Electrical wiring help please - GFCI wiring related

I need to replace a standard outlet located in a garage with a GFCI. I thought it would be straightforward but I cannot get the wiring right.

The old outlet is in a box with a switch. I cannot visualize the cables so it is very difficult to tell visually which wires are load and which are line.

The old outlet had 4 wires attached:

One black wire to a brass screw
One red wire to a brass screw
One white wire to a silver screw
A second black wire to a silver screw

The red wire is a short piggyback that goes straight to the switch. The switch controls power to the outlet.

I tried pretty much every configuration of wiring, starting out with what made the most sense to me and ended up grasping at straws trying different combinations.

There is a small indicator light on the GFCI - sometimes it was red, other times green. But even when green there was no power to the GFCI (verified both with a ground tester and a small lamp).

However, when the circuit breaker was on, there was always power to the other garage outlets, including the door opener, which, as near as I could tell, are all downstream of the GFCI outlet.

I am at a loss. Any advice?



Is the existing outlet a split outlet? In other words, has the metal connector that ties the two screws together on the brass side been cut? I’m guessing that the black wire that is on the silver screw is a neutral, despite its color. Why someone would do that is beyond me, unless both the wires on that side are in the same piece of Romex and the installer was just being lazy.

Thanks, @Chefguy.

I don’t think the metal connector has been snipped, at least as far as I can tell from the photos I took (see below).

I thought it very strange to put a black wire on silver screw.

And what the heck is up with the red wire connecting from brass screw to switch?

Obviously I’ve reached the outer limits of my “expertise.”



If the outlet & switch are properly wired, then the left side with the red wire & black wire is hot (second photo), and right side with the white wire & black wire is neutral (third photo). But if it were me I would first confirm that it’s not wired backwards. To do this, measure the voltage between the left side and ground: it should be 120 VAC. And then measure the voltage between the right side and ground: it should be close to 0 V. The hardest part is finding a good ground (which is defined as a low-resistance path back to neutral/ground busbar inside the circuit breaker box). The metal box or metal conduit might be a good ground. If not, you might need to run a temporary wire between the ground on a grounded outlet and this location, and then make the measurements.

After you do that, cut power to the outlet and remove the four wires. Also unplug everything from the downstream outlets. Turn power back on and start measuring voltages at the wires. Two pair should be 120 V: those wires go “upstream” toward the circuit breaker box, and should connect to the line side of the GFCI receptacle. The other two presumably go “downstream” to another receptacle or whatever, and should connect to the load side of the GFCI receptacle. And because the GFCI receptacle will not be grounded, you need to affix a sticker to it that says “No equipment ground”. The GFCI receptacle might come with a sticker. If not, make your own label.

Just guessing here.

I suspect the black wires may go to a light, although they appear to be going down the conduit. Follow the conduit and see what they go to.

Note the black wires are stranded, not solid. To me, this suggests they were added, likely not by a licensed electrician as indicated by both the live side and neutral side are white. My guess, the “house wiring” (HW) is typical romex-type wiring, that is, black and white insulated solid wire. The black wire of the HW is connected to the light switch. The white wire of the HW is now connected to the silver side of the outlet. Look to see if there is a ground wire (green or bare wire). If you find one, verify it as ground with a voltmeter.

I suspect the original wiring had just an outlet and someone went back and added the switch and whatever the black wires go to because they wanted it to be switched. Stranded wire is generally not used in household wiring because it is more expensive. Stranded wires are used where the wires will flex easier, they are not as susceptible to metal fatigue.

If you could get a picture of the wiring to the light switch, it might be helpful. Is it a three-way switch?

Thank you, @Crafter_Man, lots of good info.

I don’t own a multimeter (I know, I know), just a no-contact voltage tester. Could I get by using that to check the four disconnected wires for voltage per your second paragraph?

Assuming I do this, I would expect two wires to light up the tester and two to not light it up. The two that light go to the Line connections, the two that do not go to the Load connections on the GFCI, correct?

Can you shed any light (heh) on the 4-inch red wire that goes from the brass screw to the switch? I was assuming that the red wire is Load, making the white wire Line(?). So I thought I’d only have to sort the two black wires, which turned out to be easier said than done.


Get a volt meter or an electrician. Even a cheapo meter from Harbor Freight would be helpful. About $10. Home Depot/Lowes probably has a decent one for $25-$50.

Also note there is conduit coming from the top and going out the bottom. The second conduit going down is smaller diameter, suggesting that it was not installed when the original wiring was. If you could determine whether the wire from the breaker box comes from the bottom and the top goes to a light, or perhaps the wire from the breaker comes from the top and the large diameter conduit goes to additional outlets.

Finally, a GFCI outlet doesn’t make much sense if there is not a ground. You really need a good ground in the garage.

A GFCI doesn’t need a ground. It measures the current through the hot and neutral wires and trips if they aren’t equal.

If you install a GFCI and there is no ground present you are supposed to add a label to the outlet to indicate the lack of ground.

I’ve never used a non-contact tester, so I can’t really say. You really should get a digital DMM. You can get one for less than $20 on Amazon.

I am assuming one side of the switch goes upstream to the circuit breaker box, and connects to a circuit breaker. The other wire (short red wire) goes to the hot side of the outlet.

What is the rating of the circuit breaker? 15 A or 20 A?

Is there a ground wire anywhere in the box?

So thinking about this a bit more, here’s what I would do:

  1. Cut power.

  2. Unplug anything from downstream outlets.

  3. Remove switch, outlet, and short red wire. Discard all of these.

So now there should be four wires in the box. Let’s call them A, B, C, and D:

A: This is the wire going to the switch. I am not sure what color it is.

B: Black wire currently going to the left side of the outlet.

C: White wire going to the right side of the outlet.

D: Black wire going to the right side of the outlet.

  1. Make sure no wires are touching other.

  2. Turn power back on.

  3. Turn knob on DMM to measure AC voltage (200 V scale or higher), and carefully make these voltage measurements:

A vs. B
A vs. C
A vs. D
B vs. C
B vs. D
C vs. D

Now, for the above measurements, one of them should be 120 VAC. These two wires are going “upstream” toward the circuit breaker box: one goes to a circuit breaker in the breaker box, while the other goes to the neutral/ground bus bar in the breaker box. I would assume one of the wires is A, and I would further assume A goes to a circuit breaker in the breaker box. But I would want to confirm this. To confirm this, measure the voltage between each of these two wires and a known ground: one of these wires will be 120 VAC vs. ground; this is the “hot” wire that goes back to a circuit breaker in the breaker box. It will connect to the switch, and the other side of the switch will connect to the hot/line side of the GFCI receptacle. The other wire will connect to the neutral/line side of the GFCI receptacle.

So now you have the other two wires. These go downstream, to other outlets or whatever. You need to determine which one is hot and which one is neutral.

  1. Cut power.

  2. Plug a long extension cord in to one of the downstream outlets.

  3. Rotate switch on DMM to resistance mode. Select the lowest range.

  4. Verify which wire is hot (short/narrow prong) and which is neutral (tall/wide prong).

  5. Put everything back together. For the short wire between the switch and outlet, just cut it from a roll of NM cable (“Romex”). I would use 12 AWG wire. But I suppose you can use 14 AWG if the circuit breaker is rated for 15 A.

Oh, and when you buy a new switch, get one rated for 20 A, not a 15 A, regardless of the circuit breaker rating.

I don’t have a photo of the switch at this time. This is a house I inherited, a half-hour away. It did not pass inspection, no GFCI in the garage is part of the list of things to fix. I’ve already spent four hours this morning on this (although I got a workout in running up and down the basement steps).

The switch is a single pole, not a three-way. As near as I can tell, the feed from the panel comes from below. The switch/outlet in the photo seems to be the first stop on the circuit, which is why I am putting the GFCI there. There is another outlet, not shown, immediately above the shown switch/outlet combo. From there it ascends to other outlets in the garage. These other outlets had power even when the new GFCI (incorrectly) installed did not.

There is no ground wire. The circuit breaker is 15 A.

@Crafter_Man, everything you said makes sense to me, and I appreciate you laying it out step-wise as you did. But man, I thought this was going to be a 20-minute job.


Half of all home repairs take twice as long as you think they will. The other half take four times as long.

If it only takes as much time as you expected, you did something wrong.

You left out the part about 2 trips to the hardware store for each part you install. One to buy the wrong thing, and one to go back and exchange it for the right thing.

Sadly, the Perversity of the Universe prohibits doubling up and buying more than one correct part per trip. No matter how hard you try or how bulging your cart on the way out - it’ll still be just one correct part. :wink:

Or buy one of these.

Don’t I see one in one of the pics in post #3?

Oh, yeah. I was looking at the outlet, not the background.

Also, the outlet I am replacing is only two-pronged.


Yea, with only a two prong outlet, I don’t think a tester can determine if it’s wired correctly or if it’s backwards. As mentioned earlier, you need a “known, good” ground to be 100% sure. To get a ground, you could plug a long extension cord into a grounded outlet. And then make voltage measurements between the wires in the box and the ground terminal on the end of the extension cord.

If I’m seeing things correctly in the pics, isn’t that a metal two-gang box, and shouldn’t IT be grounded anyway (with a grounding screw tying the ground wire into the box), if only because of the toggle, but not for the proposed GFCI?

ETA: Also … @Mean_Mr.Mustard … did you verify that the GFCI wasn’t “tripped” when you were assessing the efficacy of your original work?

Yes, it is a metal box. I guess I just assumed it was not grounded because I didn’t see any obvious grounding wire. Maybe it is.

And yeah, it did occur to me that the GFCI may have been tripped. It was not.


OK, back to basics…

(the following is with respect to the original outlet, before I even started monkeying around with it):

  1. The red wire runs from a brass screw to a switch.
  2. With the switch on, the outlet has power
  3. With the switch off, the outlet has no power

Knowing this, can we conclude that the red wire is hot (and should be connected to LINE on the GFCI and not LOAD)?