Multi Rail PSUs are Obsolete!

[ChakkaSol]

According to PC Power and Cooling, multi-rail psus are obsolete. If PC Power says it, it must be true? Here is the link and myth statement (I do not work for PC Power or any psu company):

http://www.pcpower.com/technology/myths/#8

8. ARE MULTIPLE 12-VOLT RAILS BETTER THAN A SINGLE 12-VOLT RAIL?
With all the hype about multiple 12-volt rails (ads claim that two rails is better than one, five is better than four, etc.), you’d think it was a better design. Unfortunately, it’s not!

Here are the facts: A large, single 12-volt rail (without a 240VA limit) can transfer 100% of the 12-volt output from the PSU to the computer, while a multi-rail 12-volt design has distribution losses of up to 30% of the power supply’s rating. Those losses occur because power literally gets “trapped” on under-utilized rails. For example, if the 12-volt rail that powers the CPU is rated for 17 amps and the CPU only uses 7A, the remaining 10A is unusable, since it is isolated from the rest of the system.

Since the maximum current from any one 12-volt rail of a multiple-rail PSU is limited to 20 amps (240VA / 12 volts = 20 amps), PCs with high-performance components that draw over 20 amps from the same rail are subject to over-current shutdowns. With power requirements for multiple processors and graphics cards continuing to grow, the multiple-rail design, with its 240VA limit per rail, is basically obsolete.

PC Power and Cooling is once again leading the industry. All of our power supplies now feature a large, single 12-volt rail. The design is favored by major processor and graphics companies, complies with EPS12V specs (the 240VA limit is not a requirement) and is approved by all major safety agencies such as UL and TUV.

 

[Rabid Badger]

A single, large 12V rail is preferable for the most power-hungry systems. It takes away the worry of trying to balance the load across two or more rails.

 

[drizzt81]

without wanting to sound bitter/ jaded: I find it more than amusing that "all of a sudden" single-rail PSUs are oh-so-cool again when a couple of years ago everyone wanted a multi-rail PSU to "insulate" the CPU from voltage spikes/ drops due to the less-than-constant current draw from HDDs and Video cards.

 

[jonnyGURU]

Not to sound bitter/jaded, but I find it ironic that going single 12V rail is only considered chic after PC Power & Cooling says it is.

 

[MetalX]

I remember nay sayers on this forum a year or two ago is what kept me from going dual rail. Now I'm glad I listened and I doubt a multi rail or modular power supply will ever be installed in one of my systems; call me old fashioned.

 

[jonnyGURU]

....I doubt a multi rail or modular power supply will ever be installed in one of my systems...

Well... we didn't even bring up modular power supplies, but if you believe what Doug has to say about those, more power to ya'. I for one have no issue w/ modular. Doug's "numbers" for the amount of resistance created by a modular interface are grossly exaggerated.

As for multi-12V-rail, if you have enough rails to "go around" and they are properly laid out, then there's nothing wrong with that either.

For example, if you have four +12V rails and the CPU is on it's own rail as is each of two PCI-e connectors and everything else on a fourth, then there's no reason why that would be a "bad idea."

 

[madmat]

I wish that there was a multi rail with 36A for the mobo and drives and 3 - 18A rails for the with the CPU on one and the PCI-e's on the other two. That way if you've got a bunch of drives, fans, a pump or two and other sundry 12V items riding the 36A rail you won't have any worries at all about pushing that rail too far. That's my only gripe with multi rail power supplies. They allot 18A (on most high end versions) for a CPU that won't need it and 18A for drives that could end up needing more depending upon how much of a drive whore you are.

 

[Rabid Badger]

I''m not displeased with my quad-rail, modular PSU at all, and will be using it for quite some time. The way the modular interface is designed, it's pretty much impossible to put everything on one rail and leave another unloaded and it can shuffle power around a bit if needed. However, if I wasn't using a modular, I'd want a single rail. I don't see myself ever going back to hard-wired PSU's, though.

I wish that there was a multi rail with 36A for the mobo and drives and 3 - 18A rails for the with the CPU on one and the PCI-e's on the other two. That way if you've got a bunch of drives, fans, a pump or two and other sundry 12V items riding the 36A rail you won't have any worries at all about pushing that rail too far. That's my only gripe with multi rail power supplies. They allot 18A (on most high end versions) for a CPU that won't need it and 18A for drives that could end up needing more depending upon how much of a drive whore you are.

Um, you're asking for 1080W on the 12V rails alone? You'd be lucky not to trip the breaker if you plugged that into a regular house outlet.

 

[Redbeard]

I wish that there was a multi rail with 36A for the mobo and drives and 3 - 18A rails for the with the CPU on one and the PCI-e's on the other two. That way if you've got a bunch of drives, fans, a pump or two and other sundry 12V items riding the 36A rail you won't have any worries at all about pushing that rail too far. That's my only gripe with multi rail power supplies. They allot 18A (on most high end versions) for a CPU that won't need it and 18A for drives that could end up needing more depending upon how much of a drive whore you are.

DISCLAIMER: I'm not an engineer, and I'll clear this by our engineers to see if there are any glaring errors here, but this is how I understand it.

The thing is, nobody makes a PSU with truly electrical independent +12V rails, as far as I know. The only difference between "single rail" and "multiple rail" PSUs is the OCP point. Once your multi-rail PSU's rail hits the OCP point, it's supposed to turn off as a safety feature.

The older ATX spec required that OCP point be no higher than 18A on each rail. As a result, Corsair (and other manufacturers) limited themselves to that in their specs. However, when the ATX 2.4 spec came out, they removed that limitation due to its inability to run higher-end GPUs, and Corsair (and possibly other manufacturers) raised the actual OCP.

So while our PSU is rated at 3 rails with an OCP of 18A on each, the true OCP on any given rail is considerably higher. A lot higher.

So why even have multiple rails? It was part of the ATX spec. The concern was that when a CPU and GPU are on the same rail, they'd overload the safety measures of the internal wiring. For those of you familiar with electronics, that's the only real risk. Wires and cables from the PSU are usually 18AWG or 22AWG. A PCI-Express 6-pin connector has 3 +12V and 3 Ground wires. There's a certain safety precaution to drawing too much power over those 3 wires, due to resistance. Is it possible to draw 30A on those wires? Sure. Is it possible to draw 60A on those wires? Sure. The only difference is, at some point, that wire's going to heat up due to the resistance, and cause a safety concern.

Since a lot of PSUs run the wires together to one connection point on the PSU's PCB, that connection point becomes a point of resistance, and thus when you draw too much power over that single connection point (usually a bunch of wires soldered to a 2cm x 2cm section of PCB) that area can heat up and cause safety concerns as well.

Which is one of the reasons why "multi-rail" PSUs exist. They typically have the wires spread out to decrease the overall resistance. It's not that big a deal to have a single rail for a PSU that can do, say, 400 watts. But when you start getting up in the higher wattage areas, you're going to want something that either has multiple rails or a physical separation of the connection points on the PCB.

We could have easily spec'd the HX620 to have one +12V rail @ 50A. It's true, in a way, since there's only one +12V source. But by that rationale, everyone with a multi-rail PSU could do that.

So all the debate over single-rail and multi-rail PSUs is really kind of pointless, since effectively, we're talking about a very small difference in design, and that difference is the Over Current Protection point. If you set the OCP on each physical connection at the total amperage possible of the PSU, you've got a single-rail PSU. If you set the OCP on each physical connection to something lower for safety precautions, you've got a multi-rail PSU.

This is my understanding of the technology, and while I don't have 20+ years in the industry like Doug, (as I would have been very uninterested in Power Supplies before I was 10 years old) I think it's mostly on the ball. PC Power & Cooling makes great PSUs, some of the best money can buy. I'd never argue any different. They're not perfect, though, and some people want features that they don't provide.

 

[jonnyGURU]

Um, you're asking for 1080W on the 12V rails alone? You'd be lucky not to trip the breaker if you plugged that into a regular house outlet.

You're assuming that the +12V rails are additive. Which they never are. Come on.... take the Ultra 800W, for example. Four +12V rails that each do 20A. Does that mean there's 960W of power available on the +12V alone?

How many times do we have to go over this? "Rails" are just over current protected circuits tapped off of the same +12V source.

 

[Rabid Badger]

You're assuming that the +12V rails are additive. Which they never are. Come on.... take the Ultra 800W, for example. Four +12V rails that each do 20A. Does that mean there's 960W of power available on the +12V alone?

How many times do we have to go over this? "Rails" are just over current protected circuits tapped off of the same +12V source.

Yeah yeah. That's what I get for posting before I've had my coffee.

 

[madmat]

DISCLAIMER: I'm not an engineer, and I'll clear this by our engineers to see if there are any glaring errors here, but this is how I understand it.

The thing is, nobody makes a PSU with truly electrical independent +12V rails, as far as I know. The only difference between "single rail" and "multiple rail" PSUs is the OCP point. Once your multi-rail PSU's rail hits the OCP point, it's supposed to turn off as a safety feature.

The older ATX spec required that OCP point be no higher than 18A on each rail. As a result, Corsair (and other manufacturers) limited themselves to that in their specs. However, when the ATX 2.4 spec came out, they removed that limitation due to its inability to run higher-end GPUs, and Corsair (and possibly other manufacturers) raised the actual OCP.

So while our PSU is rated at 3 rails with an OCP of 18A on each, the true OCP on any given rail is considerably higher. A lot higher.

So why even have multiple rails? It was part of the ATX spec. The concern was that when a CPU and GPU are on the same rail, they'd overload the safety measures of the internal wiring. For those of you familiar with electronics, that's the only real risk. Wires and cables from the PSU are usually 18AWG or 22AWG. A PCI-Express 6-pin connector has 3 +12V and 3 Ground wires. There's a certain safety precaution to drawing too much power over those 3 wires, due to resistance. Is it possible to draw 30A on those wires? Sure. Is it possible to draw 60A on those wires? Sure. The only difference is, at some point, that wire's going to heat up due to the resistance, and cause a safety concern.

Since a lot of PSUs run the wires together to one connection point on the PSU's PCB, that connection point becomes a point of resistance, and thus when you draw too much power over that single connection point (usually a bunch of wires soldered to a 2cm x 2cm section of PCB) that area can heat up and cause safety concerns as well.

Which is one of the reasons why "multi-rail" PSUs exist. They typically have the wires spread out to decrease the overall resistance. It's not that big a deal to have a single rail for a PSU that can do, say, 400 watts. But when you start getting up in the higher wattage areas, you're going to want something that either has multiple rails or a physical separation of the connection points on the PCB.

We could have easily spec'd the HX620 to have one +12V rail @ 50A. It's true, in a way, since there's only one +12V source. But by that rationale, everyone with a multi-rail PSU could do that.

So all the debate over single-rail and multi-rail PSUs is really kind of pointless, since effectively, we're talking about a very small difference in design, and that difference is the Over Current Protection point. If you set the OCP on each physical connection at the total amperage possible of the PSU, you've got a single-rail PSU. If you set the OCP on each physical connection to something lower for safety precautions, you've got a multi-rail PSU.

This is my understanding of the technology, and while I don't have 20+ years in the industry like Doug, (as I would have been very uninterested in Power Supplies before I was 10 years old) I think it's mostly on the ball. PC Power & Cooling makes great PSUs, some of the best money can buy. I'd never argue any different. They're not perfect, though, and some people want features that they don't provide.

Yes, I'm well aware of this but each virtual rail has OCP (Over Current Protection) that will kick in and shut your PC down at a preset limit. What I'm talking about is pumping the OCP on the mobo and drive rails past the 20-25A or so that many MFG's use to a more realistic 36A so that if you place 7 hdds spinning up and drawing 3A each on the 12V rail along with what the mobo is drawing you won't have any chance of tripping the OCP.

And for the record, not all PSU's have one rail divided into multiple rails, some have two transformers for the 12V rail like the Tagan TG1100.

 

[Shaman]

I would take these myths coming from a manufacturer with a grain of salt. They also have other myths about 120mm fans and modular PSU's that put their own PSU's in a better light.

Do you really think they would say single-rail non-modular 80mm fan PSU's are bad?

 

[[Spectre]]

I would take these myths coming from a manufacturer with a grain of salt. They also have other myths about 120mm fans and modular PSU's that put their own PSU's in a better light.

Do you really think they would say single-rail non-modular 80mm fan PSU's are bad?

Smart man.