So. About those dual EPS12V connectors on X399 motherboards. I've got an 2950X threadripper workstation setup, and the 750W power supply only came with one EPS12V connector. Seems I forgot to check this little detail when I bought the components. The motherboard is powered via the ATX connector with its little sidecar/extension, 1x EPS12V, and 1x auxiliary PCI-E power connector. The system boots and runs just fine in this configuration, but I suppose I remain a little bit worried about underpowering the motherboard and VRMs.
I did a basic estimate, using information from http://www.playtool.com/pages/psuconnec ... ctors.html
6-pin PCIe: 75W (realistically, much more)
8-pin EPS12V: ~330W
24-pin ATX: 150W (on the 12V rail)
My (otherwise unfounded) suspicion is that the ATX connector and PCIe connector are exclusively used to power the 4x x16 slots at 75W each, leaving the two 8-pin EPS12V connectors to power the CPU. One of the EPS12V alone should therefore be plenty for a 1950X or 2950X CPU, but two of them would be appropriate for the 2990X, or a scenario involving Precision Boost Overdrive.
Does that seem reasonable? Any objections? Are there known examples of Bad Things happening in this situation?