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I see plenty of information telling my decent 850 watt (XIGMATEK MC NRP-MC851 850W) will support SLI with two 570 GTXs. However, it's 12+ rails distribution makes me think otherwise: +12V1@18A, +12V2@18A, +12V3@30A, +12V4@30A.

Now I know other items such as my CPU (i7 950) change things. My GTX 570 needs two different six pin +12 connections (38A total recommended). Right now I'm running 12V3 and 12V4 and its probably overkill. However, if I add a second GTX, how many amps do I need PER 6 pin connection?

For example if bf3 does not fail I plan to over overboard and get 2 590GTXs. I will need to know how to figure out how many amps per 6 pin, not RAIL. For example, if I have 30 amps on one 6pin#1 and 18 amps on the second and the requirement is 50 amps, I could run a single one but if both need to be 50 amps, finding a power supply with 50 amp rails isn't easy. I know the requirement is based on the system whole but how do I find out the requirement per 6 pin so I have a rough idea how much I need?

Per pin amps is important because 12v3 and 12v4 only have single 6 pin connections. Could just use a "Y" adapter?

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up vote 2 down vote accepted

As per the Nvidia spec sheet, the GTX 570 draws about 220W of power. At 12V, that's just under 20A of current for each card (and that's also assuming the card doesn't use some of the 150W it can get from the PCIe bus as well).

Even if the power was only drawn from the two 6-pin connectors, if the card draws 20A of current, that's only 10A per 6-pin, so you should be fine. For stability measures, I would use the +12V1 and +12V3 for the first card, and the other two for the second.

If you wanted to upgrade to a GTX 590 (or two), you need a new power supply since the 590 requires an 8-pin PCIe connector. The upgrade to 8 pins is due to the increased current carrying capacity of the actual conductors (now you can carry 33% more current because you have 33% more physical space to transmit electrons).

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So basically if the requirement is 20amps total, and you have two connections you need to divide the draw across both. In this case its 20 amps, so will say 30 to be safe. 15 amps per connection saying their on different rails. So in the case of 590' I would need say 35 amps per 8pin if on different rails. Thats why i was trying to figure out the requirements, I know for a 590s I need a new one and was trying to decide how big I needed to go. Thanks! – Ryan Mills Sep 26 '11 at 18:17
Yes, you can split it. There is no need to tack on an additional 50% worth of current, you can compensate for overclocking though (factor in an additional 25% current for most cases, if you are not part of most cases you know who you are already). As for a single 590, the max TDP is 365W, or about 31A at 12V. Add an additional 25%, and you need roughly 20A per 8-pin rail. – Breakthrough Sep 26 '11 at 20:32
@Ryan Mills also if you're wondering why you can split it right down the middle, it's because we assume the conductors have no resistance, and if you add another direct connection to a voltage/current source, you can assume that half the current will flow through the added conductor (assuming again that the conductors are identical - and in computer power supplies, even modular ones, they are almost always close enough). – Breakthrough Sep 26 '11 at 20:34

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