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I need to wire a 20 amp, 240 volt, single yoke, double pole breaker for a thru the wall A/C. Also reknown as "115/240". Using 12g wire running 40 feet. I've seen this done using 12/2 and marking the white neutral wire RED, as your 2nd hot. Without a neutral wire does this still pass code? I would think NO, and you must use 12/3 wire (R,B,W,G)to have a capped neutral at the receptacle. Yet maybe this particular setup isn't going to need that "smart" neutral anytime soon and isn't needed.

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    Ain't no such thing as 115/240. 120/240, yes. 115/230 sure, but you need a time machine or a lot of line loss.
    – Ecnerwal
    Commented Aug 7, 2023 at 21:10
  • Ignore our Jester
    – Traveler
    Commented Aug 8, 2023 at 0:30

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Only new light switches need to have a neutral at the switch box for smart switches.

Pure 240 volt only appliances do not need neutral, only two hot/live wires and a ground. One of the hots can be a marked white wire, a 12/2 for 20 amps. If plugged in it will use a NEMA 6-20 type plug/receptacle.

You do need a neutral if the appliance uses 120 volts and 240 volts, like common dryers, the heater is 240v but the motor that turns the drum is only 120v. For these you use NEMA type 14-20 plugs/receptacles. These use 12/3 for 20 amps.

If hard wiring without plugs, then connect the wires as they should be. Hot to hot, neutral(if used) to neutral and ground to ground. If no neutral, you can use a marked white as the second hot.

You need to check is the AC unit needs 120v and 240v or just 240v.

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So far as I can recall NEC allows a neutral-colored wire (white or gray) to be re-purposed for hot/live use in just one situation: powering a pure 240-volt appliance through cable such as NM-B ("Romex"), armored cable like MC, SO cable, etc. If using conduit with individual conductors this isn't allowed; the exception is only for cables.

Air conditioners, water or space heaters, and shop equipment like air compressors or welders are examples of pure 240-volt loads.

When an appliance requires 120/240 then neutral is required and must be white or gray. Common examples are clothes dryers, kitchen ovens and ranges.

You're correct that the "switch loop exception" for using white as a hot conductor is no longer allowed because neutral is required to be present at switch locations.

All that said, if it's convenient, go ahead and provision that neutral conductor and cap it. It might come in handy some day.

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