Are Multiple 12-volt Rails Better Than A Single 12-volt Rail?

[DasFox]

PCPower&Cooling
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.

Well what do you think? :roll:

 

[Aluvus]

Originally posted by: DasFox

Well what do you think? :roll:

The only advantage of multiple rails is safety. And many multi-rail supplies don't actually have 20 A limiters on the rails anyway, which is a drain on the safety argument.

I don't love their description, but PC Power & Cooling's main contention that single-rail supplies are better is largely correct.

Single-rail supplies make it more obvious how much current can be delivered at 12 V (especially since many manufacturers like to exploit the common assumption that individual rail ratings can simply be added together) and they avoid weird balancing issues.

 

[DasFox]

I'm thinking about getting a Silverstone OP650:

http://www.silverstonetek.com/...php?pno=op650&area=usa

Or Silverstone DA650:

http://www.silverstonetek.com/...php?pno=da650&area=usa

I like going modular, but from what I've read so far the OP650 seems to have better performance.

BUT let's not get to far off the topic, I'd love to hear more about what others think about single rails, and these Silverstone single rail PSU.

THANKS

 

[yuppiejr]

I think single rail is the way to go, less guesswork in determining what connectors are associated with which rails which makes it easier to 'size right' for your application. Some PSU companies used to sell the idea that components like fans running on the same rail as the CPU could cause some undesired draw or fluctuation that would negatively impact stability... but I believe this was pretty soundly debunked when people figured out that their 2/3 rail PSU's were actually a single 12v rail split with OCP limiters instead of two truly separate 12v supplies.

As for the Seasonic OP650 - I noticed it was in good company on this list: http://www.jonnyguru.com/forums/showthread.php?t=1460 -- which is good enough for me.

 

[MotF Bane]

I have a Silverstone Olympia OP650; it's a great power supply.

Single rails are a lot better than dual-rails. That whole "multi-rails is more stable" is bull. There's a post on XS about the whole multi-rail thing, I have it bookmarked at home. Basically, it's a marketing gimmick. Plus, you don't have to worry about too much on one single-rail, unless you overload the entire power supply. With multi-rails, it's a lot easier to put too much load onto one of the rails.

 

[DasFox]

Cool other comments?

Thanks guys, keep them coming.

I hope others will appreciate the information here, I know that this multi rail thing is going off a bit, making most of the inexperienced users to think it's, "The Thing".

ALOHA

 

[DasFox]

Well I just ordered the Silverstone Olympia OP650 from FrozenCPU, and paid them extra to sleeve it all up.

Can't wait...

ALOHA

 

[Mark R]

The 'trapped' power argument doesn't really hold water. Most multi-rail PSUs are designed with a lower total 12V current rating than the sum of the current limits.

E.g. my PSU has 4 12V rails of 18 A each - but something like a 40 A total rating. You're unlikely to get significant 'trapping' in such a configuration.

Again, the reason it's done is safety and voltage stability - the skinny gauge wires running from the PSU to the peripheral connectors can develop significant voltage drop at high currents - it's better to limit each string to a senisble current, so that high currents are only carried on the heavy buses in the PSU itself. That way you limit voltage drop. You also avoid the PSU turning into an arc welder in the event that a cable is damaged.

The point is that if you only have a single current limit of 60 A like on some high-end 'boutique' PSUs, every single peripheral connector on the PSU needs to be wired with 8 AWG cable in order to ensure adequate protection against fire. This would be expensive as well as impractical.

In fact, having multi-rails is actually a safety requirement. Several safety assessment agencies actually require low voltage cables to be limited to 240 W (20A @ 12V) in order that they can be sold to domestic end-users in certain jurisdictions. Devices that don't comply can only be sold for 'industrial' use, where it is expected that there are experts in safety and engineering who can provide appropriate safety equipment.

 

[TheWatcher1955]

Originally posted by: Mark R
The 'trapped' power argument doesn't really hold water. Most multi-rail PSUs are designed with a lower total 12V current rating than the sum of the current limits.

E.g. my PSU has 4 12V rails of 18 A each - but something like a 40 A total rating. You're unlikely to get significant 'trapping' in such a configuration.

Again, the reason it's done is safety and voltage stability - the skinny gauge wires running from the PSU to the peripheral connectors can develop significant voltage drop at high currents - it's better to limit each string to a senisble current, so that high currents are only carried on the heavy buses in the PSU itself. That way you limit voltage drop. You also avoid the PSU turning into an arc welder in the event that a cable is damaged.

The point is that if you only have a single current limit of 60 A like on some high-end 'boutique' PSUs, every single peripheral connector on the PSU needs to be wired with 8 AWG cable in order to ensure adequate protection against fire. This would be expensive as well as impractical.

In fact, having multi-rails is actually a safety requirement. Several safety assessment agencies actually require low voltage cables to be limited to 240 W (20A @ 12V) in order that they can be sold to domestic end-users in certain jurisdictions. Devices that don't comply can only be sold for 'industrial' use, where it is expected that there are experts in safety and engineering who can provide appropriate safety equipment.

Thats just not true.
You need to go to jonnyGURU.com and do some reading!!

 

[Mark R]

Originally posted by: TheWatcher1955

Originally posted by: Mark R
The 'trapped' power argument doesn't really hold water. Most multi-rail PSUs are designed with a lower total 12V current rating than the sum of the current limits.

E.g. my PSU has 4 12V rails of 18 A each - but something like a 40 A total rating. You're unlikely to get significant 'trapping' in such a configuration.

Again, the reason it's done is safety and voltage stability - the skinny gauge wires running from the PSU to the peripheral connectors can develop significant voltage drop at high currents - it's better to limit each string to a senisble current, so that high currents are only carried on the heavy buses in the PSU itself. That way you limit voltage drop. You also avoid the PSU turning into an arc welder in the event that a cable is damaged.

The point is that if you only have a single current limit of 60 A like on some high-end 'boutique' PSUs, every single peripheral connector on the PSU needs to be wired with 8 AWG cable in order to ensure adequate protection against fire. This would be expensive as well as impractical.

In fact, having multi-rails is actually a safety requirement. Several safety assessment agencies actually require low voltage cables to be limited to 240 W (20A @ 12V) in order that they can be sold to domestic end-users in certain jurisdictions. Devices that don't comply can only be sold for 'industrial' use, where it is expected that there are experts in safety and engineering who can provide appropriate safety equipment.

Thats just not true.
You need to go to jonnyGURU.com and do some reading!!

What, exactly isn't true?

The PSU ratings I quoted came off the side and spec sheet for my PSU. Are you saying they're not true?

Are you saying that heavy cables aren't required if you only have a large current limit? That's a fundamental principle of electrical safety - each wire must be large enough to withstand the highest current available from an upstream protection device.

Underwriters Laboratory (a major organization specializing in safety engineering, and providing advice to manufacturers, insurers, governments, etc.) actually devised the limit, and Intel has taken their advice and written the 20A limit into the ATX spec. The point is that UL certification is required in many jurisdictions before an electrical device can be sold. Without it, it can require extensive 3rd party testing, and may require conditions on sale before such a device can be sold.

Besides, several of the new 600W + PSUs actually do have multiple separate 12V rails which come from separate transformers. There are advantages to doing this - namely, lower cost, better efficiency and better cooling.

 

[Garster]

I have 2 PC Power & Cooling turbo-cool 510 psu's, one is an older 2 rail system and a newer single rail unit. Both have identical power specs on the label except for 12v+ rail, older one has (1)12v+ 18 amp (2)12v+ 16 amp. The newer one has one 12v+ at 34amp 38amp max, I swapped them out a few months back on a highly overclocked C2D system and didn't notice any difference between them. Single rail or dual, just don't skimp on the quality when purchasing a psu.

 

[Zepper]

Never have been, never will be period exclamation point

.bh.

 

[rise]

Originally posted by: Zepper
Never have been, never will be period exclamation point

.bh.

you Elitist bastich, you.



anyway, i have no problem with any- single, dual, multi. all specs being equal, i'd choose a single rail i guess but there are fewer options, models to choose from.

 

[myocardia]

Originally posted by: Mark R
The point is that if you only have a single current limit of 60 A like on some high-end 'boutique' PSUs, every single peripheral connector on the PSU needs to be wired with 8 AWG cable in order to ensure adequate protection against fire. This would be expensive as well as impractical.

Don't know much about ATX power supplies, huh? See, all of them have to power more than one component. Therefore, it isn't even possible for one component to draw 100% of any rail's output, as long as you're using it to power a computer. And since the one (sometimes two, if SLI'd or CF'd) component that requires the most 12v power has a dedicated PCI-E wire/connector, if not two of them, the size of the wires supplying the molex connectors wouldn't need to be all that large, now would it? Of course, the "boutique" PSU's use larger wire to supply the molex/SATA connectors, since electrical engineers design them.

Are you saying that heavy cables aren't required if you only have a large current limit? That's a fundamental principle of electrical safety - each wire must be large enough to withstand the highest current available from an upstream protection device.

Umm, no. The fudamental principle of electrical safety is that each wire must be large enough to supply the highest current that will ever be drawn from it. See the above paragraph for an explanation of why one component can never draw 100% of any rail's output, in a single 12v rail psu.

The point is that UL certification is required in many jurisdictions before an electrical device can be sold. Without it, it can require extensive 3rd party testing, and may require conditions on sale before such a device can be sold.

All PSU's sold in the U.S. are UL certified, including this one: 1,000 watt PC P&C with 72A continuous +12v rail.

 

[soydios]

A single +12V rail is simpler. Simpler means that it is cheaper, more efficient, more reliable, and able to deliver more power. While it is theoretically safer to have multiple rails, I'd much rather get a single, large rail.

 

[Mark R]

Originally posted by: myocardia
Umm, no. The fudamental principle of electrical safety is that each wire must be large enough to supply the highest current that will ever be drawn from it. See the above paragraph for an explanation of why one component can never draw 100% of any rail's output, in a single 12v rail psu.

No. The highest current that will ever be drawn from each wire is the fault current that is limited only by the impedance of the supply, wire and the protection. This fault current can develop for many reasons, including damage or incorrect installation, or malfunction of a high current transistor.

Wires need to be protected for both overload and fault conditions. A major problem with low voltage faults is that the fault impedance may not be low enough not to exceed the protection level.

Such a high current fault will rapidly reduce a motherboard, and its connections to a charred lump of ash and glass fiber, and can even melt the mounting studs. No. They don't happen often, but they have happened frequently enough to be recognised, and such faults are easily preventable by appropriate current limits.

Don't know much about ATX power supplies, huh? See, all of them have to power more than one component. Therefore, it isn't even possible for one component to draw 100% of any rail's output, as long as you're using it to power a computer. And since the one (sometimes two, if SLI'd or CF'd) component that requires the most 12v power has a dedicated PCI-E wire/connector, if not two of them, the size of the wires supplying the molex connectors wouldn't need to be all that large, now would it? Of course, the "boutique" PSU's use larger wire to supply the molex/SATA connectors, since electrical engineers design them.

Well, I've seen plenty of ATX PSUs, and all of them that I've seen have long strings of wires, with daisy chained connectors. Some of 3 or 4 connectors on one string. Don't forget that there are lots of adaptors around - e.g. ones that connect a disk drive connector to an 8-pin Molex connector for PCI-E graphics, Y-spiltters, etc.

These adaptors allow loads to be connected in ways that may not have been anticipated.

 

[Golgatha]

<div class="FTQUOTE"><begin quote>Originally posted by: Mark R
The 'trapped' power argument doesn't really hold water. Most multi-rail PSUs are designed with a lower total 12V current rating than the sum of the current limits.

E.g. my PSU has 4 12V rails of 18 A each - but something like a 40 A total rating. You're unlikely to get significant 'trapping' in such a configuration.

Again, the reason it's done is safety and voltage stability - the skinny gauge wires running from the PSU to the peripheral connectors can develop significant voltage drop at high currents - it's better to limit each string to a senisble current, so that high currents are only carried on the heavy buses in the PSU itself. That way you limit voltage drop. You also avoid the PSU turning into an arc welder in the event that a cable is damaged.

The point is that if you only have a single current limit of 60 A like on some high-end 'boutique' PSUs, every single peripheral connector on the PSU needs to be wired with 8 AWG cable in order to ensure adequate protection against fire. This would be expensive as well as impractical.

In fact, having multi-rails is actually a safety requirement. Several safety assessment agencies actually require low voltage cables to be limited to 240 W (20A @ 12V) in order that they can be sold to domestic end-users in certain jurisdictions. Devices that don't comply can only be sold for 'industrial' use, where it is expected that there are experts in safety and engineering who can provide appropriate safety equipment.</end quote></div>


I have a comment about the 8ga wire for 60A. The power supply's 12V rail will deliver that much current over several 16ga wires. According to this random chart I found with a Google search, 5-7A@120V or 840w can safely travel down a single 16ga wire over a 25ft distance (inside a PC you're talking 3ft max length typically). 60A@12V is 720w, so you have more than enough wire to transfer the maximum rated current from the PS very very safely.