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).