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.
It sounds like this guy wires 4-wire ovens and ranges all day everyday. When you do that, you run into a lot of obsolete NEMA 10 wired circuits, hot-hot-neutral no earth.
It also sounds like he wired this range, he saw 3-wire supply and "ran home to mama" as it were and his muscles just wired it NEMA 10 out of habit.
Older ovens want 120V only so customers can use common off the shelf incandescent bulbs for the oven light. (Incandescents thrive in heat.) Many of the newest ranges, especially those cross-marketed in Europe or Asia, don't need or want 120V at all. They need a Hot Hot Ground connection, or NEMA 6. (NEMA 14, Hot Hot Neutral Ground, will also do.)
If your wiring is old, then you are mistaken about the third wire. It is not ground. It is neutral, and this is a NEMA 10 wiring arrangement. If the cable is /2 USE type and the third wire is a bunch of strands that wrap around the two hots, that was legal as a neutral prior to 1989. In any other type of cable, use of the bare wire as neutral was always illegal, but widely done because NM is cheaper than USE. You can correct this flaw by reclassifying that illegal neutral as a ground, and moving it to the ground bar. If the cable is USE, you can do that too but you are better off enjoying the grandfathered neutral which you are entitled to, and retrofitting a separate ground, which you are also entitled to do.
If the cable was recently installed for the last range whichwas a fancy Eurostyle job that didn't require neutral, they wired it with /2 cable as NEMA 6. Any modern use of /2 can only be NEMA 6 (Hot Hot Ground). That would be a rather foolish thing to do since it cost only a few dollars more to pull /3 cable instead, allowing modern NEMA 14 (hot hot neutral ground) connections which most modern ranges prefer. In this case you are married to NEMA 6, which most ranges cannot use.
So either
- the neutral-required range must go,
- or you must pull new cable,
- unless your setup is NEMA 10 with USE cable, in which case your electrician probably installed it "correctly" in the obsolete NEMA 10 manner. Legal but dangerous, so for safety you should retrofit ground, and separate neutral and ground at the range.
Wait. Did you say two blacks?
Whoa. All modern cable has differing colors - black and red. Two blacks means either a) very old cable, or fasten your seat belt b) the wire is individual wires in conduit.
The Conduit wiring method allows you to add individual wires. If you need a neutral, just get a long enough white #6 THWN-2 wire, and fish it on down the conduit, you're all set! If indeed it is in conduit, then failing to include the white neutral wire was a perfectly reasonable thing to do, since it's so easy to add later.
Best Answer
I agree with Jim. Just cap the white wire in the box. You are fortunate to have a 4 wire feed, in most cases here people have the opposite problem: "Requiring a 4 wire feed for a new cooktop but only having 2 hots and a ground".