PSUs - Are Fewer Rails Better?

Help me settle a friendly argument with a fellow geek regarding power supplies.

Assume you have a choice of three PSUs of equal quality and features. Each has a total 12V output of 60 amps. The difference?

• Power supply "A" is a single rail of 60 amps.
• Power supply "B" has two rails at 30 amps each (60 amps total).
• Power supply "C" has three rails at 20 amps each (60 amps total).

My thought is a single rail is the better choice, based on no chance of overloading the amperage on any given rail. My friend says it doesn't matter; all will work equally well so long as the combined amperage draw is not exceeded.

Any thoughts on this? Thanks in advance for your input.

 

Hello,
I always thought multiple rails were better until I came across this post. I still think multiple rails are better when using a high end PSU (1000 watts). Hope it helps.

 

FYI that post you linked to is dated 2008.

 

Here's a few links from JonnyGuru (IMO, that site is THE last word in PSU info and testing).

PSU101- Electrical Basics: http://www.jonnyguru.com/forums/showthread.php?t=3263
PSU101- PC Power Supply Basics: http://www.jonnyguru.com/forums/showthread.php?t=3264
PSU101- How PSUs Work: http://www.jonnyguru.com/forums/showthread.php?t=3265
Single vs Multiple Rails- http://www.jonnyguru.com/forums/showthread.php?t=3990
PSU FAQ: http://www.jonnyguru.com/forums/showthread.php?t=1036

Yeah, it's a lot of stuff, but you'll have every answer for every PSU question.

(Personally, I think that units over 600-700w should have multiple rails as these are more likely to be running more than one high-end video card; IMO, running 2 or more video cards on a single rail doesn't seem too great of an idea, but the "folks that know" are making 1200w units with massive single rails, so......)

 

Thanks for the article links.

Besides the friendly argument, I posted this due to recent failure issues I've experienced on multi-rail Antecs. It may just be bad luck; however about 25% of the EarthWatts 380/430/500 units I've used in builds during the past year bit it within the first few months (the final straw being when the 500 in my main PC tanked yesterday in the middle of an editing project). In every case, the PSU wattage was at least 25% over the Antec website's calculated minimum requirement for the system.

I put in a Corsair HX550 today. Even if its "single rail" design isn't a true advantage, at least it had the best customer ratings for a PSU in this output range on NewEgg.

 

I think you will have to wait for Digerati to come on to put a good conclusion to this.

Put it this way though, who does just about every forum bang on about their quality and reliablity of their power supplies? Do you think it is just a coincidence that all their power supplies just happen to have a single 12v rail?

I dunno but that says to me that one is better. If you haven't worked it out i'm talking about Cosair.

 

Honestly i think its an apples - oranges, i think the power rail is a non issue, its more of a quality of psu over all that causes issues, caps and all the little components.

 

Less rails is better. Notice that the cheaper PSUs have more rails. I have a 1000W Corsair PSU, and it has 2 rails.

 

I can't remember where I read it (possibly at JonnyGuru?), but I do remember reading that a large batch of bad capacitors made their way in to MANY of the low wattage Antec power supplies (EarthWatts, Basiq, and others) about 18mon ago (I think? Maybe more, maybe less? My memory sucks!). I know a few other manufacturers were also affected by this batch of bad caps, but Antec had the most bad units hit the market. Also, most Antec units have a 3-5 year warranty, plus if you have one of the "bad cap" units, they should replace it even if it is out of warranty.

(I'll do some additional research on these "bad caps" and try to provide some proof)

 

Thanks for the tip. This is my third RMA to Antec in six months. I've e-mailed them a PDF copy of the sales receipt and am waiting for the RMA auth. #.

I generally shy away from "no-name" PSUs in builds for exactly this reason. Although the replacement unit is "free", the $10 in return postage and my time to replace it aren't.

 

For what it's worth, I'm pretty sure that PC Power & Cooling, who I consider to be at the top of the PSU market, use single 12v rails in their units.

 

Single Rail 12V 60A for me.

I remember searching this topic a while back and the inescapable conclusion was that the single rail option was the best, especially if you are an o/cer.

Furthermore and from a practical point of view, with say 3 12V 20A rails in theory (and practise) you would always have to monitor what items are on each rail and upgrading certain components (especially your VGA) could then render your psu obsolete (if you see what I mean).

Good Luck

 

Hi All,

Not much help: Rail was originally a copper bar across the back of a primative PC Board rack. Each rail was one voltage and was supplied from one source/regulator. Each PC board was connected to the proper rails. Single rail PSUs have all output voltages connected to the same point in the PSU. That is yellow to the +12, red to the +5 etc. The assumption is that a dual rail PSU has two regulators and two points for each voltage and the outputs are split between them. Triple would have three regs. Hope this helps a little.

Good Luck, Jim

 

In theory several well balanced rails are better then one single high amperage rail. But in the real world its difficult to balance the rails well. For example if you have a high end system with two video cards in sli or crossfire. Ideally there will be one rail per card and another rail for the CPU, mobo, harddrive and peripherals. Lets say each rail is 25 amps for example. If each video card uses 22 amps and the motherboard and everything else use only use 12. The "cpu/mobo" rail has 13 amps left over while the video cards are nearing the limit stock. Now take overclocking into account. Both cards will soon hit the power limit while there is a surplus on the third rail. Suffice it to say it all gets complected and inefficient quickly. With one large single rail the power goes where it is needed. For high end overclocking systems a single rail is always better. (assuming of corse that its large enough) In theory multiple rails would help keep the power cleaner when running low end motherboards and video cards that don't have good power regulation. As each rail is isolated from the other sudden spikes or power draws will be isolated to that rail. But in the real world I have seen little evidence to support this.

 

Aptly stated sir

 

This was one of my thoughts in my initial argument that a single 60A rail is better than multiple rails that add up to 60A, based on...

* I have yet to see a single PSU manual or spec sheet that breaks down how many amps are on rail "A", rail "B", etc. This is especially confusing on models where the max. draw varies from rail to rail.

* Since a typical PSU loses around 10% of its max. output per year thru aging, in theory a power supply with split rails may fail sooner. Using your example of a 25A rail connected to a 22A draw video card and a 10% per year degradation in max. output, it would be theoretically possible for that rail (and therefore the entire PSU) to fail in about 18 months when the degraded output becomes less than what the video card requires.

Is my math correct on this?

 

I have a corsair 950W with a single 78amp 12V,that's 99% of the power supplies total output on the 12V Rail with only 14 watts left over for everything else.

Over the past 10 years all power supplies and PC requirements have been steadily moving toward this,separate voltages and rails have gradually become or are becoming redundant,negative voltages,sub 12V voltages,multiple rails ect. ect because PC designers know the stability of their systems can't depend on the ability of a third party component.

In the next 5-10 years I think power supplies will only use 12V,all PC components will only need 12V and power supplies will only have single high current rail.I think they may even evolve to only have single motherboard connector on each PSU and every component will draw its power from the motherboard,eliminating the wiring mess and simplifying connection.

This places all of the voltage regulation on the quality of the motherboard and rightfully so IMO.

"You give us 12V and plenty of it and we'll do the rest,cheers power supply.":cool

So yeah I think a single 12V rail is the best,I think its the future and not much will change that.

 

Not entirely correct.



The fact the PSU is called the TX 950w is not because it has 950 watts of power, it's because it has (almost) 950 watts of power just from the 12v rail.

It's hard to explain so i hope that makes atleast a little sense!

 

My bad I was quoting from the product page where I bought it blame them :-D It didn't sound quite right when I was typing it.

http://www.scan.co.uk/products/950w...lus-bronze-85-eff-eps-12v-78a-single-12v-rail

If I say around 90% of the power is drawn from the 12V rail will that fly Burrel-D

My point still stands though,everything is moving toward single high current 12V solution.

Thanks for correcting me.

 

We had several IBM Netvista systems at the computer shop I work at. All of them ran off one massive 12V external PSU. The plugin in the back of the computer looked exactly like a PCIe power header. Kinda makes you think doesn't it? :major

 

Interesting,it makes sense really.Laptops have been doing it since the dinosaurs.