I'm replacing a 20 year old microwave that is hard wired to a junction box. There are 2 lines of a romex going into the box -- a 12/3 and a 12/2. The 12/3 is on a dedicated circuit, and the 12/2 is on a circuit that also powers the refrigerator which is right next to the microwave. Both circuits are on 20 amp breakers at the panel, and they are not connected by a handle tie.

The power requirements of the old microwave are 120/240 volt, 30 amps. The new microwave requires a 120 volt 20 amp dedicated circuit, and it uses a standard 15 amp plug (which as a quick aside seems wrong because the microwave specs indicate a max current load of 15.9 amps which should land it squarely in 20 amp plug territory...)

My plan was to install a 20 amp recep on the junction box, but I'm a little unsure of how to wire it. It seems like the box is now overpowered with the 2 circuits and the single 12/3 should be able to power the microwave by itself. Or maybe I should wire up the recep with both circuits? Looking for a recommendation on the right way to wire the new receptacle given the current configuration.

edit: the old microwave is hardwired via a proprietary GE adapter that has 2 hots (pic added). The black hot is the black from the 12/3, and the red hot comes from the wire nut with the twisted red and black from the 12/3 and 12/2.

edit 2: added a photo of the inside of the breaker. The 2 circuits in question are the 2 20 amp breakers in the off position (not adjacent and no handle tie).

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  • What is the other hot in the 12/3 used for? Sep 21, 2022 at 11:46
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    Are you saying the old microwave is a 30A device but is on a 20A, single pole breaker? Also I can't quite figure what is happening at the wire nut with twisted red & black plus a red to the old microwave. Can you clarify wither of those? You might well be able to use the existing wiring for a 20A receptacle for the new microwave but you also have a box fill problem -- if that is a standard 1-gang wall box it is likely too small for all the wires plus a receptacle, both by code requirement and just physically having the space to fit everything.
    – trawson
    Sep 21, 2022 at 11:56
  • @ThreePhaseEel both hots are going to some proprietary GE adapter (added a photo), which plugs into the microwave. I'm guessing each one powers a different part of the microwave, as I had to flip both breakers at the panel to get the clock on the microwave to turn off. Sep 21, 2022 at 15:56
  • @trawson I added some details of the current wiring as well as a photo of the adapter currently being plugged into the old microwave. The twisted red/black comes from the 12/3 and 12/2, respectively, out to the red hot on the adapter that plugs into the microwave (added a photo). The other black hot from the 12/3 goes to the black hot on the adapter. Sep 21, 2022 at 16:08
  • Based on the photos so far, it looks like the old microwave was on a 240 V circuit. When you say it was on a "dedicated circuit" do you mean it has a 2-pole breaker with a single handle? Or was the 240 V circuit installed wrong and connected to 2 different circuit breakers? Sep 21, 2022 at 16:23

1 Answer 1


Can you post a picture of the inside of the panel with the two breakers in question in the frame? That would help confirm how they have you wired.

What we can assume from your description and the picture of the box in the back of the cabinet there is that your microwave was fed with a 12/3 which landed on 2 20A breakers on the panel (that are hopefully adjacent). This created a 120/240V circuit (red, black, white, ground) that fed your 120/240V microwave (although the ampacity was less than the appliance nameplate rating and also not handle-tied which is illegal).

From that box, they pulled a 12/2 which tapped one leg of that circuit and continued on to feed 120V to your refrigerator outlet.

That explains why your fridge goes out when you turn off one of the breakers, but you have to turn off both to completely de-energize the old microwave.

What you want to do now is install a 120V 20A receptacle on that 12/3, which is completely OK.

Here is how I would do it:

  1. Disconnect the old microwave wiring.

  2. Put the red/black wirenutted pair back. That is the feed for the refrigerator receptacle.

  3. Connect your new 120V 20A receptacle to the recently-abandoned black/white/ground from the 12/3.

  4. Handle-tie those two 20A breakers because they constitute what is called a multiwire branch circuit (MWBC) and both need to be turned off to de-energize that cable and box.

Furthermore - you may want to use a single receptacle like this one pictured. It will take up less space in the box and you'll only need the one anyway since it is a dedicated outlet inside a microwave cabinet.

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EDIT: Box Fill

Looking at 314.16 and assuming that that is a standard 1-gang box which it at least 18 cu. in., you'd be allowed 8 #12 conductors. Counting the 5 current-carrying conductors + 1 for the grounding conductors + double-counting the single yoke puts us right at 8. If that box were shallower than standard, you'd need to change the box.

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    This answer might be premature. There are some circuit breaker arrangements that should never be used for MWBC, such as tandem and quad units sharing the same bus. I would proceed very carefully with an inspection of the panelboard before connecting new receps. Sep 21, 2022 at 16:54
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    @RobertChapin True that there are many configurations that make MWBC unsafe or impossible, but if this were a tandem on the same bus, his old microwave would never have worked. We can reasonably assume that since he had a 120/240V microwave there already, then the existing 12/3 is connected to both buses.
    – Chris O
    Sep 21, 2022 at 17:02
  • Fair enough, I just don't make that kind of assumption having seen a MWBC recep come out of the wall glowing red hot. Sep 21, 2022 at 17:12
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    @ChrisO I thought he was saying he had two home runs from this box, a 12/2 and a 12/3. Your explanation makes more sense. But I still have questions about the box fill, even with the single receptacle I think he may have a code issue, whether or not the physical space works -- though it depends a lot on how deep the box is.
    – trawson
    Sep 21, 2022 at 17:35
  • @trawson Yep, in/out makes the most sense given that description and picture. If both cables were home runs, the red/black tied pair would be a dead short to the panel and solve itself real quick.
    – Chris O
    Sep 21, 2022 at 17:38

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