For GPU is it better to use one cable with 2x8 pin or 2 seperate cables?

Monkeyboysteve · 25 Aug 2019 at 15:02

Probably a stupid question.

But my grapics card requires 2 8pin connectors, is there any difference in using one cable that has 2x8pin connectors over using two seperate cables from the PSU?

I have a corsair 850 HX fully modular and it's a 1080ti (in case it matters)

I posted this in the GPU forum too as wasn't sure the best place to stick it,

Minstadave · 25 Aug 2019 at 15:16

Shouldn't really matter but 2 separate 8 pins is optimal.

EsaT · 25 Aug 2019 at 17:10

Always separate cables with high power draw graphics cards.
Using single split in the end cable doubles voltage losses compared to separate cable.
And not only does it affect stability of 12V, but also ground refence of the card, because current causes voltage loss also on return path.
(+ power loss in cable quadruples heating cable more)

There have been cases of single cable causing instability/crashing in this forum, so it's not insignificant thing.
And in case of PSU with current limited wire groups current can exceed limit shutting PSU down.

Threepwood · 25 Aug 2019 at 17:20

EsaT said:
Always separate cables with high power draw graphics cards.

Any card with x2 power pins is going to be a high draw GPU!

Monkeyboysteve · 26 Aug 2019 at 10:08

EsaT said:
Always separate cables with high power draw graphics cards.
Using single split in the end cable doubles voltage losses compared to separate cable.
And not only does it affect stability of 12V, but also ground refence of the card, because current causes voltage loss also on return path.
(+ power loss in cable quadruples heating cable more)

There have been cases of single cable causing instability/crashing in this forum, so it's not insignificant thing.
And in case of PSU with current limited wire groups current can exceed limit shutting PSU down.

But then why do corsair provide cables with a 2x8 split?

Minstadave · 26 Aug 2019 at 10:13

Monkeyboysteve said:
But then why do corsair provide cables with a 2x8 split?

On a high quality PSU itll be fine to use the 2x8 cable. They'll be designed to tolerate the current draw on that single cable.

EsaT · 26 Aug 2019 at 10:22

Monkeyboysteve said:
But then why do corsair provide cables with a 2x8 split?

Because of marketroids.

Minstadave said:
On a high quality PSU itll be fine to use the 2x8 cable. They'll be designed to tolerate the current draw on that single cable.

Unless having doubled wire cross section to keep current density and voltage loss same, it's not designed for that.
And don't see much 15AWG wires...

Monkeyboysteve · 26 Aug 2019 at 10:23

Monkeyboysteve said:
But then why do corsair provide cables with a 2x8 split?

So in my Corsair hx850 I should be fine? The cables are so think it would be a massive pain using two. Especially as that would mean I would have 2 spare 8 pin connectors just flapping around looking silly near the gpu.

Minstadave · 26 Aug 2019 at 10:26

EsaT said:
Because of marketroids.

Unless having doubled wire cross section to keep current density and voltage loss same, it's not designed for that. Corsair arent going to be shipping PSUs that arent to spec.
And don't see much 15AWG wires...

It's going to be designed to tolerate the full power draw of both 8 pins on that cable, it would be unsafe it it wasnt and I doubt Corsair arent building their PSUs to spec.

Monkeyboysteve · 26 Aug 2019 at 10:39

Minstadave said:
It's going to be designed to tolerate the full power draw of both 8 pins on that cable, it would be unsafe it it wasnt and I doubt Corsair arent building their PSUs to spec.

That's what I would assume. When I asked I was thinking more about if it gave better performance to use two.

People saying these cables are unsafe seem a bit silly. Corsair would be crucified for selling unsafe power cables. Just noticed a jay2cents video where he did a comparison. He always used 2 connectors from one cable on his test bench and found only a minor difference going to two. (but it was an increase in performance)

EsaT · 26 Aug 2019 at 13:29

Minstadave said:
It's going to be designed to tolerate the full power draw of both 8 pins on that cable, it would be unsafe it it wasnt and I doubt Corsair arent building their PSUs to spec.

Technically PSU could have 100mV ripple in 12V and voltage varying between 11,5 and and 12,5V and it would be still inside ATX specification.
But that doesn't prevent it being just plain bad PSU.

Similarly cable not heating enough to melt insulation doesn't mean it's good.
And unless wire cross section was doubled, it's designed to be unable to maintain voltage stability.
That's basic electric engineering.

And voltage stability is affected by momentary current, instead of average like for heating of the cable.
While switching regulators have spiky current draw, unless having heavy enough input filtering.
Remember how Vega 56/64 can draw 50A spikes, which caused Seasonic even need to loosen OC protection "to slow fuse" in some models.