Single 2 x 8 pin PCI cable safe for RTX 3090?

[Adm_Zaxxon]

Hello!  So I'm doing a SFF build with a 3090 FE...    Oo at least, I will be... one magical day when I get my hands on one.  For now, I am wiring up my case, and seeing as it is a SFF build, space for cables is at a premium.  My PSU came with two PCI power cables that have Y ends for two separate 6+2 connectors for each cable.  My question is, can I get away with just using one cable?  I can fit the second connection but I don't really feel like finding room for all the extra pigtails is worth the space, if there is no actual concern.

 

My PSU is a SilverStone 800W SFX-L, 80 Plus Titanium (SST-SX800-LTI-USA)

 

The cable in question is attached as a pic.

 

 

If it is deemed unsafe for one cable, I may pursue a custom wiring solution, but otherwise are there thoughts on just cutting off the pigtails and heatshrinking over the ends? I may, in the short term, just mod the cables to make them less bulky. 

 

 

If anyone is interested, I'm building in a Sliger "Conswole" CL530, and this SFX-L PSU fits totally fine, as long as you figure out what to do with the extra cables.

[Helpful Tech Witch]

On a 3090? No. One 8 pin is rated for 150, plus 75 from the slot? That's only 225. And the Fe 3090 has a 350w tdp. That's 125w more you need to makeup.

 

Don't take a shortcut here

[Entropy.]

So you're saying you want to use one 6+2 pin for a RTX 3090? Not sure exactly how that would play out, but I highly doubt that you would want to find out, especially if you plan on buying a higher-power AIB card, or overclock. Since an 8 pin can deliver about 150w of power, and the slot 75w (225w out of the 350w+ tdp on the STOCK FE), I'd expect severe downclocks/throttles while the GPU is under enough load. Even the FE comes with a 2x8 pin to 12 pin adapter, instead of a 8 pin to 12 pin. Nvidia has to account for some overhead sure , but still, If they wouldn't do it, It's probably not that wise for you to do it. 

[IkeaGnome]

There's been people on here with 3080s and some 3070s that have issues with GPUs until they use two separate cables.

 

Edit:

I remember seeing a couple 3080s with that problem in the last few months. I want to say there was a 3070 or two. If there was, they're very very rare.

Edited March 3, 2021 by Voluspa
Clarified

[Adm_Zaxxon]

Thanks for the quick responses.  I'll go ahead and wire it up with both then, and start stuffing away...

[Moonzy]

uhm... are there any 3090 with 2x 8-pin pcie?

 

all the ones i know are 3x 8-pin, which for almost all PSU i know, requires 2 harness of PCIE power (since there's only 2x 8-pin on both)

 

if you can find a 3090 that uses 2x8-pin, using the daisy chained cable is fine, as it has 2x 6+2-pin at the end, so it should be capable of delivering 300W

but a 3090 draws around 350W sustained, with much higher peaks, that's why they all come with 3x 8-pin afaik

[WhitetailAni]

3 minutes ago, Moonzy said:

uhm... are there any 3090 with 2x 8-pin pcie?

The 3090 FE comes with a 12-pin to 2x 8-pin adapter.

I believe it's the only one to use only 2 8-pins.

[Grabhanem]

Btw, don't get scared by the 150W per 8-pin rating - that's an artificial restriction by the PCI SIG on the GPU end, as those physical connectors can easily deliver 300+ watts. However, you will tend to get better transient response using two separate cables, especially on very transient-heavy cards like the 3090.

[Guest]

3 hours ago, Grabhanem said:

Btw, don't get scared by the 150W per 8-pin rating - that's an artificial restriction by the PCI SIG on the GPU end, as those physical connectors can easily deliver 300+ watts. However, you will tend to get better transient response using two separate cables, especially on very transient-heavy cards like the 3090.

No.

 

If you look at the mechanical specification of "when can these connectors essentially melt and/or catch fire", then yes.

 

If you look at the "high level" spec for the pins, each pin is rated at 7A per pin.  But if you look at the detailed spec, you'll see that Molex de-rates the pins for each additional adjacent pin.  The wattage "limit" is based on acceptable voltage drop, temperature rise.  The more pins, the more power, but it's not scalable (i.e. 8 pins can not handle 2x the power of 4, etc.)

 

[Adm_Zaxxon]

This is more or less the discussion I had seen elsewhere and wasn't really sure since the "spec" wasn't listed on any of my components.  Either way, I'll just run two cables to be safe, and maybe custom sleeve the PCI to 12pin thing to save on space.

 

[Grabhanem]

On 3/2/2021 at 9:05 PM, jonnyGURU said:

No.

 

If you look at the mechanical specification of "when can these connectors essentially melt and/or catch fire", then yes.

 

If you look at the "high level" spec for the pins, each pin is rated at 7A per pin.  But if you look at the detailed spec, you'll see that Molex de-rates the pins for each additional adjacent pin.  The wattage "limit" is based on acceptable voltage drop, temperature rise.  The more pins, the more power, but it's not scalable (i.e. 8 pins can not handle 2x the power of 4, etc.)

 

This spec?

https://www.molex.com/pdm_docs/ps/PS-5556-001-001.pdf

 

This seems to indicate that 7A is the current capability after derating for 8 pins. Single pin pairs are rated to 9A with reasonable wire gauges.

[SFFDesigns]

SilverStone PP12-PCIE INTRODUCTION (silverstonetek.com)

 

Haven't checked if it's for the SX800-LTI yet (I assume it would work), but this might be what you are looking for if you use a 3090 FE? Should be easier to route

[Guest]

1 hour ago, Grabhanem said:

This spec?

https://www.molex.com/pdm_docs/ps/PS-5556-001-001.pdf

 

This seems to indicate that 7A is the current capability after derating for 8 pins. Single pin pairs are rated to 9A with reasonable wire gauges.

Right.  7 for 8 pins, 8 for 4 pins, etc.  But like I said, it still doesn't take into account acceptable voltage drop. That spec is defined by Intel, PCI-SIG, etc.  Not Molex. Molex does not define the application.  Molex's spec is based on how hot the connector can get before it starts to melt.  Not how much current you can deliver and expect the voltage to only drop 10% (assuming even 10% is acceptable).

 

Trust me.  I can plug in an 8-pin mini-fit jr into the ATE and put a 28A load on it and it will get hot, but not melt.  But the voltages will be significantly lower.  If Intel, for example, wants voltage to be +/- 5% nominal, then they can say that a connector that normally could support 16A should only support 8A.

 

Heat's not free.