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My Christmas lights keep tripping my GFI in the rain.

I've got one of those Light Show boxes with 6 different sections of lights plugged into it. When I have all 6 sections plugged in, the GFI trips immediately. However, if I try them one at a time, the GFI doesn't trip.

If there was a ground fault in any of the sections, wouldn't it always trip the GFI? What am I missing? How do I debug this problem?

Can "high instantaneous current" also trip the GFI?

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Do you have another GFCI (maybe in your kitchen) that you could try to see if it trips? If it trips one outlet but not another it's likely a faulty outlet or breaker – Steven Dec 11 '12 at 21:50
@Steven - Yes, already tried that. It worked the same as the outdoor one. – Greg Dec 11 '12 at 21:58

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GFIs don't actually measure "ground fault", they measure the difference between the current (amperage) going in and the current going out. It sounds like the current is leaking a little bit on each one, but maybe in not sufficient magnitude to trip the GFI. Try doing 2 or 3 and seeing what happens..

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I thought this might be the case, but since they detect just a few milliamp difference I figured it was unlikely. Any suggestions on how to fix such a small leak? – Greg Dec 11 '12 at 21:32
I don't know about you, but my light strings range quite a bit in age. I would suspect the older ones have a slight leak, too small to matter for one string, but add them together and they cause a trip. There's a decent chance replacing the oldest strings may solve the problem. OTOH, if you have any cheap Chinese strings, they could be suspect even if new. – bcworkz Dec 11 '12 at 22:44
Yes, we have brand new el-cheapo strings. :/ – Greg Dec 12 '12 at 4:15
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This is exactly right. Devices are allowed to have a small amount of current leakage, and since the light strings are connected in parallel the leakage current of each string is added together. Once the total current leakage surpasses the GFCI's trip level, the GFCI opens the circuit. For example. If the current leakage tolerance was 9 µA (0.0009 A), 5 strings of lights would be 4.5 mA (0.0045 A). One more string of lights pushes the circuit past the trip level (~5 mA depending on device), with a total leakage of 5.4 mA (0.0054 A). – Tester101♦ Dec 12 '12 at 12:36
@Greg Finding where this amperage loss is occurring in a string of Christmas tree lights would be like trying to find a needle in a stack of needles. It is such a tiny amount and at such a low voltage that it would be unoticeable to most people. – maple_shaft Dec 12 '12 at 12:37

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