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X399 and dual EPS12V

Fri Nov 23, 2018 9:56 pm

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 ... 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?
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Re: X399 and dual EPS12V

Fri Nov 23, 2018 10:18 pm

While that's a reasonable guess, I don't think you should just assume unless the user manual states this or you've checked everything out with an ohmmeter (with all power removed, of course).

Maybe you could hedge your bets by getting a PCIe to EPS12V adapter cable, and using that to power the 2nd EPS12V connector.

Another useful test you could do: Put some heavy load on the CPU for a few minutes (peg all of the cores), then feel the EPS12V connector. If it is noticeably warm (or hot) to the touch, that may be an indication of potential trouble down the road since it indicates that you're drawing excessive current through the contacts. This will eventually cause contact oxidation, which will increase the contact resistance, which will in turn lead to more heating in a positive feedback loop until you melt the EPS12V power connector. (And a melted EPS12V power connector will almost certainly ruin your day...)
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Re: X399 and dual EPS12V

Sun Nov 25, 2018 5:44 pm

Depends on the board, but the two I've played with both ran happily with a stock-clocked 1950X and only one EPS12v connector (and this was mentioned in the manual that both were needed only for overclocking).
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