I was confused too for a while but to copy another article:
"What is "multiple +12V rails", really?
In most cases, multiple +12V rails are actually just a single +12V source just split up into multiple +12V outputs each with a limited output capability.
There are a few units that actually have two +12V sources, but these are typically very high output power supplies. And in most cases these multiple +12V outputs are split up again to form a total of four, five or six +12V rails for even better safety. To be clear: These REAL multiple +12V rail units are very rare and are all 1000W+ units (Enermax Galaxy, Topower/Tagan "Dual Engine", Thermaltake Tough Power 1000W & 1200W, for example.)
In some cases, the two +12V rail outputs are actually combined to create one large +12V output (Ultra X3 1000W, PC Power & Cooling Turbo Cool 1000W, for example.)
So why do they do they split up +12V rails??
Safety. It's done for the same reason that there's more than one circuit breaker in your house's distribution panel. The goal is to limit the current through each wire to what that wire can carry without getting dangerously hot.
Short circuit protection only works if there's minimal to no resistance in the short (like two wires touching or a hot lead touching a ground like the chassis wall, etc.) If the short occurs on a PCB, in a motor, etc. the resistance in this circuit will typically NOT trip short circuit protection. What does happen is the short essentially creates a load. Without an OCP the load just increases and increases until the wire heats up and the insulation melts off and there's a molten pile of flaming plastic at the bottom of the chassis. This is why rails are split up and "capped off" in most power supplies; there is a safety concern.
Is it true that some PSU's that claim to be multiple +12V rails don't have the +12V rail split at all?
Yes, this is true. But it's the exception and not the norm. It's typically seen in Seasonic built units (like the Corsair HX and Antec True Power Trio.) It's actually cheaper to make a single +12V rail PSU because you forego all of the components used in splitting up and limiting each rail and this may be one reason some OEM's will not split the rails, but say they are split. Some system builders adhere very closely to ATX12V specification for liability reasons, so a company that wants to get that business but also save money and reduce R&D costs will often "fib" and say the PSU has it's +12V split when it does not.
Why don't those PSU companies get in trouble? Because Intel actually lifted the split +12V rail requirement from spec, but they didn't actually "announce" it. They just changed the verbiage from "required" to "recommended" leaving system builders a bit confused as to what the specification really is.
So does splitting the +12V rails provide "cleaner and more stable voltages" like I've been told in the past?
It is true that marketing folks have told us that multiple +12V rails provides "cleaner and more stable voltages", but this is usually a falsehood. Quite frankly, they use this explaination because "offers stability and cleaner power" sounds much more palletable than "won't necessarily catch fire". Like I said before, typically there is only one +12V source and there is typically no additional filtering stage added when the rails are split off that makes the rails any more stable or cleaner than if they weren't split at all.
Why do some people FUD that single is better?
Because there are a few examples of companies that have produced power supplies with four +12V rails, something that in theory should provide MORE than ample power to a high end gaming rig, and screwed up. These PSU companies followed EPS12V specifications, which is for servers, not "gamers". they put ALL of the PCIe connectors on one of the +12V rails instead of a separate +12V rail. The +12V rail was easily overloaded and caused the PSU to shut down. Instead of correcting the problem, they just did away with the splitting of +12V rails altogether. Multiple +12V rail "enthusiast" PSU's today have a +12V rail just for PCIe connectors or may even split four or six PCIe connectors up across two different +12V rails. The rails themselves are capable of far more power output than any PCIe graphics card would ever need. In fact, Nvidia SLI certification these days REQUIRE that the PCIe connectors be on their own +12V rail to avoid any problems from running high end graphics cards on split +12V rail PSU's.
There's less components and less engineering to make a PSU that DOES NOT have the +12V rail split up, so it's cheaper to manufacturer (about $1.50 less on the BOM, $2 to $3 at retail) and typically this cost savings is NOT handed down to the consumer, so it actually behooves marketing to convince you that you only need single +12V rails.
But some people claim they can overclock better, etc. with a single +12V rail PSU
B.S. It's a placebo effect. The reality is that their previous PSU was defective or just wasn't as good as their current unit. If the old PSU was a cheap-o unit with four +12V rails and the new one is a PCP&C with one +12V rail, the new one isn't overclocking better because it's a single +12V rail unit. It's overclocking better because the old PSU was crap. It's only coincidental if the old PSU had multiple +12V rails and the current one has just one.
The only "problem" the occurs with multiple +12V rails is that when a +12V rail is overloaded (for example: more than 20A is being demanded from a rail set to only deliver up to 20A), the PSU shuts down. Since there are no "limits" on single +12V rail PSU's, you can not overload the rails and cause them to shut down..... unless you're using a "too-small" PSU in the first place. Single +12V rails do not have better voltage regulation, do not have better ripple filtering, etc. unless the PSU is better to begin with.
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Taken from http://jonnyguru.com/forums/showthread.php?t=3990 Article is work of said site/person, i hold no responsibility to the creation of this
