recommendation for wiring a new microwave using existing circuit(s)

Question

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).

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.

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.