I simplified. I'm not saying that I would ever be pulling 84A off a single line or even all of them combined, the max I can pull is 25A (or 22A) per rail. What I am saying is that I have 84A distributed over 4 rails. OCP limits the max I can pull to 2x25A & 2x22A for each rail, respectively.
EDIT: When nVidia states one needs 30A on the 12V line, that's the same as when they say you need a 500W PSU. The card will never actually pull that much juice. When they say you need 30A on the 12V line, they assume that most people operate a PSU with one single 12v run supplying juice to ALL the components (CPU, drives, videocard etc). In reality the Card will pull maybe 15A or 170W.
Edit II: I stand corrected, the combined 12v rating is 64A..!
GA-EP43-UD3L . Q8300 . HIS 5850 . 2x2GB . Windows 7