Power supplies with multiple rails?

[feedtheduck]

I'm really confused about PSU's with multiple rails, such as this one:
http://www.newegg.com/Product/Product.aspx?Item=17-341-041&SortField=0&SummaryType=0&Pagesize=10&PurchaseMark=&SelectedRating=-1&VideoOnlyMark=False&VendorMark=&IsFeedbackTab=true&Page=3#scrollFullInfo
It says it has 4 12V rails each with 18A of power. Isn't this worse than having a single rail with all the power? I'm just concerned, that let's say my GPU uses 250W of power, which is more than the 12V*18A=216W of power that a single rail can provide. Wouldn't that be bad, because there would be more than enough power on the other rails, while this one wouldn't have enough? Can someone just clarify this for me?

 

[birrdd123]

I believe you can use a single rail and use the total amperage of the PSU. Here is a great explanation to clear it up (hopefully) for you.

http://www.jonnyguru.com/forums/showthread.php?t=3990

 

[hunter315]

That jonnyguru post is very good, i wrote up a similar one a while back, the link to it is in my signature.

As for your power concerns, it is important to realize that the connectors tend to be spread between the rails, in your case 12V1 feeds the main motherboard connect, 12V2 feeds half the 4+4 pin CPU connector, 12V3 feeds the other half of the 4+4 and half of the modular connectors, and 12V4 feeds the 8 pin CPU connector and half the modular connectors. A GPU gets power from 2 sources, it gets up to 75W from the motherboard and the rest from the PCI-e power connectors. This means that if your GPU is consuming 250 W of power it isn't pulling it all from the PCI-e connectors, it is pulling some from the motherboard and some from the PCI-e connectors which spreads your load across at least two of the rails, and since a 6 pin connector provides up to 75 W and an 8 pin connector provides up to 150 W, so a 250 W graphics card would have 2 6 pin PCI-e connectors or 1 8 pin, and the motherboard providing it so it could be split between up to 3 rails, this isn't the best setup but it is good enough that you will tend not to overload it. It is also important to know that the OCP limit is set slightly above what is stated on the label so it gives you a little bit of leeway over that 18A number in case the GPUs are drawing a bit more from the PSU connectors and a bit less from the PCI-e slot.