timer switch trips GFCI


Old 01-26-18, 08:43 AM
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timer switch trips GFCI

I'm wiring a new bathroom. I've installed a GFCI, and a timer switch on the load side of the GFCI. The timer switch has LED lights and selectable time, so it requires 4 wires, and I used 12/3 wire to the GFCI and the switch. My circuit tester shows the GFCI is OK. However, as soon as I turn on the switch, the GFCI trips.
I replaced the switch with a standard one and it doesn't trip the GFCI.
The same switch was working on the other side of the bathroom, on the same circuit (but wired slightly differently).
I measure 2-3 volts between hot and ground, and between neutral and ground, on the circuit, with the switch disconnected and the GFCI tripped. I assume that's bad. But the strange thing is that the same GFCI worked OK on the same circuit, on the other side of the room. With the GFCI not tripped, I measure almost no current between neutral and ground (around 0.013 volts, which is around what I measure when the voltmeter isn't touching anything, but it's consistent when it's on the circuit).
This is all new wiring, except for where the circuit continues upstairs through some knob and tube (of which I've gotten rid of all I can without ripping into walls).
What's the best way to figure out what's going on?

Last edited by wjquigs; 01-26-18 at 09:15 AM. Reason: new info
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Old 01-26-18, 10:56 AM
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You obviously have a fault somewhere. I'd start by taking the switch off the GFCI and wire it from the line side of the GFCI device.
Old 01-26-18, 01:45 PM
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Why three wire from the GFI to the switch ?
Do you mean two wire with ground ?

You can't use three wire to wire a GFI and light. The neutrals must remain separate.
It may be easier to just the wire the light to the GFI line side.... unless the light is in a shower.
Old 01-26-18, 02:13 PM
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Thanks. I wired it to the line side and everything is working now. But I'm curious: why do the neutrals have to remain separate (for the GFCI and the switch)? Is this true even if they're on the same circuit (which is what I have now).
Old 01-26-18, 03:22 PM
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GFCI works by comparing current going out to hot side of the load and current coming back to neutral side of the load. If there is any different, GFCI detects it as a ground fault and trips.

Therefore, you cannot have a load between the hot side of the load terminal and neutral side of the line terminal.
It would have worked if you had neutral going to the light on the load terminal, it would have worked fine. However, the light will go out when GFCI trips.

Last edited by lambition; 01-26-18 at 03:54 PM.
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