n00b power supply question

I know this is probably a really n00bish question but...


I'm builiding a new computer in a week or two and all of my time in research has gone into the main components (cpu, gpu, MoBo...) but i was wondering, if a power supply has dual 12v rails and only one is used (non sli/crossfire), does some or all of the current of the 2nd rail go to the first?

There are some PSUs that have shared 12v rails. However, from what I have come across, these are generally kinda "crappy". A true dual 12v PSU does not share amperage across the rails.

It's almost impossible to put all of the load on 1 12v rail. The way it typically works is, there is one rail dedicated to the CPU and first video card, and one dedicated to the other video card, hard drives, CD drives, and everything else. But if one of the rails has a lower load than the other, it will not be able to dedicate some of its amperage to the rail with the higher load.