As I understand it, disposals do not currently require GFCI. However, receptacles in a kitchen within 6 feet of sink, which I believe includes under a sink, do require GFCI.
As I understand the original question and comments, you have a single cable going to the switch and then to the receptacle with:
- Black = Hot (currently unused)
- Red = Switched Hot
- White = Neutral
I'll ignore ground - everything should have it, all grounds need to be connected together, etc.
I see two plausible alternatives:
My disposals are hardwired. Unless the disposal requires plug/cord installation (the ones I've dealt with do not, but I don't know what make/model you have) or local code requires plug/cord installation, this is definitely my recommendation.
- Remove the cord/plug from the disposal. The end should be in a small junction box that is an integral part of the disposal.
- Use a short piece of 14/2 NM cable to go from the disposal junction box to the wall box.
- Assuming the existing wall box is flush rather than surface mount, you may have to drill a hole in the back of the cabinet to get the cable through so that it can properly go into box. If it is a plastic box there should be integral clamps. If it is a metal box you'll knock out a knockout cover and put in a clamp.
In the box:
- Connect disposal black to switch cable red.
- Connect disposal white to switch cable white and add a white pigtail which goes to the receptacle line neutral.
- Connect switch cable black to receptacle line hot. (The same place that had the red.)
The switch will turn the disposal on/off, the receptacle will be on all the time, but the disposal will not be GFCI protected.
If you can easily remove the existing box and replace it with a two-gang box, great. But that probably isn't so easy to do, in which case add another single-gang box near the existing one. Alternatively, you could use a surface-mount metal box, which will be ugly (not inherently, but having a mix will look strange, *but it is under the sink, so who cares!).
Unless you go with a two-gang box (in which case there is more work to remove and replace the existing receptacle, but you can use simple wires (remove the outside of 14/3 cable if you want - that's legitimate for inside the box use) to connect the receptacles), you will need to run a short cable between the old box and the new box - pretty much the same as if you were going to hardwire the disposal, but a much shorter distance.
- Install a GFCI/receptacle in the new box (or 2nd half of replacement box). It doesn't have to be the same brand as the original.
- Leave the red wire as is.
- Connect the black wire to line hot on the new GFCI/receptacle.
- Remove the white wire from the original receptacle. Add two white pigtails and attach one to each receptacle's line neutral.
The original receptacle will continue to be for the disposal. The new receptacle will be on all the time. The disposal will still be GFCI protected.