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I recently purchased an older home and wanted to install GFCI outlets in the kitchen. When I opened up the outlet, there were four wires (two black and two white) in addition to the ground. I watched a few videos online about installing GFCI outlets, but they only gave guidance about doing it when there's one black, one white and the ground. Currently, the two-prong outlets have all four wires attached - the two black wrapped around screws on one side and the two white wrapped around screws on the other side. What do I do with these "extra" black and white wires on the GFCI outlets? Attach them somehow or tie them off?

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You need to determine which pair is the line, and which pair is the load, and whether the load needs to be gfci protected. The answer to those questions completely alters how you wire the outlet. If you don't know how to do this, I'd recommend getting someone more experienced to assist since a mistake can result in an outlet that will still be powered even when the gfci is tripped. –  BMitch Jun 3 '13 at 0:18
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2 Answers

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.

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Another concern that you may or may not have is to make sure there is only a single GFI outlet on each circuit - having one GFI outlet feed a second GFI outlet is generally a Bad Thing. –  John Jun 3 '13 at 18:11
    
@John Why? What would happen? –  Tester101 Jun 3 '13 at 18:27
    
I don't know from my own experience, but my local licensed electrician (Master Electrician with 25+ years of experience) passed on this tidbit during a course I was taking on residential wiring. –  John Jun 3 '13 at 18:30
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@John There are two problems with installing a GFCI receptacle on the load side of another GFCI receptacle. 1.) You're wasting money, since GFCI receptacles are expensive and the circuit is already protected by the first GFCI. C.) If there is a ground-fault, it's anybodies guess which GFCI will trip. One will definitely trip, but there is no way to guarantee which it will be. This makes locating the ground-fault, just a bit more difficult. –  Tester101 Jun 4 '13 at 13:13
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It seems as though the wires are ment for a double outlet. here is a tutorial i found on youtube. http://www.youtube.com/watch?v=YBCA4-XwXFg

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Link only answers don't work well since links can go bad over time (and youtube videos can be removed). –  BMitch Jun 3 '13 at 13:19
    
Not to mention that in that video he installs a regular duplex receptacle, not a ground-fault circuit interrupter (GFCI) receptacle. –  Tester101 Jun 3 '13 at 14:22
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