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Old November 28, 2007, 01:42 PM
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SKYMTL SKYMTL is offline
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Join Date: Feb 2007
Location: Montreal
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Originally Posted by Thund3rball View Post
I have read that multiple 12V rails is something to stay away from these days and having just one is better. I know you are the man to ask about this stuff so any comment on this, considering this unit has 4 12V rails? Thanks :)
This is a bit of a tough one since there IS no right answer. For 99% of the computer systems out there, the number of rails really doesn't matter. On the other hand things start getting a bit more complicated when you start looking at power users and overclockers.

Let's break this down a bit.

Multiple +12V rails

Multiple +12V rails will not really affect you unless you are pushing your power supply harder than it should be pushed. Take this CM 750W that I reviewed. It has 4 19A +12V rails which means that you can put a 228W (19A X 12V) load on any ONE of those rails. Since the CPU connector is on its own dedicated rail your CPU can draw up to 228W before tripping the OCP...and this is basically as much as a QX6800 overclocked to 5Ghz+ (this is an estimate) would draw.

The same goes for the PCI-E connectors. Since the GPU has been the heaviest loader of power supplies for the last few years, this area is a bit of a slippery slope so you will have to bear with me.

One of the main dangers of a multi-rail unit is bumping into the OCP circuit (it is set at 19A on this PSU) on any particular rail. This is why you have to pay VERY close attention to which rails power which component. In most good 4 +12V rail PSUs, the PCI-E connectors are broken into their own rails or if there are 4 PCI-E connectors, there are two on each rail. This is a good thing since each pair of PCI-E connectors will have access to 19A before the OCP trips.

Some other PSUs group the second PCI-E connector with the Molex or SATA connectors on the same rail. This is OK as well but you will have to start paying attention to what you attach to each rail. BUT, see below....

On the other hand it is VERY hard to trip an OCP of a good power supply even with heavy loads. Take the example of the Silverstone ST50EF in my review from ages past. Basically, the +12V2 rail is dedicated to the CPU while ALL other +12V outputs (PCI-E connectors, ATX connector, Molex Connectors and SATA connectors) is handled by the single +12V1 rail which is rated at 18A. Even with my setup in theat review (which included a pair of 8800GTS cards) I was still not able to trip the OCP.

Sure, if you are pushing for insane (5Ghz) overclocks on a 65nm Yorkfield you should not be looking at a multi-rail unit but for pretty much any application, multi rail is still perfectly matter what PCP&C says.

Single Rail Units

Like I have already mentioned, these are good for any application but are not as necessary as some people would have you think. If you need an insane amount of power in one place then it is the way to go. On the other hand, with more and more CPUs and GPUs moving to smaller and more efficient manufacturing processes they may become a thing of the past.

Hope that answers your question.....
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