PSU 12v rail question

[dave_beast]

I have this PSU http://www.xigmatek.com/product.php?productid=38&type=specification

It's got 2 12v+ rails as per below

+12V1 30.0A/0.5A +5, -5% 120mV (1 x PCIEx 6pin and 1 PCIEx 6+2pin)

+12V2 22.0A/1.0A +5, -5% 120mV (1 x PCIEx 6pin and 1 PCIEx 6+2pin)

With 504W shared between the two.

My question is, what is the better way to connect my GPU (7970) 6pin, + 8pin, is it with 1 from each rail, or both from the same rail? (30a)

Reason I ask is when overclocking to 1250mhz the system will shutdown when both PCIEx leads come from the same rail, although i'm not sure whether i'm on the 22a or 30a as there is no visible difference

Edit: 1 set from 1 rail are yellow with blue stripe, and 1 set from the other rail are yellow with green stripe! No reference on Xigmateks website as to the difference...

 

[Clav]

If the PC shuts down when you use both from one rail then the only logical step is to try one from each rail. If it still happens get a better PSU with a high quality 12v rail.

 

[Tealc]

The additional power might be popping the current limiter on one of the virtual rails.

Most multi rail power supplies these days have an electronically split 12v rail with current limiting devices on each. The limiting device isn't always set exactly at 30A or 22A and is usually somewhat higher but in your case maybe they are set exactly on the limit.

Normally a PSU will apportion the 12v for CPU and motherboard to rail 1 and then perhaps peripherals and PCI-E2 to the second rail. The CPU will draw a lot of Amps and so will the motherboard as it needs to supply up to 75w to the graphics card along with everything else so may be hitting the limit on the first rail.

Is the shut down a crash or a total system shut down as if you cut the power?

 

[dave_beast]

It was a total system shut down, but since swapping to the 2nd pair I can't recreate it. The 2nd pair seem slightly thicker if that makes sense? Now I'm just limited by the ge bios limiting the volts to 1.18v.