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Current setup is a 20A breaker that powers:

  • Above counter: 2 gang box with:
    • Left side: a switch powering over-sink light
    • Right side: a switch for the garbage disposal
  • Under sink: A hardwired garbage disposal

Desired/plan:

  • Replace the garbage disposal switch with a 20A GFCI
  • Install a 15A receptacle below sink (where I'll plug in a button air switch for a plug-in disposal + hot water dispenser)

Nothing wrong with this plan I hope?

I have questions about how to properly wire the GFCI, so that it (I think should) also protect the under-sink outlet.

As I look to install the GFCI, the instructions say:

DO NOT install the GFCI receptacle in an electrical box containing (a) more than four (4) wires (not including the grounding wires) or (b) cables with more than two (2) wires (not including the grounding wire)."

As you can see from the photos, I have 3 cables going into the box. All 3 cables have black/white/ground. Can this be installed and if so what is the correct wiring?

Please let me know if you need more information and I can edit this. In USA.

  • 2
    It's likely that those instructions assume a single-gang box of a particular volume and they're warning you about overfilling. – brhans Oct 21 at 3:47
  • Where are you planning to move the switch for the sink light to? – ThreePhaseEel Oct 21 at 22:54
  • Nowhere - sink light switch should stay as is – philfreo Oct 22 at 0:08
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When UL lists the device, they list the instructions along with it.

The reason for the "DO NOT" instruction is because the instructions in the GFCI device are written for the specific case of one cable in; and 0 or 1 cable out. UL would not approve more complex instructions to handle every case; so the instructions say to get professional help if your case is more complex than the instructions.

The way to wire it is to identify "Always-hot" (which is hot at all times) and true Neutral. Pigtail "Always-hot" to the LINE brass screw. Pigtail Neutral to the LINE silver screw.

GFCI devices have the ability to provide GFCI protection to a downline. If you want to protect downline points-of-use, then for each downline load, hook its Hot wire to LOAD brass screw, and its Neutral wire to the LOAD silver screw. Only do these in pairs - both hot and neutral of any downline.

Note this can get quite confusing where switches are concerned.

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    And the standard advice is to get the LINE side installed & working before even thinking about taking the label off the LOAD screws. Once the power is properly coming into the outlet, pull the sticker off and get everything else working. – FreeMan Oct 21 at 13:09
  • Can you explain more about identifying the “always hot” wire? Does that mean only 1 of the 3 black wires is “always hot”? If I separate all 9 wires and use a contactless voltage tester, will that make it obvious? And you mentioned switches being more complicated — since I am keeping the light switch in that box too, any additional details on wiring would be very helpful. – philfreo Oct 22 at 0:13
  • @philfreo Just put everything on LINE and call it a day. Downline protection is nice when you can do it, but it's not mandatory. – Harper - Reinstate Monica Oct 22 at 1:13
  • Since the other plug (I believe downline) is under a sink, where I could have leaks, I was thinking it would be wise to do it. – philfreo Oct 22 at 11:53
  • Then follow Freeman's advice about hooking up only 2 wires to LINE until the GFCI checks out fully operational. Once you know that's solid, leave that alone. Then hook hot+neutral of anything you want to provide downline protection to, to LOAD. If it instantly trips, you hooked it up wrong. Feel free to do 1 downline at a time. If you get one dialed in right, mark the wires, pull it off and do the other one. – Harper - Reinstate Monica Oct 22 at 15:44

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