First off, I'd like to give credit to Jonny Gerow (former star of JonnyGURU.com, now PSU project manager at BFG) for writing what I consider the definitive article on power supply "rails", one which educated me. If you want an accurate historical and mildly technical explanation of power supply rails, read his article here:
Also, I'm going to not bother mentioning the +5V and +3.3V rails, because it just confuses the matter. In this article I will talk only about the +12V rail, and single/multiple versions of it.
So let's get down to it.
"Single rail power supplies are best!"
"No, multi rail units, stupid. You've got like five times as many rails to power your parts."
"No, single rail is best, are you an idiot? Look at how much more I can overclock!"
"You're just a moron, you don't know what a rail is!"
"You're just jealous because I've got a single, massive, powerful rail, if you know what I mean."
Children, children, calm down and shut your traps. You're both wrong! It doesn't matter, at least not in the way 99% of enthusiasts think.
What is a rail?
No, now stop being a smartass.
Seriously, most people have very strong opinions on what rail distribution is best, without understanding what a rail is. The common misconception is that a rail is a part in the power supply that provides power. This is utterly and completely false. You can have two power supplies that are 99% identical and one be single rail and one be multi.
No, rather a rail is a group of traces on the PSU's mainboard that are monitored by an OCP circuit.
I said, it's a group of traces on the--
"Shut up until you can speak English."
Fine. A trace is a pathway of copper on a circuit board that carries electricity. You can see them as the faint copper-y lines running all over your motherboard. In this instance, I'm referring to the traces on the power supply that the wires in the cables are soldered to, specifically the ones carrying +12V power. With me so far?
"Yes. Speaking of tracing, I'm going to go get some stencils."
Whatever. Now, OCP is Over Current Protection. What OCP does is it monitors an output on a power supply. If the amount of current--
Read this: http://en.wikipedia.org/wiki/Electric_current
If the current going through a given output exceeds a certain amount, it will shut the power supply down.
What does this have to do with single or multiple rails?
"I was about to ask, yes."
Glad to know. The difference between single or multiple rails is this:
Multiple rail: each trace is monitored separately, so if, say, one trace goes over 25A the power supply will shut down.
Single rail: all traces are monitored all together, so if the total current going through the +12V outputs goes over, say, 60A, the power supply will shut down. Alternatively, no OCP may be present at all on the +12V rail.
Make sense? Nothing about single rail having extra power or being more stable, on either side. You could have two identical power supplies with the only difference being how the OCP chip is configured, and one could be single rail and the other multiple. Understand?
Yes you do, if you don't go back and read through again.
So which is better? Well, neither has any affect on voltage stability or ripple suppression or efficiency or anything, really, at least not to a measurable degree. So all those arguments are bogus. A single rail PSU will not let you overclock any higher than a multi rail, or vice versa. However, there is a difference.
Let's imagine you have a short circuit on the +12V and the SCP (Short Circuit Protection) doesn't catch it. If you have a single rail power supply the draw on that one +12V wire and that one +12V trace will climb until either the combined OCP point is reached, or until something burns; the cable, your power supply, your motherboard. With large units, usually something will burn before the OCP point is reached.
In a multi-rail unit, the current would climb until it hit that rail's OCP point, at which point the PSU would shut down to protect itself and your computer. Ideally you'd have a rail for every wire, but that would be impractical, so you generally see 2-8, depending on the power rating of the PSU and the design philosophy.
So multi-rail is inherently safer, correct? Yes, but early on there were some problems. You see, when multi-rail units were first introduced, the specification at the time was poorly written, and power supply engineers made a mistake; they put all the cables that power heavy-draw components like your CPU, mobo, and graphics card(s) on one rail, and all the light-draw stuff like HDDs and fans on the other. This meant that you had one rail that might be pulling, say, 24A, and another pulling 5A.
"I see! They were unbalanced, so the man inside the PSU fell off his unicycle and--"
No. If your PSU's OCP trip point for that main rail was set to 24A, then your system could easily hit that trip point when under load, causing the PSU to shut down to protect itself. That could cause you to lose data, or leave your friends without backup while playing TF2.
"Wow that sucks."
Yes indeed. The power supply was just as powerful as the single rail units of the time, but would shut down under heavy loads because of the poor cable distribution among the OCP rails.
However, that is no longer an issue. The problem units only existed around 2006; since 2008 virtually all power supplies have had intelligent cable arrangements, with only a few gaffes here and there. On a modern multi-rail power supply there is virtually no chance of tripping a rail's OCP in normal use unless you're running way too much for the power supply to handle anyway.
So final verdict? Single rail or multi rail? Well, with low-wattage units it doesn't matter. OCP on a single rail is useful up to about 40A or thereabouts, which is where most 550W power supplies fall. So with 550W and under power supplies, it's a moot point. However, with high wattage units, >45A on the +12V (650W and higher) picking a multi-rail unit will provide you with an extra layer of protection. It isn't essential, and it has no impact on the power supply's performance. However, it does provide an extra layer of safety in case you get a short circuit. And I would consider it a must for >1000W power supplies; [H] recently tested the single rail Corsair AX1200, but they had an accidental short circuit, and since the PSU's OCP is set for over 100A, the short overloaded and destroyed most of their testing equipment. So there is a danger with single rail units over 1000W.
So multi-rail is mildly better, especially with high wattage units, but it won't have any impact on your performance or overclockability.
There, does that settle it?
".... HAHA TOLD YOU MULTI RAIL WAS BETTER."
"You can kiss my rail for all I care!"
This has been your friendly neighborhood Phaedrus, signing out.