Borrowed Neutral
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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
- Location
- Canterbury, New Zealand
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.
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
- Sorta retired........
Got one in my home, they tend to get your attention.
- Location
- Chapel Hill, NC
- Occupation
- Retired Electrical Contractor
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?
- Location
- Chapel Hill, NC
- Occupation
- Retired Electrical Contractor
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
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
- Location
- 3 Hr 2 Min from Winged Horses
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).
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).
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?
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
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!
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.
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.
kwired
Electron manager
- Location
- NE Nebraska
- Occupation
- EC
I always thought if you borrowed something it is common courtesy to return it when you were finished with it.
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?
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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