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I would like to install a receptacle under the sink to control the dishwasher and garbage disposal, with a single pole single throw switch mounted in the wall above the counter top to control the garbage disposal. In the load center I would like to use a 20 amp GFCI breaker to power the circuit. I'm using #12/3 romex to feed the circuit. My question is, would it be better for me to replace the garbage disposal with a unit that has an automatic sensor that activates the disposal when the plug is inserted or turned rather than going to all the trouble of mounting a switch on the wall. Since the dishwasher and disposal don't run at the same time and the full load amps are less than 14 amps on either appliance, I'm thinking I can just use a 20 amp GFCI receptacle and save on the GFCI breaker. I would really like a professional opinion on which way proceed.

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In what jurisdiction is the property? Will you pull a permit? –  Bryce Nov 23 '13 at 17:47
    
When you say you're using 12/3 are you using 12/3 with ground, or 12/2 with ground? –  Tester101 Nov 25 '13 at 11:08

2 Answers 2

It is not permissible under any electrical code to assume the dishwasher and disposal won't be used at the same time. If they can be used at the same time, you must design for that condition. For the United States:

National Electrical Code 2008

Article 210 Branch Circuits

II. Branch-Circuit Ratings

210.19 Conductors - Minimum Ampacity and Size

(A) Branch Circuits Not More Than 600 Volts

(1) General. Branch-circuit conductors shall have an ampacity not less than the maximum load to be served. Where a branch circuit supplies continuous loads or any combination of continuous and noncontinuous loads, the minimum branch-circuit conductor size, before the application of any adjustment or correction factors, shall have an allowable ampacity not less than the noncontinuous load plus 125 percent of the continuous load.

Beyond that it's better to either hard-wire both appliances, or provide two GFCI plug positions. The code gets more complex for mixed setups. See also the answer at Can I split the direct wire to dishwasher to handle a garbage disposal as well?

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If both devices are cord-and-plug attached, you do not have to size the conductors based on the ampere rating of the devices. Only the conductors in the individual cords would be sized based on the device, not the house wiring. –  Tester101 Nov 26 '13 at 12:15
    
@Tester101 it would be inadvisable to create a situation where the two plug connected devices are known to overload the socket they are plugged into, even if that were code permissible which I don't think it is. –  Bryce Dec 4 '13 at 10:01

If you're using 12/3 with ground, non-metallic sheathed cable from the panel to the outlet, you'll have no problem doing what you want.

Materials:

  • 20 ampere double pole combination GFCI circuit breaker.
  • 20 ampere duplex receptacle.
  • Length of 12/3 with ground non-metallic sheathed cable.
  • 20 ampere single pole single throw switch.
  • 2x single gang boxes.

Procedure:

Pull the cable

  1. Install a length of 12/3 cable between the switch box and the receptacle box.
  2. Install a length of 12/3 cable between the receptacle box and the service panel.

Install the switch

  1. Connect the Bare/Green grounding conductor to the Green grounding screw on the switch.
  2. Connect the Red ungrounded (hot) conductor to the common switching terminals on the switch.
  3. Connect the Black ungrounded (hot) conductor to the switched terminal on the switch.
  4. Use a twist-on wire connectors rated for use with a single 12 AWG conductor to cap off the grounded (neutral) conductor.
  5. Tuck all the wires into the box, and mount the switch.

Install the receptacle

  1. Clip the tab connecting the upper and lower receptacles on the ungrounded (hot) side of the duplex receptacle.
  2. Connect the Bare/Green grounding conductor to the Green grounding screw on the receptacle, and to the Bare/Green grounding conductor from the cable to the switch.
  3. Connect the White grounded (neutral) conductor from the feeder, to one of the grounded (neutral) screw terminals on the receptacle.
  4. Connect the White grounded (neutral) conductor from the cable to the switch, to the other grounded (neutral) screw terminal on the receptacle.
  5. Connect the Black ungrounded (hot) conductor from the feeder to one of the ungrounded (hot) screw terminals on the receptacle.
  6. Connect the Black Ungrounded (hot) switched conductor from the cable to the switch, to the other ungrounded (hot) screw terminal on the receptacle.
  7. Using a twist-on wire connector rated for use with two 12 AWG conductors, splice the two Red ungrounded (hot) conductors together.

Install the breaker

  1. Terminate the Bare/Green grounding conductor to the grounding bus bar.
  2. Install the breaker in the panel (make sure it is in the OFF position).
  3. Terminate the White grounded (neutral) lead from the breaker to the neutral bus bar.
  4. Connect the White grounded (neutral) conductor from the circuit to the neutral terminal of the breaker.
  5. Connect the Black ungrounded (hot) conductor to one of the ungrounded (hot) terminals on the breaker.
  6. Connect the Red ungrounded (hot) conductor to the other ungrounded (hot) terminal on the breaker.

Turn on and test the GFCI breaker

  1. Switch the breaker to the ON position.
  2. Press the TEST button on the breaker.
  3. Switch the breaker back to the ON position.

If the breaker will not switch on, there is a ground-fault, the wiring is wrong, or the breaker is bad. Detect and correct the fault, and reset the breaker.

tl;dr

Here is an image showing what the finished circuit should look like.

enter image description here
Click for larger view

NOTES:

  • If using metal boxes, make sure to bond them to the grounding system.
  • You can also use 12/2 with ground, but you might find that if both devices are run simultaneously, the breaker might trip.
    enter image description here
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