Chozart

The market for power supplies (PSU) is quite confusing. Current offerings include power supplies with single or dual, and a few triple and quadruple, 12V rails. So the question is: do we need multiple 12V rails, or is one 12V sufficient? Single 12V rail power supplies are not a thing of the past: there are SLI certified single 12V rail power supplies, such as the OCZ PowerStream 520W, the PC Power & Cooling Turbo-Cool 510 Express, and the Silverstone SST-ST56ZF 560W. These are power supplies offered in the upper level of the market at prices to match ($134.99, $229.99, and $137.99 respectively, Newegg September 30, 2006).

Why did multiple 12V rails come about?
Multiple rails came about because the EU mandated that the power on a single rail should not exceed 240VA (12V x 20A). Intel then stipulated in the ATX12V v2.xx standard that if the power on a 12V rail was likely to exceed 240VA, a second 12V rail needed to be engineered. For practical purposes, a limit of 18A was used, allowing for 2A headroom. Manufacturers replied by mass-producing multiple rail power supplies, using several types of engineering. Some power supplies are well engineered, and the 12V rails are truly separated with separate transformers. However, others draw power from one single transformer and the rails are only separated after transforming takes place. The issue with multiple rails is that the power per rail is relatively low, which unfortunately often results in the use of inferior parts.

Do we really need multiple 12V rails?
The answer is simple: no, we do not. As stated above, there are high-end power supplies with one single 12V rail that are SLI certified. These single 12V rails are rated at >30A, and thus likely build with high-quality parts. This is also confirmed by Intel, who unofficially dropped the 240VA standard in spring 2005, actually re-instating single 12V rail power supply units as an accepted standard. It is interesting to note that the Siverstone SST-ST56ZF 560W (single 12V rail, 38A) is SLI certified for Dual GeForce 7900 GTX or Dual GeForce 7800 GTX 512 MB, while its “bigger” brother, the Silverstone Zeus SST-ST65ZF 650W lacks this certification, although it is certified for all other SLI configurations and delivers 42A on the combined 12V rails. The problem likely is that it has a bit of an odd distribution of current along quadruple 12V rails (13A, 18A, 16A, 8A). Thus, this is an example where the single rail PSU is preferable over the multiple-rail PSU. On the other hand, there are many really well engineered and well built multiple-rail PSU. This FAQ is only meant to point out that multiple rails are not a necessity for a good PSU.

Keep your options open, and do your research. Also, I posted another FAQ about how much current really is available on the 12V rails of a PSU, which you can find here: http://www.overclock.net/faqs/85645-...mperes-do.html

Good luck

CyberDruid

Awesome--clears up some sommon misconceptions! Your advice on this subject guided my choice for the PSU for Crunch--as stated above Dual Rails are just a mfr reaction to a change in spec...when the spec changed, high quality single rails came out to meet it.

Thanks Chozart!

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cokker

Nice FAQ, so would my PC (A8N-SLI, A64 3000+ and say 2x 6600gt's) get away with a single 12v rail @ 30a+?

Chozart

If it is a good quality PSU with a solid 12V rail, you should be fine. Check if it is SLI certified at www.slizone.com

MjrTom

another smashing FAQ :D

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timmay7

great FAQ dude, although i heard dual rails provide better stability.

claymanhb

Good stuff!

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Chozart

Initially dual rails did improve stability, esp. when well manufactured. However, technology has progressed, and the new Single Rail PSUs are rock solid!