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I have an outlet underneath my kitchen sink that serves my dishwasher and my garbage disposal. The latter's circuit is toggled by a switch above the countertop. I need to replace the outlet, and was informed by the hardware store that any outlet I purchase can be used to serve the dual circuit setup. The hardware store employees left me uncertain as to their claim. What's more, the hardware store recommended I purchase a GFCI outlet to increase safety. While I understand the benefits of GFCI, I'm even less sure of its ability to be wired the way I require.

Will any outlet suffice for replacing the outlet I have under the sink, serving two different circuits (one of which is toggled by a switch)? If not, what do I need to be aware of going back to the hardware store to get what I need? Are there any other tips that would ease the installation?

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  • Calling this "two circuits" is confusing, as they may be on the same circuit breaker, which would be one "branch circuit", even though the top & bottom of the outlet are switched separately. – Jay Bazuzi Sep 5 '11 at 17:33
  • @Jay - I can confirm these two lines run to different breakers in the box, so I believe my description above is accurate, though it really doesn't bear much on the actual question as your comment suggests... – fbrereto Sep 6 '11 at 17:51
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A GFCI is a good idea because of the location of the outlet. However, I don't think that a GFCI that can simultaneously protect two circuits even exists (and I doubt one would fit into a single-gang box if it did exist), so I believe you have a couple of options:

  1. Install GFCI breakers on the dishwasher and disposal circuits, and use a regular outlet under your sink with the tab between the screw terminals broken off. This may be what you already have; the breakers will have a Test button on them if so. If the supply for the disposal runs to the switch first then to the outlet, this is the only way that the switch will be covered by the GFCI. In your place, this is what I would do: safer and less work, albeit a little more expensive.

  2. Use GFCI outlets. As I said above, you'd need two GFCIs to protect your two circuits, so you would have to install a double-gang box in place of the existing gang-box or add another single gang box then run the wiring for one of the appliances to it instead (in your place, I'd do the latter and use a remodeling box for the new outlet). However, this means that the switch for the disposal wouldn't be covered by the GFCI, potentially risking an electric shock.

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    If you do put in two outlets, you should do it so the line comes into the first GFCI (dishwasher), then the load side of that goes to the switch and second GFCI outlet for the disposal. This way, the switch is protected as well. Of course doing this depends on where the existing wiring runs (eg, does it go from the panel to the switch, or the outlet first?) – gregmac Sep 5 '11 at 21:11
  • I think the tabs between the terminals being broken is the key I've been missing - thanks! – fbrereto Sep 7 '11 at 5:33
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    Of course, looking at this again, there's no point in the second GFCI - if you wire it the way I described, the second outlet is already protected by the first GFCI, so it can just be a normal switched outlet. – gregmac Sep 7 '11 at 21:14
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Some things to know:

As of Jan 1, 1996, two major things changed with respect to the circuits you're working on. First, the "appliance branch circuit" (the circuit from your service panel that supplies the kitchen countertop outlets) had to be GFCI-protected in new construction and renovations, and second, that same circuit could not feed the disposer or dishwasher.

The upshot is that how you deal with this depends on how your kitchen is currently wired, which depends on the build date and/or the date of last renovation. If your home was built or the kitchen last redone after 1996, the dishwasher and disposer should have their own dedicated circuit (your wiring is illegal otherwise). That makes it very simple; swap the under-sink outlet for a GFCI. A GFCI breaker is available, but the outlet type are better IMO (They're closer to the source of the fault so they trip slightly faster, they're right there to reset should a fault occur, and they don't require you to shut off your entire house to install and replace them). The downside is that you'd have to have two outlets under-sink.

If the same circuit that supplies the disposer and dishwasher also powers the countertop outlets (common in pre-'96 construction), then more than likely, the GFCI should go on one of the countertop outlets, and you should replace the under-sink outlet with a normal outlet. Specifically, the GFCI should go on whatever outlet is the furthest "upstream" on the circuit (the one that receives power directly from the panel breaker, or is the closest to that one that you wish to protect), which is usually very unlikely to be the J-box under the sink. GFCIs can only protect the portion of the circuit that is "downstream" of the GFCI; they cannot kill power to anything "upstream" in the event of a fault. This was a little tricky in my house, where I found that I have two appliance branch circuits in my kitchen, one of which powers the fridge, microwave, and one outlet, and the other powers the dishwasher, disposer, and three appliance outlets. I wanted to protect the outlets and disposer/DW, but not kill power to my fridge if something should trip while we were out of the house. I found a way to make it all work for me, but your experience may vary.

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