GFI and switch loop

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  #1  
Old 02-01-11, 01:27 PM
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GFI and switch loop

While updating a 1950 house that has ungrounded wiring, I replaced the first outlet in a circuit with a GFI outlet. Downstream from the GFI is a ceiling light controlled by a switch loop. The light worked fine until I installed the GFI, but now something about the wiring of the light trips the GFI (even with the light turned off).

Here's the setup: The ceiling box gets power from a two-conductor cable running from the GFI. The cable from the ceiling to the switch is three-conductor; it not only runs to the switch, but also supplies power for outlets downstream of the switch.

In the ceiling box, the red wire from the switch is connected to the black wire of the light fixture. All the other black wires are connected in one wirenut. All the white wires are connected in one wirenut.

This looks right to me, and it worked fine until I installed the GFI. If I replace the GFI with a regular outlet, it still works fine.

I've heard that GFIs sometimes don't work well with switch loops, but I'm also wondering if the ceiling box is wired incorrectly.

Any suggestions?
 
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Old 02-01-11, 01:48 PM
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I've read you post a few times so I hope I'm reading correctly so, here it goes.

On your first GFI, do not use the load side to feed the light box, then install another GFI post light box to feed additional downstream receptacles.
 
  #3  
Old 02-01-11, 02:09 PM
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Thanks, wirenut. I understand what you're suggesting, but I'm still unsure about a few things. Does that mean you think the ceiling box is wired correctly, and GFIs are incompatible with switch loops?

In this particular case, if I did what you suggest, I would need several GFIs, because there's another cable leaving the ceiling box that feeds downstream receptacles. In other words, this is a stream that forks.
 
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Old 02-01-11, 02:50 PM
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Is it possible this is a multi-wire branch circuit?
 
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Old 02-01-11, 03:30 PM
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No, I don't think so. The circuit originates with only one two-conductor cable leaving the panel, where it is attached to only one breaker. I haven't run across any second hot wires.
 
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Old 02-01-11, 03:54 PM
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Disconnect the light and see if it works, maybe there's an issue within the light.
 
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Old 02-02-11, 05:51 PM
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Wirenut, I think you were right after all about the multi-wire branch circuit. I finally discovered the problem was in several downstream outlets that branched off from the ceiling box, not the ones that were fed from the switch box.

I had systematically isolated every device downstream from the switch, thinking the problem was in that area because I had been doing work there (running new cable, adding receptacles, etc.). I had ignored the devices downstream from the ceiling box.

In two of the old metal boxes, someone had attempted to simulate a true ground by looping the neutral wire around a screw in the box. Just in case that wasn't enough to trip the GFI, there was also a switched outlet that seems to be controlled by two separate breakers in the panel. This is my first encounter with a multi-wire branch circuit.

I have easy access to that switched outlet from the unfinished basement below, so I'll just re-wire it.

Thanks for your help!
 
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Old 02-02-11, 06:50 PM
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In two of the old metal boxes, someone had attempted to simulate a true ground by looping the neutral wire around a screw in the box.
I'm sure you know but just for others bootleg grounds are a safety issue and need to be disconnected when found.
 
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