I'll post it here then:
We have heard information or bits of advice buzzing 'round the internet, claiming whether you should get a single-rail or a multi-rail PSU. One faction claims that they won't even touch anything that has single-rail, while the other never would use any multi-rail PSUs for compatibility reason. Well, it turns out that both of them are "fanboys". Like every controversial topic in existence, the answer lies between those two.
What are rails anyway?
In engineering terms, "rails" mean a single electrical source. This definition creates the confusion that "multi-rails" refers to multiple 12V sources, while in reality, it is not the case. Multi-rail PSUs
don't use more than one source on the 12V.
The description is based on how the protection system is configured, to be precise, the OCP (Over Current Protection). What OCP does is that if more current is flowing through a particular rail than some specified value, it shuts the whole thing down to prevent damage. This mechanism saves the PSU and your components from dangerous instances such as power surges and short circuits.
So, "single-rail" PSUs have their 12V rail protected by a single OCP circuit, or even only protected by the OPP (Over Power Protection). On the other hand, multi-rail PSUs have their OCP divided into at least two circuits, working together on top of a single 12V rail source. Each OCP is specified to its cables, or power connectors, each having its Amperage rating.
Things get more complicated from there. Multi-rail PSUs usually have those specific rails are dedicated to different things. In most cases, you have OCP channels specifically for CPU, Motherboard, and PCI-E connectors. This configuration is often described on the manufacturer's website or manual. But, some others distribute the rails differently.
Why is it configured like that?
So single-rail PSUs often have their 12V OCP rail set higher. If you have a single-rail, 1000 W PSU, you'll have the OCP set at least 83 amps. Meanwhile, a multi-rail, 1000 W PSU will have their OCP set at much lower amperages, about 40 amps. Imagine if there is a short on one of the 12V cables, it'd take at least 83 amps for that single-rail PSU to recognize that a short happening, while on multi-rail PSUs, it'd recognize the short much earlier.
With single-rail PSUs, there is a much higher chance that you will damage something before the protection kicks in. It's going to be No Bueno if you're going to feed a cable by 83 amps! And the gap gets more significant the higher you go with the wattages. A 2000 W single-rail PSUs stops at 167 amps, and you shouldn't imagine what kind of damage would a current that big do to your system.
So there you have it. Which one is better? Which one should you use for better performance? The answer is: Neither is better, but one is safer than the other. And, the higher the wattage, the more critical multi-rail is for safety.
A bit of History of this controversy
It began in the mid-00s when PCI-E GPUs started to consume a lot of power and started requiring an extra PCI-E (or a Molex) connector to run. Back then, the go-to design was to have two 12V "rails" limited to delivering 240 W each. It turns out that this design was incompatible with some high power PCI-E cards such as the Radeon X850 or the GeForce 6800 Ultra. That old rail design, paired with high-power GPUs, will shut the whole system down because it is possible for a GPU to consume more than 240 W.
So a "solution" was spearheaded by PC Power & Cooling. Instead of simply increasing the OCP trigger point, they removed the OCP altogether on the 12V rail, and then calling it as a feature that "ensures compatibility". This solution leaves a gap in the other end, thereby making the PSU less safe when a short is happening.
Then, it went all haywire when XFX decided to step this a notch by calling single-rail "the only choice". That's where all this confusion started.
Which one should I get? (TL; DR)
If you're looking for a
650 W or under PSU, there is practically
very little difference. I even would prefer single-rail units here, as some low wattage units that are multi-rail are often old designs or not enough to power a high-end graphics card. An example of this would be Bitfenix's Formula Gold and Whisper M 450-550W, or older Seasonic SS-HTs known to be incompatible with cards such as Vega 64 LC.
For
750 W - 1000 W, I would prefer multi-rail PSUs, but it also depends on the price and performance.
If you have two options that perform and are priced the same, but one has multi-rail, and the other does not
get the multi-rail one.
If you have two options that perform the same, one has multi-rail but is more expensive, and the other that does not is cheaper depends on the price delta. I'd say
if it is less than $10 get the multi-rail one.
If you have two options that perform differently, but prices are the same, then the inferior one has multi-rail, and the other does not
get the single rail one. Performance matters all the time, while protection only matters when you're getting a short circuit.
For >1000 W, I see no reason why you should get a single rail one. So long as there are no known compatibility issues,
always go for the multi-rail one.
So that's it for today. Hopefully, you don't feel dumb after reading this, and you can start debating people who recommend one after the other!
Sources:
https://www.overclock.net/forum/31-...y-single-rail-not-better-than-multi-rail.html
https://www.jonnyguru.com/blog/2012/01/23/xfx-pro-1250w-black-edition-power-supply/
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