Electrical – Wiring dishwasher and garbage disposal

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I am finishing my kitchen remodel but I had run the electrical supply lines previously, before I finished my basement, when its ceiling was exposed for routing. For some reason I can't exactly recall right now, I ran a 12/3 cable to supply the dishwasher and garbage disposal. I now think it was not the best choice because they should either be on a 15A (which corresponds to 14 AWG) dedicated circuit each or share a common 20A circuit. In other words, 20A each is an overkill and might be an overload on the side of the panel. The breaker at the panel is a double 20A where the switch is shared between the two circuits, in other words they are bother either on or off but can't be in a different state, because they share the same cable and the same neutral (I think this is called a multi-wire branch circuit).

Even though this may be perfectly legal (is it?), I prefer to go optimal and not provide more amps than can realistically be drawn. The dishwasher draws a maximum of 10.9A and the garbage disposal 5.9. I am asking for help to decide among several possible recourse options, assuming that the 12/3 cable going in cannot be changed as the distance is very long and blocked by drywall the entire route:

  1. Even though the wire is 12 AWG, replace the 20A double branch breaker with a 15A double branch circuit and the 20A rater terminal outlet, which both appliances share, with a 15A, making it effectively a 15A circuit for each appliance.
  2. Make it a single regular 20A circuit by simply not using the red wire inside the cable. Is this allowed?
  3. Leave it a 20A double branch circuit as is.

The main reason I would like to make this change is so that the main panel looks as elegant as possible and not overloaded so that a future buyer's home inspector doesn't ask any unnecessary questions. It is not so much a safety issue (or is it?) as it is about perception management.

Best Answer

The 12/3 and 20A is perfectly fine. This is a Multi-Wire Branch Circuit or MWBC. The basic idea is that by having two breakers on opposite legs (240V between them), you get 2 x 120V using 3 wires (hot/hot/neutral) instead of 4 wires (hot/hot/neutral/neutral). The catch is that the circuits have to turn off together at the breaker (your double breaker takes care of that).

From a safety standpoint, a disposal and dishwasher on 2 x 20A is really the same as if they shared a single 20A circuit. If one of the devices overloaded then the breaker would trip at the same 20A that it would trip if they were on the same circuit if the other device wasn't running at the same time. In reality, most overloads are going to be way over (short circuit or just a massive overload) and 15A vs. 20A just wouldn't make a difference anyway.

It also happens to be right now that you could put the two devices together on a single 20A circuit (sum < 16A) but if you replaced one of them (e.g., installed a more powerful disposal) you could end up with a problem. 2 x 20A = guaranteed no problem.

As a general rule, you can't mix and match circuit breaker sizes, wire sizes and devices. For example, you can't put a 15A receptacle on a 30A circuit. However, 15A and 20A are a special situation in the NEC. The end result is that, generally speaking, most devices that go on a 15A circuit (15A receptacles, a lot of built-in appliances except for ovens & cooktops) can just as easily go on a 20A circuit.

Just to be sure, I spot-checked a major US manufacturer dishwasher selected at random and the installation guide clearly states that it can be installed on a 15A or 20A circuit and plug-in or hardwired. Local codes may have specific requirements.

In this case, the one big catch is GFCI. You may or may not need GFCI on this circuit. If you do, then it gets a bit more complicated - either a GFCI double-breaker (expensive) or 2 separate GFCI receptacles instead of a duplex receptacle. Assuming GFCI is not needed, then depending on the instructions for your dishwasher and disposal, you can do one of the following:

Hardwired

  • 12/3 from breaker to a junction box.
  • Short 12/3 to switch box above counter. Black hot to incoming black, red switched hot to disposal black hot, white neutral to incoming neutral and disposal neutral and dishwasher neutral.
  • Dishwasher black hot to incoming red, dishwasher white neutral to incoming neutral and disposal neutral and dishwasher neutral.

Receptacles

  • Use a 20A duplex. Normally a 15A duplex is OK on a 20A circuit, but that is when you are thereby getting 2 15A receptacles on one 20A breaker. In this case, each 1/2 has 20A, so I suspect (can't say 100%) that you must use duplex 20A. And nothing is lost by doing that (cost is same or perhaps 50 cents different).
  • 12/3 from breaker to a junction box.
  • Short 12/3 to switch box above counter. Black hot to incoming black, red switched hot to top receptacle hot, white neutral to incoming neutral and receptacle neutral.
  • Incoming red to bottom receptacle hot.
  • Remove the hot-side tab from the duplex receptacle. Do not remove the neutral-side tab.
  • Plug disposal in top and dishwasher in bottom.

It is also possible to have one device hardwired and the other plug-in.

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