Originally Posted by N00builder
Can anyone take the time to explain to me how the 12v rails on power supply's work ie
quad 12v, dual 12v).
For instance, I have a OCZ gamestream 700w psu with 4 x 12v rails rated @ 18amp per rail. My video card requires 26amps on the 12 volt rail. I am not running SLI but in my case does the quad 12v rails add up to 72amps total, or am I only getting 18amps to my video card?
To really simplify things here, yes, the four 12V rails add up to supply your system's needs. However, you really only have a maximum of 18amps x 3 to play with. The reason is one of the 12V rails directly feeds the CPU (the four- or eight-pin 12V connector on your motherboard). The rest of the 12V rails are therefore left to combine to supply your system's needs.
So, given that you have a theoretical maximum of 54amps to play with, you also have to consider the power demands of the rest of your system. If we therefore subtract the 26amp requirement for the video card, that gives you a total of 28amps to feed the rest of your 12V needs. Keep in mind, though, that the more components you have on your system, the greater the demand on the 12V rails. HDDs, optical drives, and your cooling system(s) all share the 12V amperage output.
PSUs are immensely interesting components, in my opinion, and lately a lot of my studies are focused on this subsystem. Hopefully this information is communicated correctly here.
Hope this helps!