240V outlet code question

I need to add a 240V-20A outlet for a largish window air conditioner in my home.
I have read several places that code now requires all 240V outlets to be four wire (i.e. three wire plus ground.) This doesn't make sense to me however, since if the outlet is for a device that uses only 240V (as opposed to both 120 & 240V, like a range or drier), then the neutral wire won't do anything, other than run from the panel to the outlet. The device plugged in won't even be connected to it.
I realize that it was once permissible to wire a drier with only three wires and that now four are required, and I can understand the logic for that code change, since there really should be a dedicated safety ground, but I don't understand the reasoning behind requiring a neutral wire in a 240V-only circuit. Shouldn't two hots and a ground be all that is needed, both electrically and for proper protection?
Is what I've read incorrect? Possibly just a misinterpretation of the newish requirement for split-voltage receptacles to be four wire?
It's not just on-line or in other suspect places where I've picked up this info, I've seen it in electrical books as well. For instance, Home Depot's "Wiring 1-2-3" book states in the "Adding a 240V Receptacle" chapter:
"...Recent codes require four-wire receptacles; three-wire receptacles were once acceptable."
In the very next paragraph they then seem to contradict that however, stating:
"a 20-amp, 240-volt receptacle for a window air conditioner or other appliance requires only 12/2 cable..." (and the illustration shows a standard three-slot, 240V-20A receptacle - with two hots and a ground.)
The next section of this book is "Adding a 120/240 Volt Receptacle", so it would seem that the above quotes are meant only to apply to 240V receptacles.
I'm confused. :-(
- Jeff
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I don't know about the US Code, but from what I have read on a number of forums, all 120/240V circuits require a four wire/four prong outlet. Previously they could use a cable without a ground, and the appliance used the neutral for a ground.
I am sure that for pure 240V items, only 2 wires plus ground would be required. When the item comes with a moulded plug you will need to use a matching receptical, whether it is twistlock or some other configuration, and matching wires in the cable will be needed.

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For "pure" 220v devices, only two power wires are truly required.
In Russia, for instance, they use 220v and two prong outlets for everything.
The rest is for safety purposes.
I understand why they need two power wires and ground here. But I am not sure why they need neutral for 220volt here. The current is conducted from one energized wire to another.
i
wrote:

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On 10 Jul 2003 15:15:47 GMT, Ignoramus11661

Because, phase to neutral is 220V there, as opposed to 120V in North America.

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snipped-for-privacy@ziggzigg.nullmodem.org (Jeff Morris) writes:

Yep, that's big all right.

The simplest solution is to just pull 12/3 w/ground to the box. Then you could install any receptacle (NEMA 6-20?) you want or swap the AC plug to any plug you want (NEMA 14-20?).
I think in the future we are going to see more and more NEMA 14 plugs and receptacles, so I would probably go that route.
You could avoid the whole plug and receptacle thing by mounting a disconnect and hard wiring the AC unit into it.
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There is always the rule that a safety ground should be a dedicated wire which normally carries no current. The neutral wire may carry current if the 220V load is not perfectly balanced. Now I suppose in this case the safety ground will only make a connection to the outlet box.
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The next A/C may well require the neutral, even if this one does not (most do). Who knows what the next person may do with that plug. Even if it is not required, it would be a good idea.
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On Thu, 10 Jul 2003 08:29:42 -0400, "donald girod"

Not necessarily. Just about all 220V A/Cs are 220V through out, not requiring neutral.

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240V receptacles for an A/C does not require 4 conductors (two hot, 1 neutral and one equipment ground) like a range/oven. An outlet for an A/C may be installed (not required) with the four conductors and you would wire nut the neutral, since it is not needed for your A/C, but four conductors are not required. You may choose to do this for future applications. Typically if your A/C is 240V, it does not have a 120V fan. A oven/range needs 120/240V (or a four conductor connection, 4 prong) since the some of the equipment on the unit will use 240V and some 120V.

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See http://www.lowes.com/lkn?action=howTo&p=Improve/220outlet.html&rn=RightNavFiles/rightNavLightingElectrical

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