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I have an existing conduit coming from the ground and goes into the side my garage. Just before it goes into the wall, there is a rectangle shaped box with screws that hold a small cover on. If I remove the cover, I see 4 wires: Black, White, Red, and Green. These wires power several 2 and 3-way circuts inside the garage.

I want to "splice" into these wires (from outside) so that I can have a GFCI outlet outside the garage.

My question is, what is the best way to do this? There isn't really enough slack on the wires inside the little rectangle box to do anything. I suppose I could snip the wires, remove the little rectangle box and replace it with a large junction box, and that way I would have room to extend the wires, and route them to another outlet box which would be my GFCI outlet. Does this sound like the right way to do this?

If this is correct, how would the wiring diagram for this look?

My proposed wiring diagram - Does it look correct?

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This must be a 15A or 20A circuit. If it is 25A or larger, you are not allowed to put the usual receptacles on it.

That funny thing is a conduit body. It's made to ease pulling. You cannot splice there.

enter image description here

First, pull the wires out of there. Find the ends of the wires (i.e. at the service panel or at the next junction box where they splice) whichever is closer. Unsplice them and pull them back, all the way to the conduit body and a little past. That way you don't have to cut the wires.

Then replace the conduit body with something else. You can change it to a "Tee" conduit body, or to a junction box. A junction box is a great deal more useful. You'll need to work in the same kind of system you're already in, e.g. if PVC conduit, then either use PVC boxes, or use PVC threaded ends to go into any other kind of box.

enter image description here

Next, extend the conduit pipe system to your new receptacle.

Now put the original wires back and you're back up. Now figure out how to add the additional wires. Now let's review the rules:

  • You cannot make any splice in a conduit body, only junction boxes.
  • Each wire must have 6" from where it enters the box, and also stick out of the front of the box at least 3". At both ends.

So if you added a conduit body "tee" you'll need to run wires from the new receptacle, through the "tee" to a junction box where you can splice. Watch out that you don't overstuff your conduit, the legal max is 9-12 wires depending on several factors.

If you added a new junction box and want to make the splice there, you'll have to see if they left you enough slack for the required lengths in all locations. If not, you'll have to sacrifice one side or the other, and replace them with longer wires. Take them into the electrical supply and ask for wires 2 feet longer. They can help you match up the size and type. Also get the new wires (same size and type) you'll need for your new branch, if you're branching. If you know you'll have to sacrifice wire, you can save time at step 1 by doing that first.

You drew it with "wire nuts" or some other kind of splice tying all the wires, and a short pigtail to the receptacle. That is correct. You must pigtail the ground, always. You must pigtail the neutral because this is a multi-wire branch circuit (which have many complicated rules). You aren't required to pigtail the hot, but it can help especially if it's in a cramped location. (you can attach the pigtails to the receptacle at your desk.)

  • Please explain how you know this is a multi-wire branch circuit and what the significance of that is to the OP's plans. IIUC this means that the wiring to the garage is, or should be, protected by two linked single-pole breakers in the main panel, right? How would the OP know which of the two hot wires (red or black) he should use for the new receptacle? Would it be better to put the GFCI receptacle in the outside exposed location or would it be better to put it inside the garage and connect it so it would protect one or more receptacles inside the garage as well? – Jim Stewart May 20 '17 at 16:24
  • If that is not an MWBC, the red wire is unexplained. It has little effect on his plans if he does it as he diagrams (presuming of course that it is an MWBC and not a dryer cable!) The breaker tying requirement hit around 2008 or 2011 IIRC and he may be grandfathered. Good point, he should turn off both breakers in any case. Also good point, better to put the GFCI breaker indoors out of the weather and supply the outdoor receptacles off LOAD. – Harper - Reinstate Monica May 20 '17 at 16:37
  • OP, you have not told us that you have found the breakers (presumably exactly two of them) in the panel which control the two circuits in the garage. You will have to locate them to turn off the power to do what you want to do. What is the breaker type? I.e., are they both 20A? both 15 A? Are they just standard breakers, i.e., not GFCI nor AFCI nor combination? Are they linked to each other mechanically? The two breakers should be on different legs of your service, meaning that the voltage difference between the red and the black should be ~240 V. Is that true? – Jim Stewart May 20 '17 at 18:21
  • Another explanation for the red wire is if there is a pair of 3-way switches for the lights in the garage with one switch in the garage and one switch is in the house, allowing the lights to be switched on and off in the garage from the house as well as the garage. It is possible that the power to the garage is supplied by only one breaker and is not a MWBC. The OP should tell us how many breakers control power to the garage. – Jim Stewart May 21 '17 at 12:58
  • OP try to answer @JimStewart 's questions... surely not a 3way but could be a switched hot. Not all MWBCs have handle ties, eg PushMatic doesn't sell them. – Harper - Reinstate Monica May 21 '17 at 14:13
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I assume that this is metal conduit and the little rectangular box is a "pull port".Pull port

I do not think you can tap into wire there. I think the standard way to do this would be to tap into the wiring at a receptacle in the nearest full sized box.

  • Yes, it's similar to that. It's plastic though (the conduit also looks plastic), and a little bit larger. There is no other receptacle nearby. Only inside the garage. If I did tap into these wires how should the wiring diagram look to connect a GFCI outlet to them? There are 4 wires, black, white, red and green. I would probably leave the red wire alone and only need the white, black, and green right? – Chris W. May 20 '17 at 2:24
  • Are the studs in the garage exposed or is there drywall? Does the conduit to the various receptacles run through the studs or is it above the ceiling or ceiling joists? Are all the receptacles and lights in the garage on one circuit breaker or two? Are the breakers for the garage in the main panel in the house, or is there a sub-panel in the garage? It is time for the knowledgeable electrical people to take over on this. – Jim Stewart May 20 '17 at 2:34
  • The studs are covered by drywall. I can't tell for sure but I think the wires run through the studs for the lower outlets and up the wall, and over the ceiling for the ceiling fan. Where the wires come through the wall, they come into a junction box, and connect 3 and 4 to a wire nut, and then go out in various directions out of the box. – Chris W. May 20 '17 at 2:41
  • I wanted to paste in a pic of how I think the wiring diagram would look but I can't here. So I'm thinking I would simply snip the white, black, and green wires. Then extend them with "wire nuts" in a larger junction box (get rid of small box). Then using wire nuts, tie those two ends (for each wire) together, along with a third wire that would go to the new GFCI outlet. How's that sound? How can I paste a picture here? – Chris W. May 20 '17 at 2:44
  • Please see the diagram I added to my original comment. Does it look correct? – Chris W. May 20 '17 at 2:53

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