Our house was built in 2003 and it came with a GE 24" microwave oven that uses 120 volts/20 Amp. The microwave stops working so we went out and bought a KitchenAid convection microwave oven that requires 20 Amp, 240 volts, 60 Hertz.

The old microwave oven uses 12/2 w/g romex from a subpanel. In order to use 240 volts, I understand that I'd need to install 2-pole 20 amp breaker, and probably my best bet is to run a new 12/3 w/g romex. Use the black and red wires for hot, white for neutral, and bare wire for ground.

The problem is that it's a two-story house and it's nearly impossible for me to run a new line without ripping out some drywalls which I'm trying to avoid if possible. The house sits on a concrete foundation and the joists on the first floor's ceiling are running vertically from oven to subpanel. I've been on the attic but saw no signs of hope.

The question I have for you is: Can I use the existing 12/2 w/b romex? I'm thinking about using the black and white wires for hot and tape both ends of the white wire with red tape. Use the bare wire as neutral and tape both of its ends with white tape. Finally, share the ground with bare ground wire of the built-in oven sitting below. Just above the microwave oven, there is a built-in oven running 240v/30Amp/60Hz on a 10/3 w/g from a main panel.

So, the wiring would be:

MICROWAVE OVEN SIDE 12/2 w/g SIDE
red wire <---------> white wire taped red
black wire <---------> black wire
white wire <---------> bare wire taped white
bare wire <---------> bare wire from built-in oven (10/3 w/g)

Thanks in advance for any help.

--
Quang

stanfortyman

This is ONLY allowable if the existing cable is type SE. The bare wire in NM cable is not, and NEVER was, allowed to carry regular circuit current. PERIOD.
Frankly I am surprised that anyone is even suggesting it can.

Here is the applicable code section:

250.140 Frames of Ranges and Clothes Dryers

Frames of electric ranges, wall-mounted ovens, counter-mounted cooking units, clothes dryers, and outlet or junction boxes that are part of the circuit for these appliances shall be grounded in the manner specified by 250.134 or 250.138.

Exception: For existing branch circuit installations only where an equipment grounding conductor is not present in the outlet or junction box, the frames of electric ranges, wall-mounted ovens, counter-mounted cooking units, clothes dryers, and outlet or junction boxes that are part of the circuit for these appliances shall be permitted to be grounded to the grounded circuit conductor if all the following conditions are met.

(1) The supply circuit is 120/240-volt, single-phase, 3-wire; or 208Y/120-volt derived from a 3-phase, 4-wire, wye-connected system.

(2) The grounded conductor is not smaller than 10 AWG copper or 8 AWG aluminum.

(3) The grounded conductor is insulated, or the grounded conductor is uninsulated and part of a Type SE service-entrance cable and the branch circuit originates at the service equipment.

(4) Grounding contacts of receptacles furnished as part of the equipment are bonded to the equipment.



See, the exception is that the grounded conductor could also be used as the grounding conductor. NOT the other way around.
A NEUTRAL WAS ALWAYS REQUIRED!

donf

Standforty man,

Well I'm in serious trouble, because my place has a three wire drop and three wires com into the SEP. Two are Hot and one goes to the Neutral bus. On the neutral bus, both ground and neutrals are connected. Physically, there is no grounding bus in the SEP.

So I have only two hots and a Neutral, The house is currently receiving a 200 amp service. Now I know the at the SD panel, ground and neutral meet up. So I understand the wisdom of using the neutral bus for both ground and neutral connections. What I don't understand is how two electricians can read the same code cite and come up with two different answers both supported by the same text in the book. Sorry, that makes no sense to me.

Is there a method in place to get the NEC to explain what the paragraph is actually saying>

stanfortyman

Sorry Don. I am not following you.
How does this apply to the topic at hand? Meaning a household cooking appliance.

stanfortyman

The NEC Handbook is a BIG help in this respect.

tkrussell

Excellent, someone reads the exceptions to the exception.

Now the poster has a very clear answer.

Nicely done, Stan.

qn1234

Currently, it's a 12/2 wg (3-wires: black, white, and bare) NM cable. It's hooked up to a standard white 3-leg receptacle like the ones around the house. The old 24" microwave has a standard 3-leg plug like the one from a carpet vacuum. Had my wife wanted a 24" or 27" microwave there wouldn't be a problem. Hey, but I don't blame her; she wants something to match with the rest of other appliances.

I was aware of the voltage difference, but I was told it's going to be an easy job.

qn1234

I'm totally confused now!

First, there was hope, then little hope, then some hope, and it now looks like I'm going to have to bring out the hammer and knock down some walls. I've been staring at the walls and scratching my head.

stanfortyman

Really? I had hoped it was very clear.
What are you unsure about?

qn1234

I didn't mean to say that your posts were not clear. I think they are very helpful and make a lot of sense. It's me who lacks of knowledge about electrical wiring. After reading posts from various people I'm confused as to whether I will end up tearing down the walls, sale the unit and buy something else, or hire an electrician to do the work. I'm an average DIYer and I like to do it myself if it's feasible.

We spent some $1500 on this unit and hate to sale it at a loss. I'm afraid Sears might not want to take it back since we have it for a month now. Anyway, we rather keep this unit.

I really appreciate your and others' help.

Please keep talking. Maybe there is still a better answer out there.

donf

Well for one thing, Sears has a 30 day, free return policy. That solves the problem for you one way.

However, why are you thinking that you need to knock walls down? When I had to change the conductors from 10/3 to 8/3 for a new range/oven @ 40 amp, all I had to do was get my hand inside the the wall and pull the two staples out, then I attached a "Chinese finger" onto the the end of the 10/3 and used the existing 8/3 to pull the new wire down to the SEP.

Now I only had to pull about 20' of cable, so it went quite smoothly. However if you are the least bit unsure of what you have to do in the SEP, then under no circumstances should you attempt to do the work. You can do all the prep work, for example pulling the cable, than call the electrician to do the connections and test the work,

Regardless, though of who does the work, someone will have to get an electrical permit. Home owners can get permits, however, If I'm hiring someone to do the work for me, I would want him to do the entire job which would include pulling the new conductors and getting both the permit and the required electrical inspection.