In this case, you would simply not hook up the neutral wire. Instead you can just put a wirenut on it and tuck it neatly into the electrical box.
Typically 240V appliances require the neutral wire so that they can run the electronics at 120v or provide a plug on the appliance. In this case, these devices are hooked up to one leg of the hot and the neutral, giving 120V.
If no neutral is required, then the device can only operate on 240V/208V, though at 208V you should expect the oven to take longer to heat up.
What it means by "Connect only to a 3-wire, 120/240-volt power supply; the neutral conductor is not required for the operation of the appliance," is that the 240V needs to come from two legs of 120V service; this device won't function on 240V mains like you'd find in Europe.
Code may vary where you live
Yes. You are going to create a "Shared Neutral" circuit. This isn't difficult or dangerous, but you should do some labeling in the panel and the box itself.
First, it looks like the receptacle is a 30A/220V grounded receptacle (the wire itself may also be 30A -typical for an electric dryer- but that doesn't matter since you are using 20A breakers and receptacles). You are going to share the neutral and ground wires for these (2) new circuits by wire-nutting the two circuits in your first location (the junction inside the GFCI).
Shut off power to the 40 amp double pole breaker.
Assuming you have a red, black, white, and green/bare wire:
The red is one hot.
The black is the second hot.
The white is the neutral for both.
The green/bare is the ground for both.
Assuming that the "red" circuit will be the GFCI at this location, connect the Red to the hot\line side of the GFCI, make a pigtailed white wire to do the same. On the load side of the GFCI, run the continuation of the circuit.
Using the Black wire and another pigtailed neutral, junction these to the line that is going to the other room.
To be a good citizen: Label the inside of the box "Shared Neutral". Extra points if you "tie" the two hots together with a piece of white electrical tape with some extra length so it looks like a little "flag" that says "Shared Neutral". Double extra points if you label the cover plate so that someone sees it before they make any future modifications.
Go to your panel. If you are not comfortable working in the panel, shut off the main power and probably have a friend hold 1 or 2 flashlights for you.
Disconnect the (2) hot wires from the 40 amp double pole breaker (I assume these will be red and black). LEAVE the neutral and ground in place.
Remove the breaker and simply replace it with (2) 20 Amp breakers. Keep the same position in the panel OR make sure that the breakers are on OPPOSITE poles -as if they were a single 220 volt circuit. IT IS IMPERATIVE THAT THEY ARE NOT BOTH ON THE SAME POLE OR YOU RUN THE RISK OF SENDING 40 AMPS THOUGH THE NEUTRAL ON THE SAME POLE (If you need further explanation, please ask -this is the most important safety step). Some people will disagree and will say to use a 20 Amp double pole breaker. The problem with this is that it inaccurately appears to be (1) 220 volt circuit instead of (2) 110 volt circuits with a shared neutral. There is an exception to this rule below.*
LABEL The panel cover by bracketing the (2) circuit labels and write "Shared Neutral".
Extra points: inside the panel, tie the (2) hots together (the red & black) with a piece of white electrical tape that says "Shared Neutral".
NOTE: You will NOT be able to split the circuit after the GFCI ...so don't think you can give GFCI protection to both circuits with (1) GFCI receptacle. If you want the other circuit to be GFCI protected, you must use another GFCI receptacle in the first location of the second (black) circuit. You also will not be able to use a single pole GFCI breaker on either circuit. You must either use (2) GFCI receptacles as above. Alternatively, you may use (1) double-pole (220volt) GFCI circuit breaker -but you should still label the circuit as "Shared Neutral".
- EXCEPTION TO THE (2) 20 AMP CIRCUIT BREAKER RULE: If you want these circuits to be AFCI protected you must use (1) double pole (220 volt) AFCI circuit breaker. You should still label the circuits as "Shared Neutral".
...That was fun to write! :D
Peace,
Greg
Best Answer
Running 6/3 is future-resistant, but only a conduit is future proof
While many modern cooktops are designed for worldwide usage, running all controls and power electronics on 240V as that's what's guaranteed to be there no matter where you are, some North American market cooktops chintz out and rely on a neutral being present to run their controls, and the next owner may wish to rearrange things and put a range in, or tap an oven off this circuit for that matter -- both of these things typically require a neutral in North America.
So, running a 6/3 cable is more future-resistant than a 6/2; however, if you truly want a future-proof installation, you'd want to run a 3/4" conduit of some flavor to the cooktop location and then pull stranded THHNs through there -- ENT aka "smurf" is the cheapest option for this, just be mindful of the bending limits when installing it. The conduit option also has the benefit of being able to run the wire at 75°C, which allows you to run 65A over 6AWG copper. This is plenty of current, even for the largest cooking appliance setups.