Dual vs single rail

What's the brand and model number of the PSU?

Dual rail of 18 amps does not always mean 36 amps total more likely 30 amps. The PCIe connector is only of one of the rails so 18 would be the limit.

OCZ ModXStream Pro 500 W Power Supply, i bought it because it has a golden medal at secretshardware and it got a good price.

^+1 really depends on if you need both connectors.

ok so if i got it rigth you´re telling me if i use the second 6+8 cable i have the complete 36 amp at my disposal except for the amp needed by the cpu and other stuffs and that psu can handle an hd 7950?

36amps is what your PSU can handle max on the +12 volts. Yes that PSU can handle HD7950.

can you tell me the maximun gpu that psu can handle, i just wanna make sure of it limits because that issue of the single vs double rail got me confused. Thanks

Go here: http://www.hwcompare.com/
Compare the cards you are interested in for wattage. Divide by 12 (v) to get the amps the +12v rail should be required to handle for the respective card(s).

This can be useful http://www.realhardtechx.com/index_archivos/Page362.htm
remember the card gets 75watts through the slot itself a 6pin is additional 75 watts and the 8pin 150watts. (watts devided by volts (12) gives amps).

ok the hd 7970 takes 20 amps to be powered up and as you tell me earlier the combine 12v rail amps of my psu is 36 and minus cpu and the others there´s plenty of power then.

Yes, assuming the PSU is wired so that the 2nd PCIe cable is on the 2nd rail. Which normally would be the case.
The HD 7950 3GB requires 200W at max. That's apprx. 16.7A on the +12v rail, or 8A+ per rail. A 125W CPU (for example) will require 10.4A from those rails also. 200W + 125W = 325W just for the CPU and GPU. Other +12V devices won't be as demanding. If possible, you should try to keep your PSU at 80% or less. But not carved in stone. Remember, your GFX card will rarely reach its maximum demand of that 200W.

(most CPUs require less than 125W, but I used that example as a max)

HD 7970? Are you changing your GFX card choice now?

As it seems in the image the second cable goes on the second trail (i think so), the red one so this is wired rigth?

No, it´s just hypotetically speaking taking one of the most power hungry gpu into the play.

The colors mean nothing. But, myself, I would naturally assume a name-brand 2 rail PSU will have each of the 2 PCIe cables on different rails. You would need a schematic of the PSU to verify for sure, but it is almost a given.

Ok thanks to all for bringing light to the matter.

Best answer selected by pit_1209.

http://www.hardwaresecrets.com/article/OCZ-ModXStream-P...

+12V1: The cables that are permanently attached to the power supply.

+12V2: The cables from the modular cabling system. <--- Video card is all on this rail.