can 2 circuits share a neutral wire?

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  #1  
Old 03-21-07, 05:25 PM
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can 2 circuits share a neutral wire?

I have a house that was built and wired in the mid-60s. All wiring had been done using flexible metal conduit. The kitchen was wired with two appliance circuits. There are two outlets above the kitchen counter--each outlet on a separate appliance circuit.

A couple of months ago I replaced these two standard outlets with GFCI outlets. When I was done with the wiring, I tested each of the two new GFCI outlets with one of those 3-prong test plugs with the 3 neon lites--where the combination of the illuminated bulbs indicates the type of failure. I don't recall the exact failure indicated but it definitely indicated failure. I double-checked my wiring--no problem there.

Eventually I discovered that the wiring of the two appliance circuits shared a single run of conduit coming from the circuit breaker panel--a black wire for one circuit and a red wire for the other circuit and a single white/neutral wire shared by the two circuits. (At this point I reconnected the original outlets.)

Questions:

1) Would the shared neutral wire explain the failure of the GFCI outlets on the two appliance circuits?
2) Is the sharing of a single run of conduit by two circuits a normal practice?
3) Can I pull a 2nd neutral wire thru the same conduit so that each appliance circuit can have its own dedicated neutral wire?
4) If the additional neutral wire is allowed, is there any marking requirement to indicate to which circuit each neutral is assigned?

thanks,
Brent
 
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  #2  
Old 03-21-07, 06:54 PM
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A multi-wire circuit is a circuit that shares one neutral for two 120 volt circuits. Properly wired this is legal and safe. Improperly wired this is a fire hazard.

To use GFCIs you can either replace each receptacle with a GFCI, wiring nothing on the LOAD side of the GFCI. Or you can use a 240 vol GFCI circuit breaker.

You probably cannot simply add a neutral to the conduit.
 
  #3  
Old 03-22-07, 04:43 AM
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Originally Posted by racraft View Post
To use GFCIs you can either replace each receptacle with a GFCI, wiring nothing on the LOAD side of the GFCI.
I don't think the first solution offered is an option in my case because there are 3 or 4 additional receptacles downstream on these 2 appliance circuits--where the 2 kitchen counter receptacles are just the first receptacles on each of their respective circuits.

Originally Posted by racraft View Post
Or you can use a 240 vol GFCI circuit breaker.
The 2nd suggested solution sounds pretty good. If I understand correctly, I would simply remove the 2 adjacent 120v circuit breakers that control the 2 appliance circuits and replace with the 240v GFCI breaker?

Originally Posted by racraft View Post
You probably cannot simply add a neutral to the conduit.
I didn't relish the idea of adding a neutral wire for all receptacles on one of the appliance circuits--I'm sure I would have problems pushing/pulling the wire down that conduit between all those boxes. But besides the difficulty of it, is there some other reason why it couldn't/shouldn't be done?

thanks,
Brent
 
  #4  
Old 03-22-07, 04:54 AM
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You can always replace each and every receptacle on the circuit with a GFCI receptacle. This will always work, as long as you only use the LINE terminals. However, five or six GFCI receptacles is probably more expensive than one 240 volt GFCI circuit breaker.

If you go with a GFCI circuit breaker, remember to properly connect the neutral pigtail on the breaker to the neutral bar and to move the circuit wire neutral to the GFCI breaker.

You are not allowed to add a wire to a factory made cable assembly. If this "flexible conduit" was sold pre-wired then it must stay as is. However, if it was sold as plain conduit that wire had to be added to then you could add a neutral, provided that you do not exceed conduit fill rules.

However, as you have guessed, adding a new wire to an existing conduit is often difficult.
 
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