Two circuits in the same receptacle box?

I have a good/convenient opportunity to add another kitchen receptacle on a new circuit since I'm remodeling on the back side of my kitchen. The subject wall currently has the frig on the same circuit as the microwave. I want to add another kitchen receptacle on a new circuit, but also put the microwave on this new circuit. This will allow the frig to be on its own dedicated circuit.

Existing circuit #1: frig and microwave (duplex receptacle)
New circuit #2: new receptacle and microwave receptacle

Can I simply splice circuit #1 together in the existing duplex box for the microwave (just as a pass-through circuit), and then have the new circuit #2 power the microwave receptacle (as an end of branch)?

The receptacle box is non-metallic. I'm assuming if this is acceptable, then the pass-through circuit can just have the grounds spliced together (ie. I don't need to splice both circuit grounds together)???

 

As long as the box has enough cubic inch capacity you can add a second circuit to the box.

The grounds must be spliced together and to the devices.

 

Will the end result be 2 separate branch circuits protected by two different breakers?

 

Yes, the plan was to have 2 separate branch circuits with different breakers. However, I changed my mind a little on separating the frig and wall outlet (wall outlet has microwave plugged into it). Instead I'm going to run a new dedicated 20 amp circuit above the stove for when we add an over the range microwave (probably within the next year).

My originaly plan, described in first post, would leave 2 outlets nearly side-by-side on a short counter. That would look a little odd (and not needed) for when the microwave gets moved above the range sometime this next year. The new plan will still have the frig on the same circuit as the duplex counter receptacle (currently used by microwave), but the cost/effort to separate this circuit doesn't seem feasible. The frig would be sharing a circuit with only small appliances (smaller energy usage than a microwave).

Going back to the over the range microwave circuit... I'll need to replace the 18" tall cabinet that's currently over the stove with a 12" or 15" cabinet. So I was going to install an adjustable electrical box on a stud behind the wall cabinet and cap off #12 wire inside of it. I'll just put 4 small drill holes through the drywall to mark the location for when the new cabinet and microwave get installed (the backside kitchen wall is exposed right now due to a bathroom project so access to the wall cavity is easy). From what I can tell I just need to make sure the receptacle will be located within the future cabinet since most microwaves are designed for cords entering a cabinet for power. If I'm planning wrong, let me know.

 

If you are going to mount the adjustable box you should bring it out through the cabinet and put the blank plate ot receptacle in it. Or just leave the cable behind the wall and cut in an old work box later.