I have an opening in my cabinetry for a wall oven that has both a 120V grounded circuit outlet and a 2-wire + ground 240V circuit. My oven has a 3-wire + ground connection. Previously, the oven was connected to the 240V circuit only via a black-black, red-black, and white+ground-ground configuration. While this (obviously?) worked, it's probably not the safest configuration, nor does it sound like it's legal these days.
Assuming I could figure out which of the 240V hot wires is out of phase with the 120V circuit, could I connect the oven's 3-wire + ground to both the 120V and 240V circuits? As an example, have the oven's red, white, and ground go to the 120V outlet and the oven's black go to the 240V circuit?
Alternatively, could I leave the oven's red, black, and ground going to the 240V circuit and have the oven's white go to the 120V outlet?
EDIT: the wall oven unit is a KitchenAid oven + microwave combo unit (I don’t know the model number). The 240V circuit uses 8 gauge wire for the two hots. The oven wiring uses 10 gauge for the two hots and 12 gauge for the neutral.
Best Answer
No, you can't combine the two circuits because of NEC 300.3(B). (funny I was just answering another question about that one). It says all related wires must be together in the same cable. But also, they must be in the same circuit.
That bare "ground" from the wall
Now, if the ground wire from the wall is bare, it's probably illegal. When grounding came in in the 1960s, they did a bargain with appliance makers to continue allowing 3-prong H-H-Neutral connections, but with a big asterisk, and the asterisk was to start using 3-wire+ground cable as soon as stocks of the old cables ran out. So at least someone could convert to a safer 4-prong connection later. The legal cable types were:
Some electricians felt the other cables going out of stock was their "license" to use /2 w/ground. Wrong.
However... /2 w/ground actually is fine for a 240V-only range.
So if your cable is /2 w/ground, then something goes in the trash, either the cable or the neutral-hungry oven/range. Either replace the cable with /3 or get a range that is 240V-only.
If your cable type is SE (meshed neutral), then you have an option... but not quite as good. You can carefully insulate neutral, and then retrofit ground by running a #10 ground wire from that location to anywhere else that has a #10 or larger ground wire going back to that same panel. The list of possible appliances are water heater, dryer (if it's not in the same pickle), air conditioner, any non-flexible metal conduit, or the bare mesh copper "Grounding Electrode System" carrying ground out to the water pipe or ground rods. You need a split-bolt to tap the GES, you must not cut it.
Another very, very wackadoodle option, if you have the oven's schematics, is to re-wire the oven so the 120V loads on the oven are completely separated from the 240V power going to the oven heating elements. This may be quite easy if it has a big 2-pole contactor to control oven power. Now you can have two line cords - a NEMA 5-15 common recep with H-N-G that runs everything but the heating element -- that can go onto any 120V circuit including kitchen recep circuits........ and 2 fat hots and a ground which can go to that /2 cable. This must not have any conductors tied together or in common, except for safety ground.