Nope, I went with the SC version, no external power needed, and as I got my Molex-P4 adapter yesterday, I think I might try firing it up tonite and see what happens. I just better not let the smoke out, that would make for a Very bad evening...

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With the usb risers there are no Power Draw from the PCIE slot on the mobo.
The riser supply the gpu With Power from the molex Connector on the riser itself.

The molex Connector are rated for 11A for each pin, Equals 132W at 12V. To be on the safe side i would supply just one riser from each cable on a psu. One possible solution to Power all the GPU's would be to get a surplus server powersupply, and make up the wiring by Your self. The Power Draw for 8 GPU @ 75W is 600W. So a 700 - 750W server Power would fit the bill Nice, and the server PSU's supply all their Power on the 12V rail.

On my rig running With risers the Cards on risers have a little longer "loading period" for a New taskn than the Cards running off the MB, due to the lower bandwith. (Bus load peaks at 100% during a New task start, and Levels out at around 50% when one task is running on the card. (GTX titan card)

This is just my 2 cents on the risers. I am happy With the risers i use as the temp on the Cards have dropped since the cooling situation is much better than Cards inside a closed cabinet.

Rune, thanks much for your insight, I literally just got off the phone with my service guy at PCP&C, he said that he believed that they were rated for about the same as the 18 gauge wires in the cable, 5 amps, or about 60 watts. I thought that might be a little low and he CYA'ed it at that amount, but he also did think that I could probably run 2 of the cards off of one cable. That would get me 8 out of 10 running off of the modular Molex PSU connectors, and only have to come up with another 2 connections.

This is a modular/wired PSU, and hardwired has 2 8 pin connections for the CPU's, the 24 pin MB power and 2 6/8 pin PCI-E GPU connections. I understood that those connections are only 12v, so I could pull the 12v out of it and run them to 2 Molex connectors, just need to find a male 6 pin PCI-E connector to make the cables out of. The only concern I had about doing that, is that I thought the Molex has both 12 and 5 volt power in it, and I am not sure if the USB-PCIE adapter uses any 5v power at all, or if it just runs on 12v. If it needs both, my PCI-E to Molex adapter idea wouldn't work. Anyone know if those adapters need both voltages or just the 12v?

As this PSU is 1200w, and should cover both the dual Xeons, the MB, as well as all the GPU's with a bit to spare, I think I should be able to make this one work, I'd rather not have to try dealing with 2 PSU's in one system, I know it can be done, but I'd prefer not to if I can avoid it.

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My risers does use the 5v from the molex plug.
The 5V line does supply voltage to a small linear 3.3V regulator.
This regulator deliver 3.3V Power to the PCIE slot.

Not sure if the GPU use the 3.3V or not, and if the card use the 3.3V line i'd belive the load is very low, so i think it would be okay to split the 5V from one molex to two or 3 risers.

Another possible route would be to check the specs for the 3.3V regulator. If it can handle 12V as input, it would be a small manner to supply the regulator With 12V instead of the 5V.

Not sure if the GPU use the 3.3V or not, and if the card use the 3.3V line i'd belive the load is very low, so i think it would be okay to split the 5V from one molex to two or 3 risers.

Pin 1 Side B & Pins 2&3 Side A&B supply +12V
Pin 8 Side B & Pin 9 Side A & Pin 10 Side A supply +3.3V. Pin 10 Side B supplies +3.3V Aux power.

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Grant
Darwin NT