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I'm trying to add a GFCI outlet to an existing circuit and have run into an unexpected obstacle. All of the wiring literature and videos I've come across allude to either 12/2 or 14/2 cable with 1 black, 1 white, and 1 bare ground wire.

The existing receptacle (non-GFCI) that I plan to tap into surprised me when I pulled it out of the box in that it has 2 black and 2 white wires along with the single ground.

Should I splice the existing blacks together along with another piece to make a pigtail and then connect it to one brass screw, do the same with the whites and a silver screw, then connect the new Romex (12/2) going to the GFCI to the OTHER brass & silver screws? If so does it matter which pair of screws?

Additionally, am I right to assume the connections to the GFCI are made on the LINE rather the LOAD side since there will be no other outlets connected to it?

Any advice would be greatly appreciated.old receptacle

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    Sounds like you're pretty new to wiring. Outlets that occur along a circuit will always have two cables attached. unless they're the end device. Be sure you know enough to be safe. – isherwood May 7 '18 at 19:36
  • Correct, you should not be using the LOAD terminals unless you have a well rounded understanding of that feature, and would be able to deal with any of several surprises which can occur. That's what the sticker across those terminals is saying. – Harper May 7 '18 at 19:42
  • @harper, 95% of GFCI installations are intended to protect outlets downstream (as I'm sure you realize). I wouldn't say "correct" and leave an unsuspecting person with an unprotected circuit. – isherwood May 7 '18 at 19:45
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    @isherwood 95% of GFCI installations are installed by people with no idea that's even possible. 95% of uses of the LOAD terminals are because the homeowner has an extra black and white and they don't know what else to do with them. – Harper May 7 '18 at 19:56
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    @isherwood Well, 95% of homeowner installations. Sure, experts putting LOAD to good use to protect a large part of a circuit, I'm all for that. But I really cannot possibly endorse the idea of tricking people into using LOAD to protect parts of their circuit they don't even know they're protecting, don't know how that protection works, or what it even is. Too many potential surprises, such as a cross-circuit-wired neutral, having unrelated loads trip with no idea where the reset would be, etc. We could leave them way over their heads, and in a "gotta call an electrician" situation. – Harper May 7 '18 at 20:05
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It depends if you are just trying to connect that particular receptacle as a GFCI or you want to protect the entire circuit.

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This would be for individual GFCI's. If you are then make a pigtail for the hot neutral and ground and connect it to the Line side terminals.

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And this would be for the entire circuit. For more drawings go to: https://www.do-it-yourself-help.com

Good Luck.

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    Thank you for the reply! Yes, I'm only attempting to connect the new receptacle as a GFCI - it's in a bathroom - not protect the entire circuit; so it seems pigtailing its 3 wires to the existing duplex is what I need to do . – Gene Leis May 7 '18 at 20:02
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What you need here is a pigtail. Just buy an 8" section of #12 Romex cable at the hardware store (or three, that makes 2 feet), and shuck off the sheath. Now you have a black, white and bare wire from which to make pigtails. Put one of each on the LINE brass (black), LINE silver (white) and GROUND screws on the GFCI - and you get to do that at the bench, not in an awkward position. Two red wirenuts to join all the blacks and whites, add the bare to the existing grounds, and you're done.

You're right not to "just use the LOAD terminals"

The LOAD terminals have a purpose. But they should not be used except by someone well-versed in their usage. That's not just my opinion; there's a sticker across the LOAD terminals that says pretty much the same thing.

If you use LOAD haphazardly, you risk a lot of really weird problems that will overwhelm the inexperienced novice. For instance "problem X" can cause mystery trips whenever the downline loads are used, and the knowledge you need to fix it is too far beyond where you are now. You'll end up having to pay an electrician to do a bug hunt. I won't say what "Problem X" is, because everyone would go "Well, I would've just turned back to LaGuardia the instant the birds struck!" Yeah, right you would.

If these downline loads are in locations whose relationship with this location is not obvious, you introduce a UX problem where nobody can find the GFCI reset or even be aware that a GFCI trip is the problem, and again you end up paying an electrician to spend 2 hours, ultimately showing you where the reset is and making you feel like a fool.

So no -- use of the LOAD terminals is not an automatic, kneejerk, "can't hurt might help" thing to do.

When used skillfully, they are cheap coverage

That said, properly schooling yourself up in the functioning and proper use of GFCIs will allow you to use the LOAD terminals competently. It will prepare you for "Problem X" and its sisters. It will give you horse sense about where to locate the GFCI device(s) so it/they can be found... avoid wasting money on outdoor or kid-proof GFCIs or the "Yo dawg" problem. Tell you the times not to use GFCI, and how to layer that protection with AFCI without losing your mind. There's quite a lot of Q&A on this site about all that.

Then yes, protect as much of your house as is feasible.

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Speaking from experience, just connecting the LOAD side without considering what is downstream is going to create headaches. I have an older house with strange circuit runs and the previous owner put GFCI outlets in the bathrooms. That was fine and good, but that put the bedrooms under the GFCI as well. I fixed it by splicing the GFCIs out (I have aluminum wiring so splicing is the only safe way to connect to a GFCI anyways).

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