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First of all let me get this out of the way: Some things are not designed to work with the higher voltage and if that's the case then obviously you wouldn't attempt it with that specific device.

However, many modern devices are rated for a very wide input voltage range of 100-250 volts. The catch is these devices are still only single-hot-pole (like the 230 volt system in the EU uses only a single hot wire.)

Here in the USA, we have two-hot-pole 240 volt. Meaning there are two hot wires at 120 volts each.

Why can't I use both these hot wires on regular devices (as long as it states it supports 100-250 volts?) I would wire it as black hot to appliance black & red hot to appliance white/neutral.

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Mains guy here.

Nope. To be precise, Ms. Nope.

We often see people take a bog-standard NEMA 5-15 socket and intentionally miswire it so the (tall) neutral pin is actually connected to the opposing hot pole. That seems to work, when you plug in a computer via the common cord (NEMA 5-15P to IEC C13). But like many things that seem to work, it will kill you.

Anyway, it's rather silly to do, since there's a right way to do that very thing.

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You use the 3rd or 4th gal, officially called Ms. Nope or Ms. Winky-Nope. (they are female, after all). The NEMA 6 connector is precisely for that, and as you can see, it's the same form-factor (except for the pins, of course; those are keyed.) And that's it; done. Get a NEMA 6-15P to ICE C13 power cord, and you're good to go.

Hardwiring, yeah, that's OK

If you make the electrical connection hardwired, inside a junction box, then yes, you can attach appliance white/black to the two 240V hot wires (black/black, black/red, etc.)

You do not need to use 3-wire supply cable (black-white-red) if you only want 240V. You can use 2-wire cable (black-white) but you need to re-mark the white wires near each splice point to designate them as hot wires, typically with black tape. If your wiring is in conduit with individual wires, you can't use white for a hot; use the correct wire color.

Your wire color codes are green, yellow-green or bare = safety earthing (which is called "ground" in mains; not to be confused with electronics GND/Vss). In North America, gray and white = neutral, and all else = hot.

Don't eliminate your minimum required 120V outlets

Most houses are built with only the bare minimum number of 120V circuits and outlets - 1 circuit to a bedroom with outlets every 12', 2 20A circuits to the kitchen countertop receps. This means, if you re-wire one of those circuits to 240V, you will now fall short of the minimum number of 120V outlets. Can't do that. So unless your house was over-wired by a genius electrician, it probably means you'll need to run new circuits in the walls for the 240V.

Multi-wire branch circuits are a 120V alternative

If you are running /3 (black white red) cable anyway, this can be wired as a multi-wire branch circuit. It is capable of supporting 2 circuit's worth of power on one cable, as well as both 120V and 240V loads. However, this makes GFCI and AFCI protection more complicated, which are now required on most circuits that support 120V.

  • Thank you so much for the detailed response! My intentions with this was to use a standard PC ATX12V PSU with USA 240 volt mains. superuser.com/questions/1508139/… – he65 Dec 7 '19 at 18:52
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    @he65 you have to check the PSU and make sure it is OK with having 240V North American/Philippine style, with the former neutral being 120V above ground. The hot-ground insulation isn't the issue, it's the neutral-ground insulation. Most likely it's fine, because many Euro plugs are reversible, and you never know which side will be 230V above ground. – Harper - Reinstate Monica Dec 7 '19 at 19:12
  • When you say neutral-ground insulation, do you mean the neutral and ground not being tied together inside the PSU case (like how neutral and ground are bonded at the main panel.) I don't see this being a problem as per code the neutral and grounds should never be bonded elsewhere other than the main panel. – he65 Dec 7 '19 at 19:25
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    @he65 No, that's not an effective way to test insulation. Insulation breakdown works more like a VBO; you don't see it until voltage exceeds a certain number. It can only be tested with a megger. Given that we'd be testing the series not a particular unit, the best test I know is to have a German friend plug one in upside down :) That'll put 230V between N and G... heck when we megger things we only test to 150%, so that's plenty. – Harper - Reinstate Monica Dec 7 '19 at 20:45
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    @he65 Look up what a VBO is. It has infinity resistance up til a certain voltage, then it has very low resistance. That's how insulation breakdown is, it'll test out as infinity with a 2 volt DVM... but when you put service voltage on it, suddenly it's ground-faulting and you're like WTH? LOL. Anyway, that's academic. To do your diligence, I'd check the specs of the power supply and see what it says about insulation. If it says it's good, or if they sell it with a Schuko connector, you're solid. – Harper - Reinstate Monica Dec 7 '19 at 21:18
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Normal devices have normal power plugs associated with them, so you have to use the normal 120V sockets for them anyway. And if the device has a polarized plug, it might require a certain wire being neutral, or it will pose a safety hazard.

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    If an old 120 volt device has a polarized plug, thus possibly requiring one of the wires to be neutral, then it wouldn't be a good idea to attempt to power it with 240 volts. With that said, though, I have not seen any modern devices with a polarized plug. Most 2-prong plugs can be flipped around any which way and it'll work fine (unless of course it has a third prong for ground.) – he65 Dec 7 '19 at 20:44
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If the product has the label of a USA nationally recognized testing laboratory such as UL, cUL, or ETL, it should be fine for connection a USA standard 240 volt circuit. If the plug or connection terminals are rated 20 amps or less, I would not assume that it is ok to connect to a circuit that is rated higher than 20 amps. I don't see a request for advice about providing an appropriate 240 volt circuit, but I think @Harper etal has covered that.

  • I plan to wire up one of these receptacles with mains black hot on one side and mains red hot on the other side. i.stack.imgur.com/4JQA1.jpg – he65 Dec 7 '19 at 19:30
  • That is a 20 A, 240 V receptacle. It must be connected to a 20 A, 240 V circuit breaker using 12 AWG minimum wire size. If it is the only receptacle connected to that breaker, the entire 20 amp capacity can be used by one cord and plug connected load. – Charles Cowie Dec 7 '19 at 20:01

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