Adding a GFCI to an existing circuit. 2 black, 2 white wire problem

gfcireceptacle

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.

Best Answer

It depends if you are just trying to connect that particular receptacle as a GFCI or you want to protect the entire circuit.

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.

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

Good Luck.

Understand the circuit

A standard duplex receptacle functions as both a receptacle, and as a junction. It allows you to connect cord-and-plug devices to the circuit, while at the same time allowing other hardwired devices to be connected to the circuit. Ground-fault circuit interrupter (GFCI) receptacles are similar, however, they offer ground-fault protection to all connected devices. To offer this protection, GFCI receptacles have two specific sides.

Line VS. Load

The Line side of a GFCI receptacle is where the feed line connects, to supply power to the device. The Load side of a GFCI receptacle is used to feed other devices, while offering them GFCI protection.

Find the line

Before you can figure out how to connect the device, you have to determine where the power is coming from, and where it's going to. To do this, you'll need a non-contact voltage detector, and a few twist-on wire connectors.

Turn off the circuit using the circuit breaker or fuse.
Verify the power is off using a non-contact voltage detector.
Remove all the wires from the receptacle, and place a twist-on wire connector on each wire individually.
Turn the power back on at the breaker/fuse.
Carefully, move the non-contact voltage detector near each wire.
When the meter lights up, mark the wire in some way.
Turn off the breaker/fuse again.

_{In this procedure, only one wire should make the meter light up. If more than one wire caused the meter to light, contact a local licensed Electrician.}

Now that you've located the ungrounded (hot) Line conductor, you'll have to also locate the Line grounded (neutral) conductor. To do this, simply follow the wire you marked in the previous step back to where it enters the box. You should notice that the wire is grouped with one to two other wires. The wire you found to be hot should be black, and it should be grouped with a white, and possibly uninsulated or green wire. These wires make up the Line feeder.

Hook it up

GFCI protection to downstream devices

Connect the black wire from the Line feeder to the brass screw terminal on the Line side of the GFCI receptacle (The receptacle should be clearly labeled LINE), the white wire from the Line feeder to the silver screw terminal on the Line side of the receptacle.
Next connect the black wire from the other group of wires to the brass screw terminal on the Load side of the GFCI receptacle, and the white wire to the silver screw terminal on the Load side of the GFCI receptacle.
Connect all the uninsulated/green wires together with an extra bit of uninsulated/green wire (about 6" long), using a twist-on wire connector or crimp connector.
Connect the other end of the extra bit of wire to the green (ground) screw terminal on the GFCI receptacle.

Once you restore the power to the circuit, all the devices downstream (on the Load side) from the GFCI receptacle will be GFCI protected. If this is not the desired outcome, please follow the steps below.

No GFCI protection to downstream device

Connect the black Line feeder to the other black wire and an extra bit of black wire (about 6" long), using a twist-on wire connector.
Connect the other end of the extra bit of wire to the brass screw terminal on the Line side of the GFCI receptacle.
Connect the white Line feeder to the other white wire and an extra bit of white wire (about 6" long), using a twist-on wire connector.
Connect the other end of the extra bit of wire to the silver screw terminal on the Line side of the GFCI receptacle.
Connect all the uninsulated/green wires together with an extra bit of uninsulated/green wire (about 6" long), using a twist-on wire connector or crimp connector.
Connect the other end of the extra bit of wire to the green (ground) screw terminal on the GFCI receptacle.
Leave the sticker covering the Load side terminals of the GFCI receptacle.

_{WARNING: If you lack the tools, knowledge, and/or confidence to complete this task, please do not hesitate to contact a local licensed Electrician.}

Electrical – Why is the GFCI receptacle tripping when I add another receptacle to the circuit

Look for a ground fault

Disconnect the wires feeding the new receptacle, at both ends.
Set your multimeter (you do have a multimeter, right?) to the lowest resistance setting, or the continuity setting if it has it.
Touch one probe to the green/bare grounding conductor, and the other to each of the other wires in turn.
repeat this process for every combination of wires (G-> B, G -> W, W -> B).

With the wiring disconnected, you should not get a reading on the multimeter. If you do, it means you have a short-circuit between that set of wires. Repeat this process on the new receptacle, touching one probe to the green ground terminal and the other to each the brass and silver terminals. Again, no combination should give a reading.

Your circuit is too long?

It may be possible that the circuit is just too long, and the resistance in the conductors themselves are causing the imbalance. Try connecting the new receptacle to the previous receptacle temporarily, using short pigtails (CAUTION: Working on energized circuits is dangerous.). Turn on the power, and try to set the GFCI. Make sure nobody is near the exposed receptacles, and don't touch, or get near them while the power is on.

If the GFCI holds, there is a fault in the wiring to the new receptacle or the circuit may simply be too long.

_{^{Warning: I haven't done any calculations, research, or testing on this theory, so it could be completely false. It's just a thought, but it's fairly easy for you to test.}}

According to the documentation for the Two-Pole QO®/QOB Circuit Breaker and Ground Fault Circuit Interrupter

NOTE: To minimize nuisance tripping:...

Do not connect circuit breaker to more than 250 ft. (76 m) of load conductor for the total one-way run

Is the power on?

GFCI receptacles can only be reset when the power is on. So if you're trying to reset it with the power off, you won't have any luck.