You can retain the "big ass" 40A 8/3 cable. Here's how:
Take out the range receptacle and turn that box into multiwire shared neutral 20A circuits as you suggest. You would wire nut the black wire of a new 12/2 to either the black or red of the range cable and the whites of both wires get wirenutted. For the other new 12/2 wirenut the black to the red wire of the range if you already used its black wire. The whites are again shared. Don't forget the grounds. Then put a double pole 20A breaker in place of the double pole 40A breaker that was in there for the range. Then buy a blank 4 11/16" cover for the range box.

If his wiring is a little older, he may have a problem. My 24 year-old home uses a three conductor outlet for the 220-volt electric range. The wire feeding it has only two hots and a neutral. There is no separate ground.

Unless rontero has four conductors in that big cable of his (hot-hot-neutral-ground), he will need to run a dedicated ground for 120-Volt service.

Peace.

Marco

Thanks for the replies. Its a newer house, it has hot-hot-neutral-ground.

The original cable was not in a receptacle. It came up through the floor and was hard wired to the range. I get the idea though. I will move the 8/3 cable to the new location and wire it all together in the new box. I will just have to figure out what box to get. I'll run the other circuit up to the receptacle for the microwave.

Thanks again.

The boxes you can use are a 4" or 4 11/16" square box. I'd use the bigger box because of the fill. One more thing that I'm not sure of without looking. Will the #8 wire fit inside the lug of the 20A breaker?

One thing you might check into code wise. In this area
you're required to have Both 220v and gas available at the range.
So you might be looking at un-doing the whole thing at time of sale.

I would not even consider moving the range cable to be used for new recpetalces. Install new circuits. The microwave requires a dedicated 20 amp circuit. The counter surface requires minimum tow 20 amp circuits serving only the counter area.

Why wouldn't you considering using the cable junctioned off to two 20A circuits? That WOULD give the microwave it's own 20A circuit, as the thread starter said he would like to do.

BTW, WHERE is the requirement for a dedicate 20A circuit for a microwave?

BTW, WHERE is the requirement for a dedicate 20A circuit for a microwave?

It's in the installation instructions for one. There is also an NEC requirement that a range hood that is connected by cord and plug to be on a dedicated circuit... the over the range MW qualifies for that.

I am curious why I shouldn't consider moving that cable? The concern of the two 20A circuits supplying the counters is not an issue. They are already there.

The microwave circuit is needed according to the GE manual for that microwave. The installation manuals for both the range and the microwave range recommend a dedicated circuit for both. In fact, the original wiring had the dishwasher, garbage disposal, and OTR microwave on the same circuit. I wanted to separate the microwave off of it. The NEC requirement for hood is good to know.

article 422.16(B)(4) Range Hoods:
(5) The receptacle is supplied by an individual branch circuit.

That doesn't mean it has to be a dedicated circuit.

The NEC definition of an individual branch circuit is " A branch circuit that supplies only one utilization equipment".

Sounds pretty dedicated to me.

yep...you are correct. Too early yet. I was thinking of hard wired...

Why wouldn't you considering using the cable junctioned off to two 20A circuits? That WOULD give the microwave it's own 20A circuit, as the thread starter said he would like to do.

It's unprofessional.
You will have a hard time getting the #8 or #6 wire into a 20 breaker.
You can't bury the junction box inside the wall or behind the cabinets. It needs to remain accessible.

If the stove was going in the same place I might consider it.
You could run the stove off one of your counter receptacles if you want.

Fitting the wire into the breaker is a concern. I have a 20a breaker that it does fit but its not double pole so I am going to look for one. If I dont find it then its all off anyway.

As far as moving it goes, maybe this was confusing. The current cable just comes up through the floor. I am going to push it back through the floor, unhook it from the staples in the basement and then push it up throught he floor and into the wall where it is moving to. Then I will put a box in that wall that will contain the receptacle for the range and then run a cable up the wall to the cabinet above the range and put a receptacle in there.

You don't have to put the #8 in the breaker. Just pigtail on a #12 at the panel end just like you're doing at the other end.

It might not be up to professional standards, but it is not unsafe nor does it violate code.

I'm sure you already know this, yet juuust in case...

By using a double-pole breaker, if the either the microwave or your new oven trips, you'll knock out both. Why not just use two singles, instead?

BTW...I think your plan of re-using the 220V service is a great idea. Just be sure you get a BIG box to hold the wiring, connections, and 120V oven outlet.

Peace.

Marco

For sure you must not do it that way. If you put a 240 volt 50 amp range receptacle in, you must not put in a 15 or 20 amp duplex receptacle.
If you insist on doing it this way then do not install a range receptacle. Put a blank cover plate. You do not need a double pole breaker. You could use only the black, white and ground wires for a single circuit.

By using a double-pole breaker, if the either the microwave or your new oven trips, you'll knock out both. Why not just use two singles, instead?

Well, because the times they are a changing. To meet Section 210.4(B) of the 2008 NEC, multiwire branch circuits must have simultaneous disconnects at the point where the branch circuit originates. That means that two singles without handle ties won't cut it anymore once the new code version phases in. (I have no idea whether the OP's project is governed by the 2008 code yet. Jes sayin'...)

Here is a link that might be useful: Article re: Section 210.4(B)

joed...

I don't think rontero is putting in a 240V range receptacle. Instead, he's installing a 120V outlet on one of the phases. He's going to plug his new 120V gas range into it. The other 120V phase will be hardwired to the microwave/vent combo.

terribletom...

Thanks for the info!

rontero...

Please read terribletom's post above. Please ignore my suggestion about using a single breaker, as well.

Peace.

Marco

Here's an intriguing option: use the existing 240V wiring for one of the new over-the-range GE Advantium 240 microwave/speedcookers/convection ovens. Reuse the 120V/15A wiring for the gas stove ignition.

The Advantium 240 line was just revamped. The new ones look and function as regular microwave ovens, but with 5500w available, can also double as a regular 2nd oven, or triple as a high-speed oven that combines microwave and halogen cooking to create quickly-cooked dishes that are crispy and don't have the usual soggy microwave taste and texture.

You would need to downrate the breaker from 40A to 30A, but otherwise it should work easily.

"Reuse the 120V/15A wiring for the gas stove ignition."

There is no existing 120V cc't to reuse. That's the OP's problem. You can't pull a 120V 12/2 cc't off an 8/3 without breakering down everything to 20A.