So why do they split up 12V rails?
With the demand on +12V becoming greater and greater, Intel decided it would be "safer" to split the duty of supplying +12V across two rails. It's "safer" because inexpensive transistors capable of supplying more amperage (say more than 34A) at any kind of decent efficiency (70% or better) are subject to blowing up. I guess that's not very “safe.” ;-)
To split the duty up between two (or more) +12V rails, one can use cooler running, cheaper transistors to supply the power. Furthermore, this isolates devices on one rail from another, so EMI introduced by lighting inverters and drive motors can be isolated from sensitive components like the CPU and video card.
Some people have questioned the principle of multiple 12V rails.
And for good reason! But I don’t think multiple 12V rails in general should be shunned. But it’s best to know what rails go where when considering using a multi-12V rail power supply with a high end system.
ATX specifications only say that the CPU (the 2x2 4-pin connector) is put on a separate rail from the ATX connector (the 20 or 24-pin) and the drive (also used for fans, lights, etc.) power connectors. They also specify that no one rail should have more than 20A available on it (that’s their “safe” limit, so to speak.)
So if you breeze through reading that, you would say “Ok. The CPU gets it’s power from the 12V2 and everything else gets it’s power from the 12V1.” But then you realize there’s a problem with that. 20A for just a CPU, even a dual core or even a dual CPU, is overkill. And 20A may be enough for some drives, lights, fans, etc. But what about PCI express video cards that regulate their voltage from the 12V rail via an auxiliary 6-pin connector? High-end video cards can easily tax 7A or more EACH off of the 12V rail. 20A leaves zero overhead.
Unfortunately, some power supplies adhere to the “quick read” version of the ATX standard and put everything but the CPU on one rail. This is where everyone seems to be running into problems. Fortunately, some other power supply companies have gotten creative with rail distribution. I’ve seen power supplies with the PCI express connectors on 12V2 and even some with one PCI-e connector on each of the two 12V rails. THESE are the kind of dual rail power supplies you need to look for.
Some power supplies have more than two rails. The Antec NeoHE, for example, has three. Two modular connectors are labeled for 12V3 use. These are the two ports one should plug their PCI-e connectors into. Other power supplies have four 12V rails. These typically adhere to a standard other than ATX called “SSI” but PCI-e is taken into consideration by keeping the PCI-e off of the same rail as all of the drives. Even if a PCI-e is plugged in using a typical drive Molex, that rail is still separate from the ATX connector, and the 2x2 4-pin connector.
