Recently I was installing a couple of utility plugs and some bare-bulb lighting in the crawl space. There are two entrances to the crawl, with a 3-way switch and a plug at each. My first thought was to run a single circuit and feed the plugs and lights simultaneously–power into the first box, pigtail, feed both a GFCI receptacle and a three-way switch, run 12/2 and 12/3 down to the second box and feed a second receptacle as load and a second 3-way switch, then out to the lights. The lights worked but the GFCI wouldn’t stay on. A dim bulb went on in the back of my mind… I remember from somewhere that a 3-way switch will always trip a GFCI… something like that… came up on another job somewhere. I was in a hurry and only about 20 feet from the panel so I pulled in a second circuit and finished the job that way, but can someone clear this up for me?
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It can be done. GFIs work by electrically comparing the feed to return currents. When the difference gets to around 15ma it trips. The trick is to make sure the GFI 'sees' both the feed and the return currents. Getting the GFIs on the common side of each 3-way switch will do this.
This would make the GFI on the home run side of the circuit constantly on, like a regular receptacle is. It would also make the GFI at the other end only on when the lights are on. Probably not a problem if you only need the power when your in the crawl space.
Alternately you would need to run a second cable as a receptacle feed and tie this into the existing circuit ahead of the first 3-way. This would allow both GFIs to stay hot if the lights are on or off.
Let me see... in your alternate scenario, power into first box, feed GFCI, 12/2 over to second receptable, pigtail there and feed the first three-way switch (it's in the second box), then 12/3 back to the first box/second switch, then the lights from there. All switches/lights downstream of the two receptables. Is that what you're describing?
"When the difference gets to around 15ma it trips."
4-6 ma.
6 ma is enough to give you a permante dirt bath. But that would probably have to be the worst case like left arm to right leg.
Don't know anything about 3 ways and GFIs - Just had a thought.
If your lights and plugs are on the same circuit, and you blow a breaker, you're in the dark.
Might be worth putting the lights on a different circuit.
An ounce of mother is worth a pound of clergy. -- Spanish proverb
I wouldn't expect to do anything in there that would blow the breaker... not taking a tablesaw or large compressor in there. I did figure that having the lights take off of the GFCI would be dumb, so I didn't do that... althought it appears to be the way that it would work. Anyway, got 2 circuits now and I'm keepin' em.