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I have a shop wired without GFCI outlets. I decided that was unsafe so purchased three GFCI outlets, picked the nearest outlet and wired as described. When I powered it up a huge POP occurred in the metal box. What?

The original wiring has three outlets and two pull-chain lights plus a fluorescent light. I have disconnected all of these - open wired except white wires are still connected - hmm maybe that is the problem, the failure is in the neutral somewhere. (I'll check that by disconnecting neutrals as well. I did it on several, but not on all.)

I pulled the wiring apart again and can see no evidence of damage. So I hooked up only the line side to check operation - it works. But the other side is showing only a few ohms so I disconnected all of the outlets on the line until the short went away. Then I tried to hook up the line again without the short. The GFCI refuses to work, so I tried a different one - same result. The wires should be open. Can the GFCI determine that it has a load when there is only wires?

The building was wired in 2005 probably before the new code and it has a ground rod and two sets of ground terminals, but they are connected together and the white side is connected to the main box - could that be involved somehow?

The wires ground (bare) wire and the white wires are separated throughout and passed the tester check before the change. (Can't test them now.) But I wanted to verify that the wiring had not been destroyed, so I hooked it back up as it was and it works ok now. So what gives?

Update 7/13/2018: I discovered the enemy and it is me. I had forgotten that I had added an outdoor outlet complete with GFCI a few years ago. As it happens, the GFCI was on this circuit and as I traced it back, the box for the circuit was protected by a gob of materials I had stored there.

As I mentioned in another post, I had found an additional outlet as I had continued to trace the circuit back, but with every thing disconnected, I had a short black to white on one line. I finally traced it back to a junction box and found the feed from there that was failing was one that I thought had power on it. But unfortunately, I cut a wire assuming that it was the bad (shorted) wire and was going to wire a new circuit around it when I found this junction box and then the outdoor outlet with the GFCI so now I have to fix my damage. Life lesson: don't begin to fix until you know what you are fixing. I would have been done now had I not jumped the gun and cut the wire. It had to be it, right? No, it wasn't. I cut the wire too close to the wall so that I cannot repair it so now I will have to replace the wire as I was going to do and so needlessly if I could have figured out how to find it earlier. Even now I am guessing about where the cut wire comes out - it just has to be that one right there - but what if I am wrong again? I have no way to check - the two wires are about 8 feet apart - it could have been routed anywhere. So here goes...

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    The three GFCI outlets are for three separate lines. This one happens to have three outlets and some other stuff. I planned to just replace the first in the series and hook the next one up to the load on the first one. This first one was to determine if I could do it on this building at all. The pictures and descriptions indicated yes, but so far not so good. – JayDubyah Jul 10 '18 at 20:48
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First, GFCI trips are a fairly quiet snap. The big loud "pop" might actually be a terminal screw arcing against the steel box.

Second, backstab connections (which you jab in and it grabs the wire) are well known to be unreliable, and you're not allowed to use them twice, because the spring is sprung and now they're even more unreliable. How do you keep it straight? Easy, never use backstabs ever.

As far as neutrals and grounds, some folks treat them as if they are mixable and interchangeable. Both amongst each other, and amongst other circuits that may be in the vicinity. It's OK for grounds to intermingle with other grounds, since their one job is safety. However neutrals must not mix with grounds or neutrals from other circuits, because their one job is returning normal current from their partner "hot" wire. They are not fused so they depend on that not to overload. Also, GFCIs depend on this for how they work.

This problem can also show up as a neutral screw rubbing against a ground wire or metal box.

And what can really getcha is an MWBC, two hots sharing a neutral by design. If there's an extra hot in any of your cables, that's what that is. MWBCs are safe if wired corrctly, but do not play nice with GFCIs.


Now I don't know exactly what "as instructed" told you to do. GFCI's have a set of terminals called LOAD, and a novice thinks "sure, they're the second set of terminals like most sockets have". Nuh-uh. They are covered with a piece of tape that says "do not use unless you know what you're doing" in as many words. The LOAD terminals confer GFCI protection to all of the circuit downstream. But if that circuit's wiring has any defects like I discuss above, that will result in instant trip.

The short answer is don't use the LOAD terminals at all. 90% of homeowner use of the LOAD terminals is because their old socket had an extra set of hots and neutrals going to it, and they didn't know what else to do with those wires. That doesn't count as "knowing what you are doing". Not using the LOAD terminals requires a new skill: the pigtail and a few extra supplies.

The long answer is take your wiring all apart and figure out how each cable connects to different boxes. Then come off the LOAD terminals to the next receptacle only, unhooking any wires that go beyond that. Power up and test. If it passes, power down and add the next segment. One at a time. Eventually, adding sometimg will cause it to trip. That is the problem item.

If everything hooks up, then fit everything back in the junction box one device at a time. Power up and test again. This is where you detect screws contacting metal boxes and stuff like that. If you put them all back together at once, and it trips, you'll have no idea why.

Divide and conquer.

If you caught the bolded text above, you now realize that a socket downline of the load terminals of a GFCI is protected by that GFCI. Putting another GFCI there is wasted, it's playing a "yo dawg" joke on yourself. You may also want to rethink whether you want lights GFCI protected, since a GFCI trip will plunge you into the dark with hands inches from a spinning saw blade.

  • "as instructed" means my interpretation of the instructions that came with the outlet. – JayDubyah Jul 10 '18 at 20:34
  • I have done GFCI outlets successfully previously. I understand that the load terminals are for down-line. The method that I used so far did not include disconnecting the white wires, but I did disconnect all of the black wires physically. I was able to get the short to disappear. Late last night, I reinstalled the old wiring to see if it is failed. It did not trip the breaker, but this morning I put the tester on it and it showed all three lights on. - not good, I plan to proceed as you suggested. (When I get too tired I tend to make bad mistrakes) – JayDubyah Jul 10 '18 at 20:43
  • The neutrals and grounds thing: the box has two separate terminal strips but they are connected together via the metal of the box. It looks like the box might be isolated by connecting the copper ground wire, which now connects to the neutral bar, to the ground bar and disconnect the grounding pin on the neutral bar. There are three wires that come into the two hot terminals of the main CB and the third wire is connected to the white neutral bar. Would the GFCI be happier if the grounding pattern was changed to separate the white from the earth ground bare copper? Neutral/gnd connect at panel. – JayDubyah Jul 10 '18 at 20:59
  • No, you shouldn't be altering anything upsttream of the GFCI. I wasn't even thinking of the service panel. – Harper - Reinstate Monica Jul 10 '18 at 21:20
  • I re-examined the wiring and found another outlet on the line. I have all of them apart now and found one run that appears to be shorted black to white. Not quite sure why it decided to POP now, but it probably is a good thing. The wire that is shorted is between the second and third outlet on the string - odd. I hadn't touched anything in there at all and it had been working right up until I installed the GFCI 50 ft away from it. – JayDubyah Jul 11 '18 at 5:26

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