GFCI Receptacles, Shared Neutral

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dpenbert

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I am a little unclear of why it is acceptable to share a neutral between a circuit that has GFCI receptacles on the load and a circuit that has normal receptacles. For example, in a kitchen, there is a duplex receptacle for a gas cooktop and (3) GFCI duplex receptacles serving countertop. If the cooktop is on circuit #17 and the GFCI convenience receptacles are on circuit #19, is it ok to run a homerun consisting of circuits 17,19,one neutral, one ground? The homerun into the cooktop receptacle box and then circuit 19,the shared neutral and ground over to feed the GFCI receptacles. I have heard two different stories of how this isn't right and how it is just fine. Can someone explain why this is or isn't acceptable?

Also, what is the difference if a GFCI breaker was used for the receptacles? Why would you not share a neutral?

I have designed shared neutrals for GFCI receptacles for some time now and separate neutrals for GFCI breakers but I don't understand the explanation..

Thanks,

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David-St. Louis
 
Re: GFCI Receptacles, Shared Neutral

Originally posted by dpenbert:
If the cooktop is on circuit #17 and the GFCI convenience receptacles are on circuit #19, is it ok to run a homerun consisting of circuits 17,19,one neutral, one ground? The homerun into the cooktop receptacle box and then circuit 19,the shared neutral and ground over to feed the GFCI receptacles.
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Yes. GFCI receptacles work fine supplied by a multi-wire circuit.

GFCI circuit breakers have a separate terminal for the neutral wire which must be run with the hot wire.
 
Re: GFCI Receptacles, Shared Neutral

Actually it is as simple as "what goes out must come back" and "if it didn't go out it can't come back".

GFCIs only look downstream, they don't really care what they are connected to. That is why receptacles can be fed from circuits with shared line side neutrals. But if you think about it, a circuit breaker is always fed from a source that also has shared line side neutrals.

So the problem is always with the load side of the GFCI. "False" trips will occur if there are any shared load side neutrals between "hot" conductors that do not start at the same GFCI device whether it is a breaker or a receptacle.
 
Re: GFCI Receptacles, Shared Neutral

Originally posted by dpenbert:
I am a little unclear of why it is acceptable to share a neutral between a circuit that has GFCI receptacles on the load and a circuit that has normal receptacles. For example, in a kitchen, there is a duplex receptacle for a gas cooktop and (3) GFCI duplex receptacles serving countertop. If the cooktop is on circuit #17 and the GFCI convenience receptacles are on circuit #19, is it ok to run a homerun consisting of circuits 17,19,one neutral, one ground? The homerun into the cooktop receptacle box and then circuit 19,the shared neutral and ground over to feed the GFCI receptacles. I have heard two different stories of how this isn't right and how it is just fine. Can someone explain why this is or isn't acceptable?

Also, what is the difference if a GFCI breaker was used for the receptacles? Why would you not share a neutral?

I have designed shared neutrals for GFCI receptacles for some time now and separate neutrals for GFCI breakers but I don't understand the explanation..

Thanks,

------------------
David-St. Louis
Click to expand...
David:

The explanation lies in an understanding of how a GFCI works. The GFCI circuitry measures the current on the hot and neutral conductors and compares them. In the absence of any ground faults, the current returning through the neutral is the same as the current going out the hot conductor. If there is a ground fault, then some current is returning through the EGC or through ground. If the difference in current flow between hot and neutral exceeds 6ma the GFCI trips.

So, in your design where you have a shared neutral to the cooktop and extend the neutral and circuit #19's hot to the GFCI the GFCI is comparing the current on the neutral that flows through its internal receptacles and any load side receptacles to the current in the #19 hot. Absent any fault, they will always be equal. So this solution works.

On the other hand, if #17 is a standard breaker and #19 is a GFCI breaker sharing the same neutral, then the total neutral current is NOT inherently equal to the hot current on only the GFCI circuit and the GFCI cannot function properly. I'm not entirely clear on how you would design such a circuit in any event, since the neutral on the GFCI breaker circuit returns to the breaker so that the comparison can be made. If the shared neutral returns to the neutral bar, no neutral current would be seen by the GFCI breaker. If the neutral returned to the GFCI, it would see ALL of the current from both circuits. Either one would obviously not work.

Hope this helped.

Mike

[ January 20, 2006, 03:04 PM: Message edited by: Mike03a3 ]
 
Re: GFCI Receptacles, Shared Neutral

I think those explanations help me out. I thought along the same lines I just had trouble explaining it. Any other input is still welcome.

Thanks

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David-St. Louis
 
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