 NEC

300.3(B) Conductors of the Same Circuit. All conductors of
the same circuit and, where used, the grounded conductor
and all equipment grounding conductors and bonding conductors shall be contained within the same raceway, auxiliary
gutter, cable tray, cable bus assembly, trench, cable, or
cord, unless otherwise permitted in accordance with
300.3(B)(1) through (B)(4).
 

roger

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Electrician
I thought this is esentially what a mwbc was. Two circuits that share a common neutral. I guess I am wrong about this. Can you explain a mwbc then?
Even though a MWBC "shares" a neutral with it's ungrounded conductors and can be considered multiple circuits it is only "one" circuit in reality, so the neutral is not borrowed, it is part of the "one" circuit.

Roger
 

mull982

Senior Member
Even though a MWBC "shares" a neutral with it's ungrounded conductors and can be considered multiple circuits it is only "one" circuit in reality, so the neutral is not borrowed, it is part of the "one" circuit.

Roger
Can you please give an example of a mwbc?
 

roger

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Fl
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Electrician
Can you please give an example of a mwbc?
Say you run a 12/3 to a kitchen counter top to supply the required two SA circuits, the neutral would be a part of the circuit and not borrowed.

It would serve the purpose as shown below.



Roger
 

adelle

Member
A MWBC works on the principle of phase cancellation. As the two hot leads are 180 degrees out of phase, a 12awg neutral can handle 40amps because it's only "sees" 20amps at a time.

It gets a lot more complicated in three phase circuits!

Interesting story:

In my house, a 1990 build, They used tandem breakers through out and a lot of MWBCs. They wired BLACK-RED-BLACK-RED all the way down. You pros here will quickly see the problem! I fixed it myself but this was a large tract builder and was fully inspected! I'll bet every other house in the tract is done the same way!
 
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Barndog

Senior Member
Location
Spring Creek Pa
[QUOTE

In my house, a 1990 build, They used tandem breakers through out and a lot of MWBCs. They wired BLACK-RED-BLACK-RED all the way down. You pros here will quickly see the problem! I fixed it myself but this was a large tract builder and was fully inspected! I'll bet every other house in the tract is done the same way![/QUOTE]

the shared the neutral with the Black and the Red which added the two currents on the neutral. which if they were fully loads could have caused some problems.
 

mull982

Senior Member
Say you run a 12/3 to a kitchen counter top to supply the required two SA circuits, the neutral would be a part of the circuit and not borrowed.

It would serve the purpose as shown below.



Roger
In the picture example that you showed if this were a (3) phase system and these two phases were connected to the neutral as shown would this be considered a mwbc or borrowed neutral?
 

roger

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Location
Fl
Occupation
Electrician
In the picture example that you showed if this were a (3) phase system and these two phases were connected to the neutral as shown would this be considered a mwbc or borrowed neutral?
It would still be a MWBC even though the current wouldn't cancel the same as it does in the single phase scenario.

Roger
 
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