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

Absolutely 1000% NO!


Lets get the facts.
Is this new oven 240v or 120/240v?
Meaning does it even require a neutral? A straight 240v load does NOT require a neutral and you CAN use your 12/2 to power it.

Does it have a plug? What does it look like or say on it?

Like above, you may not need a Neutral, Reidentify the white as hot at both ends, Green or bare for ground. The ground must run with the supply. If you have a Neutral on micro,
You will need new wiring. The plug will tell all.

Thank you both stanfortyman and stratmando for quick responses. Here's the exact text from the electrical section of the product's specifications:

20 Ampere, 240 Volts, 60 Hertz (1/second). AC-ONLY. USE COPPER WIRE ONLY. A FOUR-WIRE OR THREE-WIRE, SINGLE PHASE ELECTRICAL SUPPLY REQUIRED. A TIME-DELAY FUSE OR CIRCUIT BREAKER AND SEPARATE CIRCUIT IS RECOMMENDED.

The product is listed here: KitchenAid®: Microwave Ovens

If you clicked Installation Guide PDF link on the bottom of the page the 3-Wire cable installtion shows both white and bare wires from the unit are wired together.

So I wire them up as followed?

FROM MICROWAVE: FROM POWER BOX:
Black <----> Black
Red <----> White (taped red)
White + Bare <----> Bare

Thanks,
Quang

I would say wire like your last post. New construction and remodel require 4 wires(seperate neutral and ground.
If I couldn't get extra wire there, I would do 3 wire.

In AC circuits, the voltage on the black wire carries power to the appliance and the white wire is the intended return path to ground. If the appliance you're powering has a metal chassis, it'll usually have a third wire (the bare copper wire) connected directly to the chassis. That way, if the conductor in the black wire accidentally shorts to the chassis, the bare copper wire will carry the current and (hopefully) allow the circuit breaker to trip before you're electrocuted.

Because of this design, it can be dangerous to use a "cheater plug" to allow you to plug a three-wire cord into a socket with only two contacts. (In this case, the safety ground is missing, and you're putting yourself at risk of electrocution.)

Quang,

Do Not Wire you Outlet as Directed above!You will be putting 120 VAC directly to ground and you can kiss the new oven good bye.

First: Why do you sat your house is a three wire supply?

Second, If the Convection Oven will draw no more then 20 Amp, then yes you can use the existing wire, however, because the vendor is asking you to use slo-blow breakers, my immediate concerns are that the oven will draw more than the given 20 Amps. Look at the label on the back of the oven, does it give a Mim and Max Amperage. If it does, use the max amperage and the wire for that amperage along with the proper out let.

Third; Two wire can be used to do what you want. First you need a 20 amp single throw dual pole circuit breaker. Next, the breakers need to be close the Black lead in the box. That will mean that you will have to move a CB. If you don't have 2 empty slots together.

You place the black wire on the first breaker. Place the White wire on the second breaker. Wrap a small amount of Black or Red tape on the whits wire to flag that this wire is not a return wire. Next, the bare wire has to go over to the Neutral bus bar. At the receptacle end, Black would go to Black, White, with tape on it would go to the second power pole, It might show "white" on the receptacle or red. The bare wire must go to neutra connectionl.

The oven's should be grounded to it's own frame.

If you feel the least bit queasy about working around the main utility, it means you are smart. Call a licensed electrician and have him pull the required permit. When the work is finished, the work will be inspected. Keep the inspection certificate,

Just so you know, even if you disconnect the main breaker to the house, there is still electricity on the main lugs. More than enough to turn you into a bowl of soup, should you hit the wrong wires?

I don't mean to sound rude bit this is VERY incorrect.

A) The white is not always the "return" wire. Sometimes it IS a hot, or ungrounded, conductor, and acts just like a black or red for instance.

B) The white, or neutral, is NOT a return path to ground. This is a LONG perpetuated MYTH. Voltage is NOT seeking ground, it is seeking it's source.

According to the installation instructions that unit IS a 120/240v feed. The cable you have IS NOT appropriate. You DO need two hots, a neutral AND ground.

You can use 12/3wg NM cable. DO NOT use the 12/2.

Stan,

Okay, I see you are a licensed Electrical Contractor, dose that also carry with it a Master's or Journeyman's electrical licence? That's a real question, by the way. In Virginia, I can become an Electrical contractor but that would not allow me to pull permits. Only licensed Electricians can do that.

I fully understand that if the owner switches to 12/3 then he would have and over wrapped cable containing Black, Red, White and Bare.

But to do that, the poster will have to run a new cable from the SEP panel upstairs to the kitchen. He does not want to do that.

Regardless, He will need to change the breaker out, in favor of SPDT 240 20 amp, correct. Also, the existing 12/2 cable can be used by using the black and white (wrapped in black or red tape at both ends) to handle the two 120 legs of power and bare would then connect to the Neutral at the SEP panel?

Espically since it is 20 amp rated conductors. Am I still within code guide lines?

Next, why change out the conductors if what is already there can safely be used to carry the required power to the needed load?

Also, I read the Installation instructions and they list 20 amp max, but ask for slow-blow fuse or citcuit breakers at both ends. I've never heard of installing two breakers for one circuit, is that allowed by code and where would you put the breaker at the load end?

I'm not sure why this is even important.
In my state there is NO statewide license. If a locale requires a license you must acquire that license. Some areas do reciprocate though.




He does NOT have a choice.



NO. This is NOT equal to two 120v legs. It is simply a 240v circuit.
Even though each leg is 120v to ground, there is NO neutral to give you 120v, so the 120v is irrelevant.
This will require a 2-pole 20 amp breaker. Breakers are not rated in SPST or DPST terms. There are single pole, 2-pole and 3-pole breakers. They are ALL single throw.



Again, because there is no neutral present in the existing cable if the cable is used for 240v.
A 12/2 cable can be used for 120v or 240v but NOT both at the same time.
He needs a 120/240v circuit because there are BOTH 120v and 240v loads in the same appliance.



I did not notice that in the instructions. I have no idea how they would require a breaker at the load end or where it would be placed.

Actually, the instructions are for 27" and 30" microwave ovens. I think the 120v is referred to the 27" model and 240v is for the 30" model. If you looked at the top/left of page 5 where it describes on how to use 3-wire connection, it seems that I can use 12/2 w/g NM cable.

There is a Convection Oven right below the Convection Microwave Oven that I'm struggling with wiring. The Convection Oven uses 30 Amp 10/3 w/g.

I need to arrange breakers with two new ones since I ran out of room. Basically, I need a [20,20/20,20] and [15,20] breakers. 20/20 means dual pole.

I understand this part.

Good suggestions.

Thanks,
Quang

Could someone please take a moment and read page 5 of the installation instructions (found here: KitchenAid&#174;: Microwave Ovens)

And see if I can install this microwave oven using the existing 12/2 wg NM cable. I understand that I need to install a dual-pole single throw 20 Amp breaker. I just want to find out whether I can get away with the existing 12/2 gw NM cable or I need take out a hammer and start making holes on drywalls which I'm trying to avoid doing.

Thank you very much for your help in advance.

Quang

Look at the "3-wire" instructions. It states:
2. Connect the 2 white wires (D) and the green (or bare) ground
wire (of the microwave oven cable) using a UL listed wire
connector.

This is for an EXISTING older "3-wire" 120/240v circuit. Such as for a range. YOU DO NOT have this type of circuit.

You DO need 12/3.

NEITHER of these ovens is straight 120v. This is right on page 2 of the instructions:

When a 4-wire or 3-wire, single phase 120/208 or 120/240
volt, 60 Hz, AC only electrical supply is available, a 20-amp
maximum circuit protection is required, fused on both sides
of the line.


I see now about the "both sides" comment. This does not mean both ends of the circuit. It simply means both legs of the circuit. This just means a two-pole breaker.

NO, NO, NO! The bare or green wire can never be used as a current carrying conductor. The neutral (white) wire is carrying as much current as the black wire in a 120v circuit or, in this application, as much current as the difference between the black and red wires. The safety ground is connected to the chassis of the appliance to clear a fault (blow the breaker) when it occurs. If you think about it, if you use the ground as a neutral, if it gets lifted or broken somewhere down the chain of outlets or in the panel, that appliance chassis/case/bare metal can become hot and be a shock hazard.

Stanforty,

The question about the difference between a Contractor's License and Master Electrician's license was for my education only because I do not know what the difference(s) if there are any, not to cause angst.

The OP has stated that his home is a three wire drop. Which means two hots and ground or neutral. Correct? We all know that White is not always used as Neutral, just look at any switch loop, correct so far?

If Black and White are used to carry the voltage to the appliance outlet, they will deliver 240 VAC to the receptacle. All so, we know that you cannot take a 120 tap off the circuit from the breaker to the appliance. The only place the tap can be placed is inside the appliance itself, which is a common arrangement, correct?

All I am saying is that there really is no need to change the conductors unless the amperage is requires a change.

Black on one breaker, White (wrapped with Red tape) on both the breaker and the receptacle. Bare ground serves as the Neutral return. This is an "Grandfathered" acceptable circuit on a three wire serve drop.

Mr. Miller, I have never said to put power to the ground or bare wire. If you go backwards on my first entry I specifically pointed out the error.

In the previous post, I made a mistake; I meant to say 12/2 wg. I edited and I think you knew I meant to say 12/2 wg and not 12/3 wg.

The part I don't understand about the 3-wire connection where the neutral wire and green (or bare) are twisted as mentioned in the product's instructions is that it sounds dangerous. I suppose the ground is connected to the cover of the oven. So with this connection, wouldn't it shock me if I touch the cover?

Before I go any further with this question, please tell me what wires you have coming from the breaker to the receptacle. Are they Black, Red, White and bare or are they Black, White and bare ground?

Also, I'm going to pull my code book (2008) and make sure that my suggested connection method will allow for Black to Black, White with Red Tape to the Red wire, their white and bare connection to your Bare which is connected to the Neutral bus bar, not the ground bus bar inside the SEP panel.

If I'm out of step from the rest of the electrical world I'll let you know.

Please let me know how many wires are coming from your wall. Personally, I do not understand their tying White (Neutral) to ground. Neutral and ground serve two different purposes until they get to the Service Disconnect Panel.

Gentlemen electricians, regarding using a bare conductor as the neutral and equipment ground, for "counter mounted cooking units", as quoted in the NEC, please refer to the Exception for Section 250.140 Frames of Ranges and Dryers, (Both 2005 and 2008 editions).

For a home with an existing 2 wire cable with ground, the bare is allowed to be used as the neutral and equipment ground per this exception.

Since this is a counter cooking unit, and it has an existing 3 wire cable, (2 wire with ground), I see no issue with connecting it as per the 3 wire system shown in the instructions.