I had to be honest I have never seen a 4 12v rail supply before I always buy Corsair which is single rails.
For the price it seems fantastic but 18a per rail just doesn't seem enough to power most graphic cards.
How exactly does this work out?
I checked out and it seems OCZ does it with a lot of their supplies so there has to be a method to the madness.
I wish the search function was a bit better on this site...I couldn't find my answer so forgive me if it was asked before.
With most power supplies with more than 1 12 volt rail, a bit of balancing things are in order to not overload any particular rail. I agree with you on single rail power supplies being better in this matter. I would recommend the TX650 or TX750 from corsair myself.
the number of rails is a bit insignificant really, what matters is how the cables (and power load) are distributed among those rails.
With single rail it's easy...
the ocz psu the op linked is distributing the load pretty well.
both PCI-e power cables have their own rail, one is for cpu and one for mobo and drives etc...
Jedecs pcie power specs allow 150W to be drawn from the 8pin pci-e, 75W from the 6 pin and 75w from the slot on the mobo. total 300W for a card to use... 4870X2 uses max 270W and it seems the new nvidia gtx295 is pretty much the same. Cards with two 8pins are not allowed.
max TDP for CPUs is 130W, so far... heavy OCing can increase it quite a bit but 216W is still a lot. Drives use like 10W each, mobo chipset around 20W or so, plus that 75W 'reserved' per full lenght pcie slot
So having 'just' 18A per rail is fine, and thats the Jedec spec for atx power supplys anyways. The specs advised manufs to use more rails to provide the extra wattage if needed rather than using one giant rail. Don't know why, really...
oh and that ocz had 675W combined so thats 168.75 per rail if evenly distributed (like that's ever going to happen lol)
More rails are generally used as marketing, however, a well built single rail and well built multi-rail have no pros or cons against each other that i know of. Its also possible that this has started to come about because of the EPS12V standard states the "recommended" setup as multiple rails. Starts 6.4 on pg 20 http://ssiforum.oaktree.com/pdfs/EPS12V%20Spec%202_91.pdf
which is about 10-11amps on the 12v rail. 18amp on a 12v rail is more than enough to cover this.
However, also take into account that the PCI-e slot itself will provide power to the card through the motherboards 20/24pin connector. This will (hopefully in good PSU's) be using a different 12v rail to the one being used on the PCI-e power connector from the PSU. PCI-e1.1 specs provide 75w and PCI-e 2.0 provides 150w through the slot. In theory you should be able to power a HD4870 off a PCIe2.0 slot, however, the circuitry on the PCB doesnt allow it as it requires the 6/8 pin PCI-e power attached. 6pin provides 75w as per specification, 8 pin should provide more however i cannot find a figure for it.
So does an extra ground wire or two allow the connector to support more current? Sure.
Now also remember what the PCI-SIG specification actually is for the 8-pin PCIe. It really only suggests ONE extra ground for increase current. The 8th pin is actually for a +12V sense. What's supposed to happen is as current increases, resistance increases and voltage drops (V = I*R). The +12V sense is supposed to feed voltage information back to an IC in the PSU and tell it when the +12V drops out of spec. The PSU is then supposed to compensate for that resistance with an increase in voltage at the source.
Can the 8-pin PCIe connector as it's implemented today really deliver TWICE the power of a 6-pin? No f'ing way...