15 ms·

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

by trollbridge 8mo ago

doubled112 8mo ago
I’ve actually had half of my dryer outlet fail when half of the breaker failed. Can confirm.
amluto 8mo ago
Sometimes. 240V circuits may or may not have a neutral.
jcgrillo 8mo ago
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.
- ycui1986 8mo ago
  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 8mo ago
    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.
- amluto 8mo ago
  I assume the device has two separate PSUs, each of which accepts 120-240V, and neither of which will backfeed its supply.
- HWR_14 8mo ago
  They make connected circuit breakers for this use case, where one tripping automatically trips both.
Salgat 8mo ago
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.
- reactordev 8mo ago
  This. It definitely comes in at a higher voltage.
  - amluto 8mo ago
    Sort of? It’s 120V RMS to ground.
    - razingeden 8mo ago
      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.
      - amluto 8mo ago
        > I’m dealing with a 75 year old house that’s set up this way I can’t tell what practice you’re referring to. Are you perhaps referring to older wiring that connects large appliances to a neutral and two hots but no ground, e.g. NEMA 10-30R receptacles? Those indeed suck and are rather dangerous. Extra dangerous if the neutral wiring is failing or undersized anywhere. But even NEMA 10-30R receptacles are still 120V RMS phase-to-ground. (And, bizarrely, there’s an entire generation of buildings where you might find proper 4-conductor wiring to the dryer outlet and a 10-30R installed — you can test the wiring and switch to 14-30R without any rewiring.) The exception for residential wiring is when the neutral feed from the utility transformer fails, in which case you may have 240V phase-to-phase with the actual Earth floating somewhere in the middle (via the service’s ground connection), which can result in phase-to-neutral and phase-to-ground measured anywhere in the house varying from 0 to 240V RMS. > wet bathroom floor A GFCI receptacle adds a considerable degree of safety and can be installed with arbitrarily old wiring. It’s even permitted by code to install one with no ground connection as long as you label it appropriately — look it up in your local code.
0xbadcafebee 8mo ago
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.
- fc417fc802 8mo ago
  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 8mo ago
    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.
- briandw 8mo ago
  Correct me if I’m wrong, but doubling the volts doesn't change the amps, it doubles the watts. Watts = V*A.
razingeden 8mo ago
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
projektfu 8mo ago
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.

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

doubled112 8mo ago

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

amluto 8mo ago

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

jcgrillo 8mo ago

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.

ycui1986 8mo ago
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 8mo ago
  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.
amluto 8mo ago
I assume the device has two separate PSUs, each of which accepts 120-240V, and neither of which will backfeed its supply.
HWR_14 8mo ago
They make connected circuit breakers for this use case, where one tripping automatically trips both.

Salgat 8mo ago

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.

reactordev 8mo ago
This. It definitely comes in at a higher voltage.
- amluto 8mo ago
  Sort of? It’s 120V RMS to ground.
  - razingeden 8mo ago
    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.
    - amluto 8mo ago
      > I’m dealing with a 75 year old house that’s set up this way I can’t tell what practice you’re referring to. Are you perhaps referring to older wiring that connects large appliances to a neutral and two hots but no ground, e.g. NEMA 10-30R receptacles? Those indeed suck and are rather dangerous. Extra dangerous if the neutral wiring is failing or undersized anywhere. But even NEMA 10-30R receptacles are still 120V RMS phase-to-ground. (And, bizarrely, there’s an entire generation of buildings where you might find proper 4-conductor wiring to the dryer outlet and a 10-30R installed — you can test the wiring and switch to 14-30R without any rewiring.) The exception for residential wiring is when the neutral feed from the utility transformer fails, in which case you may have 240V phase-to-phase with the actual Earth floating somewhere in the middle (via the service’s ground connection), which can result in phase-to-neutral and phase-to-ground measured anywhere in the house varying from 0 to 240V RMS. > wet bathroom floor A GFCI receptacle adds a considerable degree of safety and can be installed with arbitrarily old wiring. It’s even permitted by code to install one with no ground connection as long as you label it appropriately — look it up in your local code.

0xbadcafebee 8mo ago

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.

fc417fc802 8mo ago
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 8mo ago
  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.
briandw 8mo ago
Correct me if I’m wrong, but doubling the volts doesn't change the amps, it doubles the watts. Watts = V*A.

razingeden 8mo ago

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

projektfu 8mo ago

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.