I have three electrical outlets in my garage which appear to be connected to one circuit breaker.

I am assuming the outlet closest to to the panel is first in the three receptacle circuit.

How can I test this assumption?

If it is, will replacing that receptacle with a GFCI receptacle give protection to the other two receptacles in the line ?

  • You have to understand how it all works, and if you do understand you wouldn't need to ask this question.
    – Hot Licks
    Commented Sep 21, 2017 at 22:20
  • 8
    @HotLicks You could say that about every question on every site; if you understand how it all works, you wouldn't need to ask the question.
    – Beska
    Commented Sep 22, 2017 at 13:11
  • 2
    That explains why HotLicks hasn't asked any questions... Commented Sep 22, 2017 at 18:46

5 Answers 5


Finding the first

There's really no substitute for opening boxes, unhooking wires and turning the circuit on and taking measurements.

You open up your believed-closest box, and remove the wires you think are downstream (away from the panel). Tape off the loose wires so they can't short against anything, and plug a 3-light tester or nightlight. Then go turn the circuit on, and see what lost power. If all the other receptacles lost power, you're in the right box. Otherwise try another box.

If the receptacle still has power, you've identified the supply wires. Otherwise touch the loose wires with a non-contact voltage tester and see if you can find the lit-up hot, or just power down and try a different pair.

Usually there are only 2 pairs. If there are more than 2, you may need to disassemble a pigtail. This is also a good time to do housekeeping like make sure your wires are on screws (not unreliable backstabs), upgrade to the $3.00 receptacles instead of the 50 cent builder grade cheapies, and upgrade to 20A (T shaped neutral) sockets if the circuit is indeed 20A and you expect to use those.

GFCI+receptacle devices are very confusing because they are doing two things. Let's break it up.

What is a GFCI?

A GFCI is a black-box which inputs hot(s) and neutral on its "line" side ... and then it outputs protected hot and neutral on its "load" side.

enter image description here

That's it. To signify the protected side, I use brown and gray wire in conduit work, but you can do the same by tagging wires with brown and gray electrical tape. It can be a circuit breaker, or a dedicated black box. If that's hard to visualize, look at this.

enter image description here

This is a GFCI device in its purest form: a "deadface" GFCI. You can only hook this up just like the diagram: supply wires to LINE and downline load(s) to LOAD (otherwise what's the point?)

Since you have first location and supply wires identified, do exactly that. Pigtail the loads if necessary, though some GFCIs use screw-and-clamp to allow 2 wires on a screw.

Be prepared to install a box extension if the box is too small to fit the GFCI device and the wires.

How do you get sockets at the deadface location? Well, heh heh, I don't seriously expect you to use a deadface when you could use a GFCI+receptacle... it's just better for illustration. But here's the gotcha: Since the sockets are pre-wired to the LOAD side, it's possible to miswire a GFCI+receptacle, attaching power feed to the LOAD side - in which case the outlets will be energized but the GFCI will not protect.

How to test for GFCI functionality

Use a simple GFCI tester. Most of them come as part of a 3-light tester, in which case get a simple one**.

enter image description here

You just push the button, and it induces a hot-ground fault which should trip a GFCI protective device. If it isn't obvious, put a "GFCI Protected" sticker on the receptacle.

enter image description here

Now, this external tester will not work if the receptacle is not actually grounded. (there is no way to induce a ground fault without a ground). In that case, get a 3/2 prong "cheater" and extend the ground wire/tab with other wires, to reach a place with an actual ground. Plug the tester into that and it should work/trip.

enter image description here

Once it passes, also put a "No Equipment Ground" sticker there.

enter image description here

Now if this is a GFCI device and testing it also trips another one, then most likely it's fed off that other GFCI's LOAD outputs, which makes it redundant. Remove it and put it somewhere that matters.

** Especially avoid the ones that have digital logic to "help make the lights make more sense": you are much better off with a simple one and interpreting the lights yourself than using the silly legends. The words describing the light combinations, I call them the "magic 8-ball". But the lights are useful.


You can test it as follows:

  • turn off the circuit breaker
  • confirm that each outlet is now dead (outlet tester or non-contact tester)
  • remove cover of closest outlet
  • confirm no power in the box with non-contact tester
  • there should be at least two cables in the box, one line in and one or more carrying power downstream
  • if there is only one cable, either they are not on the same circuit or it is not the first outlet on the circuit; your plan won't work unless you find the first outlet on the circuit
  • if there are multiple cables, the black wires should be connected, either by both being attached to the same side of the outlet, or by a wire nut or other connector and a pigtail to the outlet
  • if multiple cables attached to outlet, remove one of the black wires, prop it so it does not touch any other wire or the box
  • turn on breaker
  • carefully test bare wire and black wire remaining on outlet to see which one is hot
  • that one is your LINE and will be connected to the LINE terminal on the GFCI; the other black is LOAD, the connection to downstream outlets or fixtures
  • if the black lines are pigtailed, you need to separate them and follow the same steps to see which is LINE (hot) and which carries power downstream
  • be sure to turn the breaker off again before proceeding
  • the white wire from the same cable as the hot black also needs to be connected to the neutral terminal of the LINE side of the GFCI, and the other whites to the LOAD side

Yes, you can use the first outlet in a circuit to protect downstream outlets if they draw their power from the LOAD side of the GFCI outlet.


Yep, that's how it works. But, according to regulations (The Code) you must put a "GFCI Protected" sticker on the normal looking outlets. This is so someone can hunt-down the tripped outlet after they find there's no tripped circuit breaker. Yeah, the GFCI stickers dry-out and fall of in just a couple of years, but you were legal when you started...the code is perfect, the code is perfect, the code is perfect.

  • GFCI outlets typically come with several stickers, so it is easy to comply with the code in that regard. Commented Sep 21, 2017 at 17:21
  • Welcome back @Iggy. Please keep in mind that the rules about being nice are still in place. Rude comments will be flagged and removed.
    – Tester101
    Commented Sep 21, 2017 at 17:47
  • Should I notify you when I notice you turning a blind-eye again?
    – Iggy
    Commented Sep 21, 2017 at 20:20
  • @Iggy Yes... If you see inappropriate behavior, please flag it for moderator attention.
    – Tester101
    Commented Sep 22, 2017 at 16:12

If it is, will replacing that receptacle with a GFCI receptacle give protection to the other two receptacles in the line ?

Depends on how you wire it. If you connect the downstream outlets to the "load" terminals then yes they will be protected.

However to do that requires a few things.

  1. You need to figure out which is the first receptacle in the line.
  2. Within the box for the first receptacle you need to figure out which wires are the incoming supply and which wires are the supply going out to downstream devices.
  3. You need to ensure that hot and neutral wiring downstream of the GFCI are seperate from all other wiring. This means you really want to inspect the whole circuit looking for any evidence of borrowed neutrals, multiwire bruanch chircuits etc.

OTOH if you pigtail the wires, connect them to the feed side of the GFCI and ignore the load side of the GFCI then only that one outlet will be protected.


The one thing I would add is that 'code' may restrict the number of plugs you can put on the load side of the GFCI outlet. When I built my garage I had to remove 1 or 2 outlets from both of the circuits I had wired because code only allowed for two outlets on the load side of the GFCI device. In his post yesterday, bib stated "Yes, you can use the first outlet in a circuit to protect downstream outlets if they draw their power from the LOAD side of the GFCI outlet." Make sure you follow this advice. If you wire it wrong you'll think you have protected outlets when you really don't.

  • 1
    Can you cite the relevant section of the code (NEC?) which says this?
    – mmathis
    Commented Sep 22, 2017 at 14:24
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    No I cannot. The garage was build circa 1998 and the inspector (a city employee) most often referred to CA code (where he had previously worked). The guy was a bully and vindictive but I digress. At the time it didn't occur to me to ask for section and verse of the NEC. In fact you are the only one that has ever done so, which I appreciate. It forced me to learn more about the NEC. I ran across many opinions but no one could quote a relevant section of the NEC. Based on what I found, there is no limit, so I got taken to the woodshed by a inspector who didn't know what he was talking about. Commented Sep 23, 2017 at 6:09

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