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Old 02-01-2011, 05:43 PM   #1
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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.

Suggestions?

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Old 02-01-2011, 07:53 PM   #2
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GFI and switch loop


Most likely, you have a ground-fault in your neutral wire downstream of the GFCI device. It may or may not be located in your ceiling light. While this won't trip a breaker or blow a fuse, the neutral wire is a current-carrying conductor, and as such a fault to ground can introduce some hazards.

You've probably had this problem all along, but did not notice anything until the GFCI device was installed.

What to do?

Troubleshoot the rest of the circuit, here is one method:

Disconnect the rest of the circuit from the ceiling light by taking apart the other 2 wire-nuts with the black and white wires in them. Separate the loose wires, and then test your GFCI again.

IF it no longer trips, then the problem is not in your light, but further downstream in the circuit. Make those wire nuts back up, and then open up the next outlet, and separate the wires there and repeat the test.

Sooner or later you can determine where the bad spot is at using this method.

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Old 02-02-2011, 07:57 PM   #3
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GFI and switch loop


Thanks, kbsparky. You hit the nail on the head. I thought I had systematically isolated all the possible problem areas -- but I had only done that for the devices 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, and your post made me realize that.

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 don't know if that was enough to trip the GFI (after all, those boxes are not grounded) -- because 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.

Thank you for your help!
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Old 02-03-2011, 04:42 AM   #4
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GFI and switch loop


If its old armor cableits possible the neutral might be making contact with the metal jacket. Remove the GFCI and install both wires on the line side. You really shouldn't have your light on the load side of a GFCI anyways. That will stop your GFCI from tripping and the outlet will still be protected.
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Old 02-03-2011, 05:21 AM   #5
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GFI and switch loop


Quote:
Originally Posted by dSilanskas View Post
If its old armor cableits possible the neutral might be making contact with the metal jacket. Remove the GFCI and install both wires on the line side. You really shouldn't have your light on the load side of a GFCI anyways. That will stop your GFCI from tripping and the outlet will still be protected.
If he does that it will ignore the fact there is a ground fault and the down stream outlets will not be protected. In his second post the OP says he found a bootlegged ground using the neutral.
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