I am replacing a receptacle (rec) in the kitchen by a GFCI. The receptacle (I think strangely) is controlled by a switch (light) on the exhaust hub (where there is also a fan switch). When I took the rec off I found three lines coming out of the box. I forgot exactly where these were connected. Here is a diagram of the box wires: enter image description here

When I switch the breaker back on the pair C gives me voltage only A and B not. Should I connect "C to line and A and B to load? A has grounding wire with it. It looks a new pair black and white are distinguishable, unlike B and C that have old indistinguishable rubber and cloth coating. The triple wires A are I think connected to the exhaust light switch (I am thinking that because these are new wires...) I actually do not like the idea of this particular receptacle being controlled by the adjacent exhaust hub light switch, so I connected B and C to GFCI (without checking which was line etc) and did not connect A, except its ground wire. The GFCI just had that little green light on, but the receptacles did not seem to have had electricity. So, now I want to do it properly.

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    Can you post actual photos of the insides of the boxes involved please? – ThreePhaseEel Oct 21 '20 at 23:01
  • Actually, it would not have helped, since two pairs of vires have been shortened to minimum and it is hard to see them even when you take a photo. But I solved the problem, see below. – Rado Oct 24 '20 at 3:14

OK, I have solved the problem. First of all GFCI (ground fault circuit interrupter, or something like that) is a rather pretentions name for a receptacle with a fuse. I would call it RF. I know of no studies that show how these fused things improve safety over any ordinary receptacle. Nonetheless the electrical code seems to require them if the box is 6 feet from water source (like kitchens and bathrooms).

If the box where one wants to install RF has just a pair of wires, just connect them to the "line" screws and leave the "load" screws covered. If there are two or more pairs of wires in the box, then one should first find out which pair is the "line pair" feeding the other lines downstream from it. You should have turned off the current to the box you are working on and exposed all the wires in the box. Keep track of which are paired wires (neutral and hot). Turn the electricity on again and use a voltmeter to test each pair for current. In my case, pair C had current, A, B did not, so I were to connect C to "line" and A and B to "load" screws (or tuck the wires in the holes as in my case since the wires were too short.

I did not keep track of where the wires were in the original receptacle (silver or brass buttons) so I had to determine which in pair C was the hot wire. And no I could not do it by color of wires insulation since the wiring (at least for B, C is as old as the house -- over 70 years. That means that the insulation is rubber and is then covered in cloth and in the dirty state they were in, it was impossible to determine color). In this case, use the voltmeter again. Test one wire by touching it with one electrode while touching the metal box or ground wire with the other electrode of the voltmeter. Repeat this with the other wire. One of them should show current and that is the hot wire. You should connect it to the yellow bras button while the other goes onto the silver screw. RF has grounding and I connected the ground wire from A to this grounding. That was it, turn the power back on, press test or reset button and the little green light came on. In addition I used the little thingy that shows whether your connections were made properly.

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    You have GFCIs completely wrong -- a GFCI has nothing to do with overcurrent the way a fuse or regular circuit breaker does; instead, they are entirely focused on matching "current going out" with "current coming back", in order to cut power when they don't match, as this mismatch indicates that current is going places we don't want (like through you!) – ThreePhaseEel Oct 24 '20 at 3:58
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    Re: studies: while it seems that the most salient data on GFCI shock prevention comes from studies done before everything got posted on the Internet, this CPSC memorandum provides a reasonable, if rather time-bounded, overview of the data on GFCI effectiveness from an economics standpoint – ThreePhaseEel Oct 24 '20 at 4:08
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    I hope that you learned the most important lesson of all from this: never take apart all the wiring in a box without first taking pictures and labeling everything so you know what's what and where it goes before you disassemble. At a minimum, you should be able to reassemble to the state (even if non-functional) things were in when you started so if you have to hire an electrician, he doesn't have to spend his time (your $$) figuring everything out. – FreeMan Nov 23 '20 at 13:15
  • ThreePhaseEel thanks for the useful information – Rado Nov 24 '20 at 16:43
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    FreeMan you can say that again. I have to get into the habit of photographing or diagramming things as they are before I take them apart. It is a habit I do not have. And yes, sometimes the original wiring is not good, but even that needs to be recorded. – Rado Nov 24 '20 at 16:45

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