A typical U.S. 240V circuit is actually just two 120V circuits. Fairly trivial to rewire for that.

It's more accurate to say that the typical 120V circuit is just a 240V source with the neutral tapped into the midpoint of the transformer winding.

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reactordev 14 hours ago | parent [-]

This. It definitely comes in at a higher voltage.

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amluto 11 hours ago | parent [-]

Sort of? It’s 120V RMS to ground.

	▲	razingeden 9 hours ago \| parent [-]
		yes, this is accurate for US and “works” but it’s against code here. you’ll get mildly shocked by metallic cabinets and fixtures especially if you’re barefoot and become the new shortest path to ground. old construction in the US sometimes did this intentionally (so old, the house didn’t have grounds. Or to “pass” an inspection and sell a place) but if a licensed electrician sees this they have to fix it. I’m dealing with a 75 year old house that’s set up this way, the primary issue this is causing is that a 50amp circuit for their HVACs are taking a shorter path to ground inside the house instead of in the panel. As a result the 50 amp circuit has blown through several of the common 20amp grounds and neutrals and left them with dead light fixtures and outlets because they’re bridged all over the place. If an HVAC or two does this, I’d advise against this for your 3200 watt AI rig. EU, you don’t want to try to energize your ground. They use step down transformers or power supplies capable of taking 115-250 (their systems are 240-250V across the load and neutral lines. Not 120 across the load and neutral like ours.) in the US. you’re talking about energizing your ground plane with 120v and I don’t want to call that safe… but it’s REALLY NOT SAFE to make yourself the shortest path to ground on say. a wet bathroom floor. with 220V-250v.

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projektfu 8 hours ago | parent | prev | next [-]

Yes, if you have a 240V US split-phase circuit you could make a little sub-panel with a 40A breaker feeding two 20A 120V circuits and plug the two power supplies into each side. (1600W would need a 20-A breaker because 13.3A would be too much of a 15A circuit). But it would probably make more sense to just plug them both into the same 40A 240V circuit. If you use NEMA 6-20, make sure you label it appropriately and probably color it red.

In Europe, you could plug the two power supplies into an appropriately sized 240V circuit.

In an apartment you can't rewire, you could set it up in your kitchen, which in the modern US code should have two separate 20A circuits. You will need to put it to sleep while you use appliances.

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razingeden 9 hours ago | parent | prev | next [-]

A US circuit is.

But this is re: European 240/250 which is 240 between its load and neutral

I’d say don’t energize either systems ground plane, but , really, don’t do this in EU

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0xbadcafebee 13 hours ago | parent | prev | next [-]

I think you're forgetting the wires? If you have one outlet with a 15-20A 120V circuit, then the wiring is almost certainly rated for 15-20A. If you just "combined" two 120V circuits into a 240V circuit, you still need an outlet that is rated for 30A, the wires leading to it also need to be rated for 30A, and it definitely needs a neutral. So you still need a new wire run if you don't have two 120V circuits right where you wanna plug in the box. To pass code you also may need to upsize conduit. If load is continuously near peak, it should be 50A instead of 30.

So basically you need a brand new circuit run if you don't have two 120V circuits next to each other. But if you're spending $65k on a single machine, an extra grand for an electrician to run conduit should be peanuts. While you're at it I would def add a whole-home GFCI, lightning/EMI arrestor, and a UPS at the outlet, so one big shock doesn't send $65k down the toilet.

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briandw 11 hours ago | parent | next [-]

Correct me if I’m wrong, but doubling the volts doesn't change the amps, it doubles the watts. Watts = V*A.

	▲	0xbadcafebee 6 hours ago \| parent \| next [-]
		Yes; I assumed 30A was minimum requirement for 240V service in US. Apparently I was wrong, 20A 240V is apparently normal. So in theory you could use a pre-existing 20A 120V circuit's wiring for a 240V (assuming it was 12/3 cable). And apparently 4-wire is now the standard for 240V service in US? Jesus we have a weird grid.
	▲	subscribed 11 hours ago \| parent \| prev [-]
		Doubling the volts halves the amps. P = I * V indeed.

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fc417fc802 12 hours ago | parent | prev [-]

I think you might've misread GP. (Or maybe I did?)

He's not saying you would use it as two separate 120v circuits sharing a ground but rather as a single 240v circuit. His point is that it's easy to rewire for 240v since it's the same as all the other wiring in your house just with both poles exposed.

Of course you do have to run a new wire rather than repurpose what's already in the wall since you need the entire circuit to yourself. So I think it's not as trivial as he's making out.

But then at that wattage you'll also want to punch an exhaust fan in for waste heat so it's not like you won't already be making some modifications.

	▲	projektfu 8 hours ago \| parent [-]
		The wiring (at least in the US) to the 120V outlets is just one half of the split-phase 240V. If you want to send 240V down a particular wire, you can do that, by changing the breaker, but then you lose the neutral. You also make the wires dangerous to people who don't realize that the white wire is now energized at 120V over ground. (Though it's best to test to be sure anyway, as polarity gets reversed by accident, etc.) Live wires should be black or red.

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doubled112 16 hours ago | parent | prev | next [-]

I’ve actually had half of my dryer outlet fail when half of the breaker failed.

Can confirm.

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amluto 16 hours ago | parent | prev | next [-]

Sometimes. 240V circuits may or may not have a neutral.

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jcgrillo 16 hours ago | parent | prev [-]

If you actually use two 120V circuits that way and one breaker flips the other half will send 120V through the load back into the other circuit. So while that circuit's breaker is flipped it is still live. Very bad. Much better to use a 240V breaker that picks up two rails in the panel.

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HWR_14 4 hours ago | parent | next [-]

They make connected circuit breakers for this use case, where one tripping automatically trips both.

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amluto 11 hours ago | parent | prev | next [-]

I assume the device has two separate PSUs, each of which accepts 120-240V, and neither of which will backfeed its supply.

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ycui1986 13 hours ago | parent | prev [-]

i am guessing, without any proof, that, when one breaker fails the server lose it all, or loose two GPUs, depending on whether one connected to the cpu side failed.

	▲	fc417fc802 12 hours ago \| parent [-]
		GPUs aren't electrically isolated from the motherboard though. An entire computer is a single unified power domain. The only place where there's isolation is stuff like USB ports to avoid dangerous ground loop currents. That said I believe the PSU itself provides full isolation and won't backfeed so using two on separate circuits should (maybe?) be safe. Although if one circuit tripped the other PSU would immediately be way over capacity. Hopefully that doesn't cause an extended brownout before the second one disables itself.