Confusion regarding correct power cables usage on GPU with two 8-pin

[Vortagh]

Hey guys and gals, I have a power cable question.

 

I just set up my new Ryzen 5600 system and a buddy gave me his old Vega 56 to keep me running until I can finally grab a 3080 for a realistic price in a few months. The system is powered by a 750W BeQuiet BQT-P9. 

The issue I'm having is that I have no idea what cable to use. The Vega 56 needs two 8-Pin connectors. I have two PCIe cables from the PSU, that come with 1x 8-Pin and 1x 6-Pin. I have been reading up on the matter and I am totally confused, because I find contradictory information in the PSU's manual and internet threads on the general subject, but also because a lot of the information I am finding is ten years old, and/or talks about GPUs with the need for only one power plug.

 

Can I just use a 6 to 8-Pin adapter on that cable? The additional cables are just ground and are split off existing cables anyway, so it makes absolutely no difference, whether it's a true 8-Pin, or an adapter, but I'm worried about power draw of that card. Rated or not, the Vega seems to be able to pull close to 300W from what I read, which seems to be the max on a single cable. And I rather not, if that's true. (no OC, but I am going to use it as much as I can, graphics power wise)

 

I could obviously use two cables, but I somehow get the feeling from the BQT P9-750W manual, that they don't want me to do that. Granted, they don't forbid it in the little graphic they have at the end of the manual, but it confuses the shit out of me. To clarify:

 

PCIe 1 to 4, all use two 12V rails, both of which are 25A.

PCIe 1 and 2 use 12V3.

PCIe 3 and 3 use 12V4.

Yet the diagram says, that a single card should be run from PCIe 2 and PCIe 2 only. Not 1. Not 3. Why not PCIe 1 though? It's the same rail. 

And if I wanted to do SLI, I should use PCIe 1 and 3. THAT makes sense, you don't want to pull power for two GPUs from the same rail. But why can't I just use PCIe 1 for a single card?

 

 

So, what should I do? I'm totally frustrated right now. I want to just plug it in and play Shadows of the Tomb Raider, not make pretty words on board - but when it comes to running PSUs in a wrong way, I'm a bit careful.

Can I just take two cables, put them in PCIe 1 and 2 (obviously only those, because using 1 and 3 would be two different rails, which would be a different kind of bad) and use their 8-pin only? 

Or should I just use an 6 to 8 pin adapter?  

 

Halp? Thanks!

[LIGISTX]

10 minutes ago, Vortagh said:

Hey guys and gals, I have a power cable question.

 

I just set up my new Ryzen 5600 system and a buddy gave me his old Vega 56 to keep me running until I can finally grab a 3080 for a realistic price in a few months. The system is powered by a 750W BeQuiet BQT-P9. 

The issue I'm having is that I have no idea what cable to use. The Vega 56 needs two 8-Pin connectors. I have two PCIe cables from the PSU, that come with 1x 8-Pin and 1x 6-Pin. I have been reading up on the matter and I am totally confused, because I find contradictory information in the PSU's manual and internet threads on the general subject, but also because a lot of the information I am finding is ten years old, and/or talks about GPUs with the need for only one power plug.

 

Can I just use a 6 to 8-Pin adapter on that cable? The additional cables are just ground and are split off existing cables anyway, so it makes absolutely no difference, whether it's a true 8-Pin, or an adapter, but I'm worried about power draw of that card. Rated or not, the Vega seems to be able to pull close to 300W from what I read, which seems to be the max on a single cable. And I rather not, if that's true. (no OC, but I am going to use it as much as I can, graphics power wise)

 

I could obviously use two cables, but I somehow get the feeling from the BQT P9-750W manual, that they don't want me to do that. Granted, they don't forbid it in the little graphic they have at the end of the manual, but it confuses the shit out of me. To clarify:

 

PCIe 1 to 4, all use two 12V rails, both of which are 25A.

PCIe 1 and 2 use 12V3.

PCIe 3 and 3 use 12V4.

Yet the diagram says, that a single card should be run from PCIe 2 and PCIe 2 only. Not 1. Not 3. Why not PCIe 1 though? It's the same rail. 

And if I wanted to do SLI, I should use PCIe 1 and 3. THAT makes sense, you don't want to pull power for two GPUs from the same rail. But why can't I just use PCIe 1 for a single card?

 

 

So, what should I do? I'm totally frustrated right now. I want to just plug it in and play Shadows of the Tomb Raider, not make pretty words on board - but when it comes to running PSUs in a wrong way, I'm a bit careful.

Can I just take two cables, put them in PCIe 1 and 2 (obviously only those, because using 1 and 3 would be two different rails, which would be a different kind of bad) and use their 8-pin only? 

Or should I just use an 6 to 8 pin adapter?  

 

Halp? Thanks!

The negative wires are actually what increase the power handling from 75 to 150 on a 6 to 8 pin. So, yes, those matter.

 

I wouldn’t recommend using a single 6 pin to 8 pin. Dual 6 pin to 8 pin is fine though. 
 

I am confused why you don’t think you can use 2 cables. Do you have 2 8 pin cables, most PSU’s these days do, or at least they are 6+2 pin (which is an 8....). You definitely need to use 2 cables, if you get a 3080, your going to need a single 8 pin and either 2 more single 8 pins or a 8 pin that daisy chains to a second 8 pin as most aftermarket cards use 3 8 pin connectors. Point here is your going to need more than 1 8 pin cable no matter what your trying to do here. Your PSU l doesn’t have more than 1..?

[Vortagh]

13 minutes ago, LIGISTX said:

The negative wires are actually what increase the power handling from 75 to 150 on a 6 to 8 pin. So, yes, those matter.

 

I wouldn’t recommend using a single 6 pin to 8 pin. Dual 6 pin to 8 pin is fine though. 
 

I am confused why you don’t think you can use 2 cables. Do you have 2 8 pin cables, most PSU’s these days do, or at least they are 6+2 pin (which is an 8....). You definitely need to use 2 cables, if you get a 3080, your going to need a single 8 pin and either 2 more single 8 pins or a 8 pin that daisy chains to a second 8 pin as most aftermarket cards use 3 8 pin connectors. Point here is your going to need more than 1 8 pin cable no matter what your trying to do here. Your PSU l doesn’t have more than 1..?

 

Hi, thanks for the quick reply.

 

I have more than one PCIe cable, yes. I am confused, because of a diagram in the PSUs manual. See the attached images. It tells me to use one cable, which would require the use of an 6 to 8 adaptor. 

 

However, I have no personal issues with using two PCIe cables. It just isn't covered and some threads I found online confused the hell out of me, with contradicting statements. 

If I do, I assume I should plug the cables in PCIe 1 and 2, as opposed to any other combination. Correct? That's another source of confusion.

 

BUT: Asus states that the card draws "above 300W" which obviously is more than one rail can provide at 12V 25A. 

 

[Ralf]

37 minutes ago, Vortagh said:

using 1 and 3 would be two different rails, which would be a different kind of bad

You are already using two rails. One rail comes from the 24-pin on the mobo(75W) and the other from the PCIe 8-pin(150W). I would use PCIe 2 and 4 for any power hungry cards, so it doesnt trigger OPP.

But I wouldn't use this decade old PSU wih a Vega56, let alone with a 3080 that costs $699+

[LIGISTX]

13 minutes ago, Vortagh said:

 

Hi, thanks for the quick reply.

 

I have more than one PCIe cable, yes. I am confused, because of a diagram in the PSUs manual. See the attached images. It tells me to use one cable, which would require the use of an 6 to 8 adaptor. 

 

However, I have no personal issues with using two PCIe cables. It just isn't covered and some threads I found online confused the hell out of me, with contradicting statements. 

If I do, I assume I should plug the cables in PCIe 1 and 2, as opposed to any other combination. Correct? That's another source of confusion.

 

BUT: Asus states that the card draws "above 300W" which obviously is more than one rail can provide at 12V 25A. 

 

Treat it as if it’s 2 GPU’s. Do either of the bottom two. The bottom pic shows 2 GPU’s can be plugged into that middle plug, so plug both 8 pins into that. This diagram is confusing AF I agree, but it’s a simple deal, and your thought process is correct. You want to be on different rails than other parts, and looks like that middle plug can support 2 GPU’s all on its own, so it’ll be plenty good for a single card. 

[Vortagh]

1 and 3 it is then. Thanks.

 

The PSU is nine years old, true, but it has never seen anything above a 780 and it due to me never having any time at all, it has barely seen any use in those years. I am most likley going to switch it out at some point, before the 3080, but that's nothing I'm doing tonight - or next week - nor am I worried about the age of it. If it didn't have the power, it would be something else.