GFI Breaker on a multi-branch circuit


Old 03-07-07, 09:35 PM
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Join Date: Mar 2007
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GFI Breaker on a multi-branch circuit

I pulled the ceilings off my 100 yr. old house to insulate and discovered that the lights share a common neutral-- but some lights are on one breaker and some are on another-- the two circuits are opposite poles--

one of the two circuits has too many receptacles and lights on it (5 lights and 6 outlets)-- I want to put a GFI breaker on this circuit b/c the wiring is old, there are no ground wires at the lights/ receptacles (but the circuit box is grounded)

I am thinking, though, that a GFI breaker won't work in this situation b/c the neutral and hot may not have the same current flow even during normal operation of lights and appliances.

Is this correct or am I misunderstanding how a GFI works? Could I expect a GFI outlet to work on the circuit in question? Would it protect lights and receptacles downstream even if they used a different neutral?

The wiring is all single wire-- no knobs and tubes are visible, but the wires themselves are older.

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Old 03-07-07, 09:41 PM
Join Date: Sep 2000
Location: United States
Posts: 18,497
A multiwire circuit (two hots on opposite poles sharing a common neutral) is technically one circuit.

GFCI does not require grounding.

To provide GFCI protection on a multiwire circuit, you need to use a double-pole 120/240 volt GFCI breaker. They can be pretty pricy (and a bit hard to find), but you may have no other realistic option if you want GFCI, without making substantial modifications to the circuit wiring. However, see the paragraph after next for other options, which may or may not be better for you.

A single-pole GFCI breaker won't work in this situation.

A GFCI receptacle will work if you only use the LINE side and not the LOAD side (i.e., no downstream protection). This may mean that you need one GFCI recepacle at each outlet. If the neutral is not shared beyond any one point, you could then use the downstream protection beyond that point.

You didn't really say if you plan to address the problem of too many lights and receptacles. In fact, you didn't even say if you were having a problem with it.
Old 03-07-07, 09:59 PM
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Join Date: Feb 2006
Location: North of Boston, MA.
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Gone this far

If the ceilings are down..... Why not rewire? Cost is no excuse. Access is awsome. Given what you stated.....Crazy not to.

Sooner or later you will need to. Easy now.
Old 03-07-07, 10:00 PM
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Join Date: Mar 2007
Posts: 2

Thanks for the reply-- I suspected as much regarding the GFI breaker-- I don't think that the double switch is feasible b/c the breakers are 1 and 7 on my board so they are not even right next to each other.

I am not having a problem, because we haven't moved in yet, but I think I probably won't have a problem even when we are in there-- I just don't want the problem to be a fire so I thought I'd get the best protection.

I don't know what happens to the neutral after it goes back downstairs, and I am not sure at all how there the receptacles are wired, I just know a lot of them are on the same hot wire. Its harder to trace the neutral

I will probably put GFI's in the outlets and not count on any downstream protection.
Old 03-07-07, 10:06 PM
Join Date: Sep 2000
Location: United States
Posts: 18,497
Sounds like a reasonable approach.

If you do decide to use a double-pole breaker, you can move the breakers around in your panel to get these two hot wires together.

To help make sure that you don't make anything worse, read a few books on home wiring before you do anything (if you haven't done so already). It's pretty easy to screw things up if you don't fully understand what you are seeing, and if you don't know proper techniques.

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