[michaelt]

Most likely what they actually mean is:

This server has two IEC C20 connectors, rated for ~16 amps, each feeding a PSU rated for 1600W (i.e. 16A @ 100v)

If you're plugging in to 110v you shouldn't plug them both into the same outlet, as a 20A circuit can't supply 32A.

As each PSU is rated for 1600W you'll have to plug both in to get 3200W even if you're running on 220v - although they'd only draw ~7.2A each in that case.

US Residential 220v dryer outlets are usually wired one-circuit-to-one-outlet, and multi-way adaptors are discouraged. So although plugging two 7.2A loads into a single 20A feed would work from a current perspective (and indeed it's common in Europe), I don't know how easy it is to do legally.

If you're in a data centre with a 3-phase 220v power you probably know what you're doing. Your UPS guy will probably thank you if you split your load over two phases instead of putting the whole load onto one phase.

[oceanplexian]

Imagine dropping $15k on this but not wanting to spend $800 on an electrician to properly wire a 50A circuit so you run extension cords across the room creating a fire hazard.

As for the datacenter (I’ve racked many things with A/B power) the entire point is redundancy which this defeats the purpose of since each PSU is not properly rated. Seems incredibly bizarre to me in so many ways.

[michaelt]

> As for the datacenter (I’ve racked many things with A/B power) the entire point is redundancy which this defeats the purpose of. Seems incredibly bizarre to me in so many ways.

Yes - often for the data centre you'd end up with something like [1] with 4x 2700W power supplies, providing redundancy and ample power at the same time. It does mean you need four 220v power feeds though.

[1] https://www.supermicro.com/en/products/system/gpu/4u/sys-421...

[andersa]

Why is it a fire hazard if the extension cords are properly rated for this load?

[oceanplexian]

Extension cords are only supposed to be used for 90 days or so, you're technically violating the NEC if you're using them in a permanent installation.

[bombela]

You can feed a US outlet the split phase 240V and get two 120V@20A each.

It used to be done in kitchens in the US, back when appliances were power hungry. I have done so in my workshop for the same reason.

Houses are wired in split phase 240V, with the neutral in the middle. That is, you have two opposite 120V phases, around the same neutral.

This is a clever way to double the power, while adding a single wire.

In the US the standard outlet receptacle has two outlets. Bring the same neutral to the two outlets, and assign one phase per outlet (outlets have metals tabs you can break off, you don't need any extra wiring).

At the panel, you have a dual breaker. One breaker per phase, with a physical linkage forcing them to trip and arm together at once.

As a benefit; but very unsafe; you can make up a Y that plugs into the two 120V outlets, and gives you a single 240V receptacle. This is unsafe because if you plug only one of the 120V plug, the other one has now 120V on its exposed phase prong! On the other hand, I have both 240V@20A and 2×120V@20A anywhere in the shop ;)

[amluto]

Skip the Y hack and do it in style, legally!

https://store.leviton.com/products/duplex-receptacle-outlet-...

[bombela]

I am aware of this. But then I have a single 120@20 vs a single 240@20.

With my setup I have 2×120@20 always available, and 240@20 for the occasional welding.

I could assign a different 120 phase to every other outlet but then I would need some clear identification.

The two phases are assigned to the top and bottom outlets the same way all around the shop. If I need to run two high amperage machines, I only have to remember to use one bottom and one top outlet.

[mindslight]

If you're talking about a workshop and anticipating that much ad-hoc power usage, I'd just put two dual 6-20 receptacles side by side rather than splitting one. And then since you're actually creating the premises wiring, stick an (L)14-20R next to them in parallel and get rid of your need to fuss with hacky combiner cords. At least that's what I plan to do when I have the time for such luxuries.

[bombela]

I only very rarely need 240V, if I had permanently mounted 240V machines or frequent needs, I would do exactly what you propose.

[oasisbob]

GFCI requirements will interfere with the legality of many modern-day multi-wire branch circuit plans, yeah?

[amluto]

You can get a two-pole GFCI breaker for this purpose. The prices are a bit silly.

[bombela]

two poles breakers, 2×120V@20A, $USD:

    - $20
    - GFCI, $115
    - GFCI + AFCI, $115

Yes it is expensive, but it can also save your life.

[amluto]

Many things can save your life. Most of them don’t have a UL-provided monopoly making it quite unpleasant for anyone to compete to produce the version that fits where you need it.

A 1-pole GFCI breaker, a 2-pole GFCI breaker, and an ordinary GFCI outlet all have the same clever pair of coils, the same IC, and rather similar trip mechanisms. Yet the costs are quite different, and the costs get _really_ absurd if you want a breaker that trips at a level other than ~5mA.

And yes, they’re all very much worth using. I do wish that electrical codes would at least start encouraging the use of GFCIs for 240V outlets, which might encourage manufacturers to start making them, and those would actually be able to compete with each other.

[andersa]

These are not redundant PSUs, each PSU powers different GPUs in the same machine. Are you sure connecting them to different phases is a good idea?

I've been looking for a proper answer to this for a while, because I want to build a similar machine with 8 GPUs (~4500W max load) which would need to be split between two 16A 230V circuits.

[michaelt]

The transformers in the power supplies provide 'isolation' between the input and output - which means you can connect the outputs together, even when the inputs are on different phases.

Are you planning to build such a machine for your personal home use? If so you should be aware that (a) you might find server hardware hasn't thoroughly tested compatibility with things like suspend; (b) you might find games haven't thoroughly tested compatibility with multi-GPU setups; and (c) you might find the idle power consumption is 200W or more, even while doing nothing.

[andersa]

It's for personal use, though it would not run any games, it would be for running offline inference and other experiments. Probably not a smart purchase, but a fun one...

That is good to know multiple phases can work. Perhaps there would still be a fire risk in case of a short? Like somehow bridging the circuits > breakers don't trip?

[DSMan195276]

Keep in mind GPUs (and the rest of the computer) run on DC, not AC, so there is no phase by the time it comes to your computer. The PSU will step down the AC to the right voltage and then rectify it into DC, and they do that independently so whatever phase they started with shouldn't matter.

Something to keep in mind though is that (at least with consumer-grade PSUs) it is not safe to simply tie the outputs together, even if both PSUs produce 5V, 12V, 3.3V, etc. The voltages will be slightly different and connecting them together will cause current to flow back into one of the PSUs.

You can still use this setup though, the key is that the GPUs do not (or should not) connect the motherboard voltage provided via the card slot to the voltage provided via the power connector. This detail allows you to safely power the motherboard from one PSU and power the GPU from another one, you just have to be careful not to mix connectors on the same card between different PSUs (if it has multiple). Additionally the motherboard should be entirely powered from a single PSU.