Shared Neutral

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  #1  
Old 11-10-04, 07:15 PM
guydude
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Shared Neutral

I have a 14-3 cable connected to two single pole 15 amp breakers at the panel One circuit is on one buss and the other is on the other buss. Both circuits have the white neutral being shared by the two circuits.

On one of these circuits, I want to install a 15 amp GFI breaker. Can this set-up be done? If so, what is the procedure to accomplish this? Is it as simple as moving the new GFI breaker to the same buss as the other circuit?

Thanks
 
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Old 11-10-04, 07:49 PM
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GFCI devices are designed to measure the current from the hot to the neutral.Any imbalance in this current will cause the GFCI to trip. A multi-wire circuit such as this will have a current imbalance because there is 2 hot legs & a shared neutral,therefore a GFCI breaker cannot be used,but you can use a GFCI receptacle on the circuit.
 
  #3  
Old 11-10-04, 09:00 PM
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You cannot use a single-pole GFCI breaker, but you can certainly use a double-pole GFCI breaker. Such breakers are a bit pricy. That will protect the whole circuit. To use one, you may need to rearrange the breakers if these two breakers are not currently one above the other.

Or you can also use a GFCI receptacle if you attach all the wires to the "line" side and not any to the "load" side. That will protect this one outlet, but won't protect any others. So you'd need a separate GFCI receptacle for every location you want to protect.

If you can recable to eliminate the shared neutral, then other solutions are possible.
 
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Old 11-11-04, 06:19 AM
mb32
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John,
Why couldn't he install additional recepticles on load side of GFCI ?
As long as there is no connection between neutral on load side of GFCI and shared neutral the GFCI should not see an imbalance.
Is this correct?
Just wondering
thank you.
 
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Old 11-11-04, 06:25 AM
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mb32,

Additional receptacles can be installed on the load side, as long as the neutral is no longer shared.

I believe that John did not address this because the the terminology tends to confuse people, and because many multi wire circuits are multi wire their whole length, and thus continue the shared neutral the whole way, making GFCI receptacles unable to provide downstream protection.
 
  #6  
Old 11-11-04, 06:52 AM
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John N. and Ampz,
In light of this post, I am a little confused and concerned that I headed for a problem.
Iím setting up a sub panel. I will be putting in a 40amp 240v breaker in the main box and pulling two hot, one neutral and one ground to an exterior back yard sub panel. In the sub panel I will be hooking up one hot wire to each buss, a neutral to the common bar which is not grounded and the green to a grounding bar which is grounded to the box. I also intend to install two single pole 15amp GFCI breakers. One line will control an outside light on which I will also install an in line GFCI protected outlet, so I can plug in a transformer to service 24v exterior lighting. The other will control a series of outside outlets.
The common will be wired directly from the main neutral bar to the sub panel and the ground to the sub panel grounding bar.
If I hook up all common to neutral including the GFCI breaker curly tails, wire the outlets to the GFCI, black to breaker, white to neutral, will I encounter a problem with the GFCIís tripping?
Also, should I use a ground strap on the neutral bar to the box? Or will this also cause a problem?
 
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Old 11-11-04, 07:03 AM
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At your sub panel do not connect the neutral bar to the ground bar or to the panel itself. The neutral must remain isolated.

If I read your post correct, you will have two GFCI breakers in the sub panel. One will power the series of outside receptacles, the other will power the outside light and a receptacle in the light. As long as you keep the wiring from these GFCI breakers separate you will not have a problem.

I would not use a GFCI receptacle on the light. That would put two GFCIs in series (the receptacle and the breaker). While this is not against code, it is overkill and leads to confusion.
 
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Old 11-11-04, 07:11 AM
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rcrainman,

The problem with shared neutrals and GFCIs is _only_ when the shared neutral is 'downstream' of the GFCI. Almost all homes are suppled with power using a shared neutral, and this doesn't cause a problem for GFCIs. It is only when you have a shared neutral after the current imbalance sensor that things get screwed up.

At your sub-panel the neutral must remain isolated from ground. That is why you have a separate ground and neutral bus in the subpanel. Do not install the neutral bonding strap; this will connect ground and neutral and result in voltage and current on your ground conductors.

-Jon
 
  #9  
Old 11-11-04, 07:16 AM
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RACRAFT you stated,
"I would not use a GFCI receptacle on the light. That would put two GFCIs in series (the receptacle and the breaker). While this is not against code, it is overkill and leads to confusion."

Isn't this tantamount to having serveral GFCI outlets hooked in a series?

Will this setup trip either GFCI?
 
  #10  
Old 11-11-04, 07:27 AM
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Jon, thanks for your input but I don't know what this means? Can you walk me through an installation depicting this example?
"It is only when you have a shared neutral after the current imbalance sensor that things get screwed up."
 
  #11  
Old 11-11-04, 07:40 AM
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When you have multiple GFCIs in series, a ground fault will trip any or all of the GFCIs that are upstream from the fault. In your situation, the GFCI receptacle on the light may trip, or the GFCI breaker may trip, or both may trip. The confusion arises because someone may not realize that a second (or third) GFCI exists, and may not understand why the GFCI receptacle in front of them is either not tripped, or does not reset.

A GFCI works by watching the current on the hot wire and on the neutral. When the current is different by more than a minimal amount, the GFCI trips, and shuts off power to the load side of the GFCI, which includes the receptacle itself.

A multi wire circuit is a circuit in which the neutral is shared by two different hot circuits. In a residence the two hot circuits are opposite of each other. If you have an identical current draw on each hot wire, the current on the neutral is zero. A difference of 1 amp between the hot wires leads to a neutral current of one amp. A 240 volt GFCI breaker is designed to examine both hot wires and the neutral, looking for lost current. A 120 volt GFCI receptacle is designed to examine only one hot wire and itís neutral. If you continue the shared neutral on the load side of a GFCI, or on the load side of two GFCIs, the neutral current that the GFCI (or each GFCI) sees is never (under normal conditions, except when the total current is zero amps) equivalent to the hot current being drawn through GFCI, so it will trip.
 
  #12  
Old 11-11-04, 08:31 AM
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Understand!
Thanks!
 
  #13  
Old 11-11-04, 09:52 AM
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Just say "NO" to sharing the neutral, and all your problems go away. Use two 14/2 cables rather than one 14/3 cable. In my original post, I assumed that this 14/3 cable was already installed and inaccessible. If you still have a choice, simply don't use the 14/3.
 
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