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+12V rail on PSUs, what does it power?

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October 24, 2013 3:09:03 AM

hi everyone

my question is about the +12V rail, i know that high-end Graphic cards draw their power from this rail. then i recently knew that haswell CPUs also get their power from this rail (through the Mobo VRM). am i right until now?

i have an "XFX PRO1050W Black Edition Full Modular 80PLUS Gold" PSU with 87A on its only one +12V rail. i bought a EVGA GTX 780 SC ACX (the recommended amperage on the +12V rail is 42A".

so i assumed that i will be able to get another 780 in the future as SLi (42*2=84), then i realized that my i5-4670k will draw about 17A when OCed on load. (169W / 12V / 0.8 efficiency => 17A)

so in total i will need 42A+42A+17A = 101A on the +12V rail?

can anyone explain to me? am i right?

More about : 12v rail psus power

a b ) Power supply
October 24, 2013 3:14:19 AM

Although I can't explain the theory (hopefully somebody else can), I can tell you that a 1050W PSU will be plenty for 2x 780s.
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October 24, 2013 3:21:27 AM

mi1ez said:
Although I can't explain the theory (hopefully somebody else can), I can tell you that a 1050W PSU will be plenty for 2x 780s.


i know that the overall power is more than enough, my question was about the amperage capacity on the +12V rail, will it be enough to power up 2 780s? plus the OCed CPU? will the CPU draw its power (17A) from the same rail?
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a b ) Power supply
October 24, 2013 3:31:01 AM

You're figuring the efficiency in the wrong order. The DC output of the unit is competely independent of the the AC->DC efficiency. To max out a 1050w PSU that's 80% efficient you'd have to be pulling 1313w from the wall. The EVGA recommendation is what they estimate you need to run your entire system, plus a generous margin. The reality is that the average enthusiast system with an EVGA GTX 780 SC ACX would only need ~32A on the 12v rail, so they tell you 42, just to cover their rear.

What it comes down to is the fact that you could run three of that card in your system and heavily overclock the whole thing with the PSU you're running.
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October 24, 2013 3:49:48 AM

cuecuemore said:
You're figuring the efficiency in the wrong order. The DC output of the unit is competely independent of the the AC->DC efficiency. To max out a 1050w PSU that's 80% efficient you'd have to be pulling 1313w from the wall. The EVGA recommendation is what they estimate you need to run your entire system, plus a generous margin. The reality is that the average enthusiast system with an EVGA GTX 780 SC ACX would only need ~32A on the 12v rail, so they tell you 42, just to cover their rear.

What it comes down to is the fact that you could run three of that card in your system and heavily overclock the whole thing with the PSU you're running.


thanks cuecuemore, things make sense now.
i've been looking around and found that a singe 780 draws about 230W (or ~20A) only. add to that the OCed CPU (169W or 14A).

Much appreciated. :) 
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