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Help me understand the +12v rail

post #1 of 8
Thread Starter 
Hi

I was looking through graphics cards and saw a card that in the description said "System Requirements: 550 W or greater power supply (with a minimum of 38 amps on the +12 V rail)".

my psu has 4 18A +12v rails. does that mean I can run graphics cards up to 72A or 18A?

Thanks
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post #2 of 8
The 6/8 pci-e powers will be off different rails. What is the card?
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post #3 of 8
The best way to deal with this is to ignore the whole single rail / multi rail thing. But in the case of PSUs that have multiple rails, such as yours which has four +12V@18A, you can't add them up for a total of 72A obviously (72A@12V=864W and you have a 700W PSU). Look under the 18A rails on your PSU... it says 675W for all 12V rails combined. That's your 12V capacity. Divide the watts by 12 volts to get your maximum combined ampere rating.
post #4 of 8


going to guess you're thinking about upgrading to a GTX 570...

If you had a single 12v rail, it would need to be able to push 38A, that's for the card and the entire system... but you have a multi-rail PSU meaning you should have 2 6pin PCI-E power connectors... and if you look past the 38a requirement... it says... or...
2 x 6 pin PCI Express power connector or 4 available hard disk power connectors


you're good...
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post #5 of 8
Each of your 4 +12V rails comes from pretty much the exact same source: after the big transformer, the output of the rectification stage.

All the molex, CPU 4/8 pin power, 24-pin ATX motherboard connector, and PCIe power connectors use +12V. Somebody wired those up into four groups, the four +12V rails. Each group has its combined current draw monitored such that if it goes well above 18A, the over-current protection will kick in and shut down the power supply. Unless a bunch of monkeys set up which connectors correspond to which rails, there's no way the protection will kick in no matter how you plug your stuff in--unless there's a short. It's designed to be a safety feature that catches shorts before something nasty happens.

So all you should look at is the combined +12V rating, 675W. 675W / 12V = 56.25A. You've got plenty.
post #6 of 8
Thread Starter 
Quote:
Originally Posted by mikeaj View Post
Each of your 4 +12V rails comes from pretty much the exact same source: after the big transformer, the output of the rectification stage.

All the molex, CPU 4/8 pin power, 24-pin ATX motherboard connector, and PCIe power connectors use +12V. Somebody wired those up into four groups, the four +12V rails. Each group has its combined current draw monitored such that if it goes well above 18A, the over-current protection will kick in and shut down the power supply. Unless a bunch of monkeys set up which connectors correspond to which rails, there's no way the protection will kick in no matter how you plug your stuff in--unless there's a short. It's designed to be a safety feature that catches shorts before something nasty happens.

So all you should look at is the combined +12V rating, 675W. 675W / 12V = 56.25A. You've got plenty.
ok but i suppose i couldn't run 2 in crossfire? 38*2=76 (above 56.25)
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post #7 of 8
I have no idea what graphics card you're looking at, but 38A is a conservative recommendation for the whole system based on having no idea (1) if your PSU "rated" at 38A can actually do 38A, (2) what your CPU is, since they vary widely in power consumption, and (3) how many hard drives (and really high-speed fans, etc.) you are using.

The only graphics cards that approach 38A on +12V by themselves are maybe dual-GPU models like GTX 590 or HD 6990, if overclocked a whole lot. A competent midrange GPU like a HD 6850 uses maybe like 10A in a game max, for brief peaks (maybe less).

Check the sticky thread for a quick estimate:
http://www.overclock.net/power-suppl...alculator.html
post #8 of 8
Thread Starter 
Quote:
Originally Posted by LiNERROR View Post


going to guess you're thinking about upgrading to a GTX 570...

If you had a single 12v rail, it would need to be able to push 38A, that's for the card and the entire system... but you have a multi-rail PSU meaning you should have 2 6pin PCI-E power connectors... and if you look past the 38a requirement... it says... or...
2 x 6 pin PCI Express power connector or 4 available hard disk power connectors


you're good...
Thanks for the reply! I havent seen that spec sheet before, the one on the side of the box has a few specs missing (as shown at bottom). I wasnt thinking of upgrading to that specific card but I was just using it as an example. I'm going to get ati as i have a amd system.

Pic of box specs:
ROG
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ROG
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CPUMotherboardGraphicsRAM
Intel 3770K 4.8GHz ASUS Maximus V Formula EVGA GeForce GTX 680 FTW+ 4GB Corsair Dominator 16GB 
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