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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.

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  • What make and model is your oven, and what gauge are the wires in question? – ThreePhaseEel Aug 17 '20 at 3:22
  • I updated my post with answers to your questions. – Bryan Aug 17 '20 at 3:41
  • 240v is 2 each 120v circuits so there is no such thing as a 120v out of phase with the existing 240. Notice the 240v comes from a breaker that is in 2 positions say breaker 1&2 a odd and even this is L1 & L2 split phase 240 between the 2 but 120v to neutral. We need more info on the size of the existing wiring and breakers plus the requirements to tell you how to hook it up or if safe with what you have. – Ed Beal Aug 17 '20 at 3:48
  • What gauge are the wires on the 120V circuit? – ThreePhaseEel Aug 17 '20 at 11:40
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    I’m not sure of the wire gauge used in the 120V circuit. I haven’t taken the receptacle out of the wall. I can take a look today and let you know. – Bryan Aug 17 '20 at 12:17
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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:

  • SE service entrance cable (in small sizes) -- two hots and a bare neutral in a mesh around the hots. This is made for the service entrance or lateral from the utility, which supplies 2 hots and a bare neutral. If this is the type of cable you have, then it's legal to wire N+G to the bare mesh Neutral... but only because you are grandfathered.
  • /3 no-ground cable -- insulated white neutral. Deprecated because /3 has ground now.
  • /3 w/ground cable, when stock ran out of the above cables. The ground went unused.
  • NEVER legal: /2 w/ground cable misusing ground as neutral. Ground can't be re-tasked to be neutral.

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.

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  • Yeah, the ground wire on the 240V circuit is bare. I was really hoping I could just run the oven’s neutral to the 120V receptacle. I had thought about the option of rewriting the oven, but figured leveraging the two circuits available would be easier. I’ll ask the appliance repair person about the option of rewiring the oven. I’m assuming even a unit with just an oven would still need a neutral since the control panel is likely just 120V but maybe not. – Bryan Aug 17 '20 at 4:12
  • @Bryan Oh, you definitely can't poach neutral like that. That could overload neutral - and neutrals don't have breakers! Yeah, if there is not a neutral in that cable, it can't power a neutral-requiring oven. – Harper - Reinstate Monica Aug 17 '20 at 4:16
  • OK, so a few more questions then... 1) how/why was the oven working wired to the /2+gnd before the remodel, and 2) how is rewiring oven to have the 120V side completely separate from 240V side any different neutral-wise? Isn’t the neutral just used by the 120V side of things as the oven’s wired now? – Bryan Aug 17 '20 at 5:20
  • 1) becuase we're talking about safety, not function. If you toss safety out the window, lots of things are possible. Where we're at is our old buddy NEC 300.3. All wires related to a circuit must be in the same cable, which has the side effect that currents are equal and opposite in any cable, beneficial in several ways. 2) what's relevant is how the current is traveling. With the modifications, everything but the heating elements takes power and returns it via the 120V cable. The heating elements take and return via the 240V cable. – Harper - Reinstate Monica Aug 17 '20 at 5:30

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