Pcie split cables on single rail psus

Rockbox · 8 Mar 2022 at 10:16

I read somewhere that single rail psus meant you didn't need to use 2 separate pcie cables for gpus, and that daisy chained/split cables were fine.

However, the ltt techquickie says only the 5.5v and 3v are split from the 12v, making me think the 12v is under heavy load regardless of single or multi rail.

I still use 2 separate cables for my card. Theoretically, I have 6 cables (3 split) but I take it I can't run dual 3090s with them at 850w.

mickyflinn · 8 Mar 2022 at 21:24

Do you mean your want to run 2 x 3090 on a single 850w psu ?

Rockbox · 8 Mar 2022 at 22:09

No. I wanted to know whether split pcie cables mattered on single rail psus, and why nvidia recommends separate cables.

Dg834man · 9 Mar 2022 at 03:26

8 pin pcie cable has a rating of 150w if you use both connectors of a single pcie cable the max draw is still 150w, split rail single rail has nothing to do with it.

Rockbox · 9 Mar 2022 at 15:12

OK, thanks. Is it strictly 150W? Why can the 320W 3080 and 350W 3090 work on 2 cables?

SonicSW20 · 9 Mar 2022 at 15:19

Rockbox said:
OK, thanks. Is it strictly 150W? Why can the 320W 3080 and 350W 3090 work on 2 cables?

Because the PCIe slot can supply up to 75W.

150W + 150W + 75W = 375W from 2x8-pin and the slot while remaining in spec.

Rockbox · 9 Mar 2022 at 15:22

OK. This rumoured 800W Nvidia Lovelace part, how is it possible? 5 cables + slot?

SonicSW20 · 9 Mar 2022 at 15:25

Cooling it will be a bigger challenge than powering it!

ASMB · 15 Mar 2022 at 01:23

Rockbox said:
OK. This rumoured 800W Nvidia Lovelace part, how is it possible? 5 cables + slot?

There is going to be a new standard of 12 pin pcie cable on the psu, ASUS Confirms 12-Pin PCIe 5.0 Power Connector for Next-Gen Graphics Cards - The FPS Review

Tetras · 15 Mar 2022 at 03:08

Rockbox said:
No. I wanted to know whether split pcie cables mattered on single rail psus, and why nvidia recommends separate cables.

One reason is that even though single rail PSUs are less likely to trip (due to spikes) than dual rail PSUs, you're still drawing a lot of power through a single cable (and there are twice the connectors, if the PSU is modular) and that produces extra demand (and resistance) through the wires.

You're right that the 12v rail is under heavy load regardless, but they're designed to deal with that and how demand is so variable with modern GPUs (assuming they're not group-regulated).