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The truth about multi-rail PSUs - Page 2

post #11 of 19
You look at the specs of the PSU what the max combined load on the 12V rails is.

What Ropey listed was how the power is distributed across the rails. It's not a 'calculation' that adds up to 750W, since the values listed are the maximum loads on those rails. You have to look at it from the perspective that that PSU can deliver 60A on the 12V rail (which equals 720W on that rail alone), given that the other rails combined don't draw more than 30W (thus the total power draw is 720 + 30 = 750W, the limit of the PSU itself).

In practice,you will have some load on the 3.3V and 5V rails, and possibly some on the other rails, and thus the total max load on the 12V rail might be a bit less than 12V. This obviously depends on the components on your rig.

Having that all said, it's known that PC Power & Cooling traditionally underspecs their PSUs, thus that thing can likely deliver more than the rated 750W, and possibly even more per rail than the specs mention.
    
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post #12 of 19
Chozart you psu geek
    
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post #13 of 19
So if I purchase a PSU with "750W" and 56A onthe 12V rail (s). Then, I hook up a graphics card that sucks up...say, 34A. Would I need to put connectors from TWO different "Rails" on, to give it the necessary juice? Or Could I just hook up two PCIe connections on the same line, because its all coming from one source anyway?

This is how the M12 is hooked up, I believe. Its 4 12v Rails that terminate on the same output post.
    
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post #14 of 19
Well.. first, you won't be able to power a card that draws 34A with just two PCI-E power connectors. Those connectors are rated at 75W max, and then an additional 75W through the PCI-E slot itself. Thus, total max for a card with two PCI-E power connectors is 225W (or 18.75A).

But the main point is that the PCI-E power connectors likely are from different rails. Thus, just hook 'em up, and be happy.

Personally, I'd get a different PSU though, like this 750W Silverstone for $209.99: http://www.newegg.com/Product/Produc...82E16817256006

Or, probably even this 850W version for $279.99 (four PCI-E connectors, distributed across three of the four rails, 12V1 dedicated to the CPU): http://www.newegg.com/Product/Produc...82E16817256007

And all in all, it does not matter too much given the fact that the rails share the same source. Regardless, I would not pump more than the max load over a single rail, since the components of that rail (after the split) are not designed for that much current.
    
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post #15 of 19
Couple things:
1) PCIe slots provide up to 75w.
2) PCIe power cables provide up to 150w.
3) No card consumes 34A. No card even consumes 20A, although some(like the x1950XTX and 8800GTX) come very close.
     
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post #16 of 19
So in reading the case of my PSU, seeing as it says 12v1 - 14A || 12v2 - 16A -- that would be 'dual rails' but it cant handle that much, can it... Kind of disappointing, but this thing came with a Q-Pack case (lan party type cube thing) so I figured it was mostly junk. (its an Aspire)
    
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post #17 of 19
I've known this for a long time. either way my PSU is known as one of the most solid units ever built, and I've tested it as such...so I could care less.
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post #18 of 19
Quote:
Originally Posted by Chozart View Post
Well.. first, you won't be able to power a card that draws 34A with just two PCI-E power connectors. Those connectors are rated at 75W max, and then an additional 75W through the PCI-E slot itself. Thus, total max for a card with two PCI-E power connectors is 225W (or 18.75A).

But the main point is that the PCI-E power connectors likely are from different rails. Thus, just hook 'em up, and be happy.

Personally, I'd get a different PSU though, like this 750W Silverstone for $209.99: http://www.newegg.com/Product/Produc...82E16817256006

Or, probably even this 850W version for $279.99 (four PCI-E connectors, distributed across three of the four rails, 12V1 dedicated to the CPU): http://www.newegg.com/Product/Produc...82E16817256007

And all in all, it does not matter too much given the fact that the rails share the same source. Regardless, I would not pump more than the max load over a single rail, since the components of that rail (after the split) are not designed for that much current.
Dam Bro, you sell PSU for living? hehe
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post #19 of 19
Quote:
Originally Posted by Sideburns View Post
I've known this for a long time. either way my PSU is known as one of the most solid units ever built, and I've tested it as such...so I could care less.
The Seasonic S12 might actually be an exception to the rule with true dedicated rails
    
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