How so? The OCP trip-point for quality multi-rail 750W units is typically over 50A. So unless the motherboard's rail is powering 600W+ all by itself, it's not going to be an issue.
If the motherboard's rail is trying to pull 600W+ all by itself, that's exactly where OCP will save you because it probably means there's a short somewhere. In contrast, a single rail PSU might keep supplying power until something gets fried.
I think you are missing my point, most high quality rails are limited to either 18A, 22A or maybe 25A. Lets do the math:
(75/12)x3=18.75A (potentially)
That is purely for 3 graphics cards, it also has to power the motherboard and and drives that are powered on the same rail.
It leaves very little room for the motherboard and any drives that also need 12v to run.
I know that not all graphics cards will draw the full 75w from PCIe but unless you can measure it for youself it is definitely something that needs to be considered otherwise you might end up with the computer shuting down when playing games. Different cards vary greatly.
I read an artical a few years ago where they measured the 12v demand from about 35 graphics cards (pre Fermi) and found that running Furmark the actual draw varied very differently. One card that stuck out was the 8800GT which actually drew 88W from the PCIe slot (above 75w spec).
I highly doubt that the 600W figures you say would come into play as most 18/22/25W railed PSU's will shut down before they get to 50W on a single rail (600/12=50) otherwise what is the protection there for?
This is why only certain PSU's are rated for SLI and other are certified for tri SLI. I realise they can't rate every PSU but I understand why only some can handle it.
A single 12v rail would be far less complicated when running Tri SLI as it wouldn't be as limited to either:
18A (216W@12v) too little to run Tri Sli.
22A (264W@12v) leaving 39W for the rest of the motherboard and drives
25A (300W@12v) leaving 75W for the rest of the motherboard and drives which might actually be possible but still pushing the rail close to the limits when stressed.
It's probably a case of trial and error but if is does shut down whilst gaming it doesn't mean that 750W isn't enough, just that the power needs to be distributed correctly.
I am usually very suprised when the graphics PCIe rails are rated for much more than the Motherboard and CPU rails due to PCIe slots potentially drawing more power than the graphics cards ever will.
Only 2x 8 pin PCIe connectors can potentially consume 300W by themselves.
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