GFCI Shared neutral

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102 Inspector

Senior Member
Location
N/E Indiana
Occupation
Inspector- All facets
Never been asked this question -
Can a circuit be run to a bathroom with 12/3 and use the black for GFCI receptacle branch and red for general lighting and bath fan with a shared neutral. I do not believe this set up will work, but never really checked into it. Any comments that will help me look smarter?
 

kwired

Electron manager
Location
NE Nebraska
Never been asked this question -
Can a circuit be run to a bathroom with 12/3 and use the black for GFCI receptacle branch and red for general lighting and bath fan with a shared neutral. I do not believe this set up will work, but never really checked into it. Any comments that will help me look smarter?
Yes, it is called a multiwire branch circuit and the black and red must be on opposing ungrounded supply conductors so that the neutral only carries unbalanced current.

Anything on the load side of GFCI must tie to the load terminals of the GFCI and keep all line side wiring off the load side terminals or you will have troubles with the GFCI (and this is not dependent on code it is just how they work).
 

102 Inspector

Senior Member
Location
N/E Indiana
Occupation
Inspector- All facets
I guess I was thinking that the multi-wire would not work with the GFCI. Admittedly, understanding the GFCI protection is not a science that I fully understand, but would the neutral load of the general lighting wire cause false tripping of the receptacle or is it stand alone and only recognizes what is load side of the receptacle?
 

goldstar

Senior Member
Location
New Jersey
Occupation
Electrical Contractor
I would like to add, as long as you're asking this question, that you cannot "cascade" the neutral (I.e. Wire into the load side of the GFCI and then back out). You have to splice 2 white wires to the feed neutral (pigtails) and then connect those wires to both the GFCI receptacle and lighting circuits respectfully. By the same token you cannot use the load side hot or neutral for any other purpose other than another receptacle in that bathroom. You also have to use #12's for the lighting circuit. Overkill IMHO but yes you can do it.
 

Sierrasparky

Senior Member
Location
USA
Occupation
Electrician ,contractor
I guess I was thinking that the multi-wire would not work with the GFCI. Admittedly, understanding the GFCI protection is not a science that I fully understand, but would the neutral load of the general lighting wire cause false tripping of the receptacle or is it stand alone and only recognizes what is load side of the receptacle?

As long as a Single pole or wall box style GFCI is not used in the circuit before the MWBC is split off. If so any current from the second circuit flowing on the neutral not part of the circuit that wich the GFCI is protecting will cause the the GFCI to trip.

The entire current path for a load must flow though the circuitry of that GFCI in order to work.
 

al hildenbrand

Senior Member
Location
Minnesota
Occupation
Electrical Contractor, Electrical Consultant, Electrical Engineer
. . .would the neutral load of the general lighting wire cause false tripping of the receptacle or is it stand alone and only recognizes what is load side of the receptacle?
Here. Think of it this way.

Think of a GFCI breaker connected at a panel. The bus and the neutral of the panel are "like" a multiwire branch circuit in that there are two hots and a shared neutral.

The GFCI breaker is attached to one hot and the shared neutral of the panel. The protected branch circuit from the GFCI breaker is a hot an a neutral. To work and have the GFCI protection, the LOAD side wiring of the GFCI breaker has to not by crossed over to other hots or other neutrals.

In the situation you ask about in your opening post, everything works if the GFCI receptacle LINE is taken from one hot and the multiwire home run neutral, and then the LOAD side wiring of the GFCI receptacle (if there is any) is kept from connection (crossed over) to the hot or neutral of other circuits. Any thing plugged into the receptacle of the GFCI is only a hot an a neutral, so that is not crossed over.
 

102 Inspector

Senior Member
Location
N/E Indiana
Occupation
Inspector- All facets
Thanks for the explanation and better understanding. It is the simple things that make you stop and question yourself. I can remember this in the future.
 

Dennis Alwon

Moderator
Staff member
Location
Chapel Hill, NC
Occupation
Retired Electrical Contractor
As long as the neutral to the lighting circuit is not connected to the load side of the gfci this is not an issue. I think many people just assume the neutral to go thru the gfci receptacle-- if so then it would not work as the gfci will see current returning on the neutral only causing it to trip.
 

GoldDigger

Moderator
Staff member
Location
Placerville, CA, USA
Occupation
Retired PV System Designer
As long as the neutral to the lighting circuit is not connected to the load side of the gfci this is not an issue. I think many people just assume the neutral to go thru the gfci receptacle-- if so then it would not work as the gfci will see current returning on the neutral only causing it to trip.
Either I am not understanding your statement or you need to look at it again.
If an ungrounded conductor is extended through a GFCI receptacle, then the corresponding neutral must also extend through the sensing coil of that GFCI.
But that neutral must no longer be part of an MWBC. The wiring from there on must involve only that hot and that neutral.
If the extended circuit must continue to be an MWBC, then neither hot nor neutral will come from the load terminals of the GFCI and it will not provide any protection to that extended circuit.
 

John120/240

Senior Member
Location
Olathe, Kansas
Lets try this: 12-3 home run. Home run RED to out going BLACK. Home Run BLACK to LINE side GFCI. Home Run Neutral (White) pig tails so you have three wires under wire nut with one tail which goes to Line side GFCI. The other White is your Out Going Neutral.

Tie downstream GFCI protected recptacles to GFCI Load. Any Neutral load on the RED circuit goes thru the Whites under the wire nut which is ahead of the GFCI.
 

Dennis Alwon

Moderator
Staff member
Location
Chapel Hill, NC
Occupation
Retired Electrical Contractor
Either I am not understanding your statement or you need to look at it again.
If an ungrounded conductor is extended through a GFCI receptacle, then the corresponding neutral must also extend through the sensing coil of that GFCI.
But that neutral must no longer be part of an MWBC. The wiring from there on must involve only that hot and that neutral.
If the extended circuit must continue to be an MWBC, then neither hot nor neutral will come from the load terminals of the GFCI and it will not provide any protection to that extended circuit.

You are not understanding or I am not clear- Trust me I do know how it works but obviously I wan't clear
 

electricalist

Senior Member
Location
dallas tx
I have heard others say they ran a 12/3 for sabc circuit, so the (Mr. john 120/240)
red is one circuit and the black is the second and the neutrals wirenutted ahead of the gfci. Black/neutral goes to one gfci (line )red/neutral can go to another gfci(line)
 

black_thorn

Member
Location
Pa
Another thing you need to do is use a double pole breaker or tie 2 breakers together that are sharing the neutral. Both circuits must turn off together when sharing a neutral.
 

black_thorn

Member
Location
Pa
Worst code change in my lifetime.

I think it's a good idea. Especially in panels where neutrals aren't marked with the circuit numbers. Makes working on circuits a lot safer. You might turn a circuit off and think the neutral isn't carrying a load, then go to separate the neutral and it's still carrying the load of another citcuit. Not a safe situation. Especially in commercial and industrial settings.
 

kwired

Electron manager
Location
NE Nebraska
I think it's a good idea. Especially in panels where neutrals aren't marked with the circuit numbers. Makes working on circuits a lot safer. You might turn a circuit off and think the neutral isn't carrying a load, then go to separate the neutral and it's still carrying the load of another citcuit. Not a safe situation. Especially in commercial and industrial settings.
Last two code cycles also require identifying which ungrounded conductors the neutrals go with though. A single cable or single raceway with one MWBC in it is good enough for this identification, but multiple MWBC's in a raceway need to be some sort of grouping or other effective means to ID which neutral goes with which ungrounded conductors.

I'd need to look but I think 2014 even requires this identification at junction boxes and other points where the conductors are accessible, and not just at the source end of the circuit.
 

black_thorn

Member
Location
Pa
Last two code cycles also require identifying which ungrounded conductors the neutrals go with though. A single cable or single raceway with one MWBC in it is good enough for this identification, but multiple MWBC's in a raceway need to be some sort of grouping or other effective means to ID which neutral goes with which ungrounded conductors.

I'd need to look but I think 2014 even requires this identification at junction boxes and other points where the conductors are accessible, and not just at the source end of the circuit.

200.4 B requires identification of neutral or grouping of wires in an enclosure.
 

ActionDave

Chief Moderator
Staff member
Location
Durango, CO, 10 h 20 min from the winged horses.
Occupation
Licensed Electrician
I think it's a good idea. Especially in panels where neutrals aren't marked with the circuit numbers. Makes working on circuits a lot safer. You might turn a circuit off and think the neutral isn't carrying a load, then go to separate the neutral and it's still carrying the load of another citcuit. Not a safe situation. Especially in commercial and industrial settings.
That can happen with any circuit, old or new, labeled or not.
 

electricalist

Senior Member
Location
dallas tx
One of my regulars is a sheriff's dept and the mwbc s that feed cells , lights, and offices. All start in diff cells. Then split off. Twice someone took the gfci out and seperated the home run neutral from the others and left the hots together. Burnt up lots of things.
My guess is they checked each neutral to the gfci till it reset then put the others back together.
I'm supposed to find a way to crimp the neutrals permanently.
 
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