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Thread: +12V Rail vs +5V Rail and Split Rails

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    Default +12V Rail vs +5V Rail and Split Rails

    In modern systems (i.e.: P35-based motherboard with nVidia 7 or 8 series graphics card, lots of hard drives and 2 optical drives), how important is the +5V rail compared to the +12V rail?

    The reason I'm asking is that my current ModStream 520V can put out 260W on the +5V/+3.3V rails, whereas my new Tt Toughpower 750V (still in the box) can only put out 180W on those rails. Also, the +12V is dividing into four chunks on the new power supply, while it is only a single rail on my old one. I take it that the huge increase in total watts/amps in the combined +12V capacity (basically a doubling) makes up for it being divided into 4 separate rails? Hope my questions aren't too noobish; I really want to learn how to evaluate power supplies.

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    The 5V rail barely gets any use at all in a modern system. I'd say less than 10A if you only have a couple hard drives.

    The Modstream has such a high combined 3.3V/5V rating because Topower (the OEM) was stuck in the past when they designed it. It's an older design more suited to 5V oriented systems with a little bit of tweaking to get the 12V capacity up to a useful level for a more modern rig.

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    Thanks for the quick reply! I have 4 hard drives and 2 optical drives. By scaling your answer out though, that should still take less than 30A (unless your answer already factored in the opticals, in which case it would be 15A), which is what the Tt is capable of putting out. And it's not like they're all used at once anyway. So that should be fine I'd assume.

    What about the split +12V rail question though? Let's say I get a big hulkin' 8800GTX in the future. My concern is that, though the combined +12V rails in the Tt Toughpower are twice as large as the single one in the ModStream, each individual one is only half as big. I'd think that would be a step downwards! Now, reason tells me they wouldn't do it (and Jonny wouldn't have rated it so highly) if it really ended up being worse than a single rail of half the power, so it must be better. So does the graphics card get all 4 rails simultaneously, or is there something else I'm missing as to how dividing the +12V power into 4 doesn't gimp it? Thanks!

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    Quote Originally Posted by farfromhome View Post
    What about the split +12V rail question though? Let's say I get a big hulkin' 8800GTX in the future. My concern is that, though the combined +12V rails in the Tt Toughpower are twice as large as the single one in the ModStream, each individual one is only half as big. I'd think that would be a step downwards! Now, reason tells me they wouldn't do it (and Jonny wouldn't have rated it so highly) if it really ended up being worse than a single rail of half the power, so it must be better. So does the graphics card get all 4 rails simultaneously, or is there something else I'm missing as to how dividing the +12V power into 4 doesn't gimp it? Thanks!
    Each rail is limited to what its rated output is. If it's the TT Toughpower 750w that I think you're talking about (is there more than one) there's 18a on each 12v rail, which would provide up to 216W each.. An 8800GTX requires two PCIe plugs so you could hook two 12v rails up and have a theoretical limit of 432w, which is well over teh 230ish watts required.

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