Combining 12v rails?

[Ralf]

With multi rail PSU's like the Straight Power 11, would it be possible to combine 12V gpu rails with 2x8pin to 8-pin cable without issues? So that you could push cards with single 8-pin without triggering OCP.

[Phas3L0ck]

Should be perfectly safe. Most new power supplies only have a single 12v rail anyhow, and the wires/cables are just a breakout method.

If your PSU's nameplate/label has one single spec for how much current can travel across 12v for your model, it's all good.

[OrionFOTL]

23 minutes ago, Ralf said:

With multi rail PSU's like the Straight Power 11, would it be possible to combine 12V gpu rails with 2x8pin to 8-pin cable without issues? So that you could push cards with single 8-pin without triggering OCP.

It's not only "possible", that's how you're supposed to use them. That's the whole point of multi-rail PSUs. Connecting devices using multiple rails, so each individual cable is protected from short circuits while still allowing the device to use large amounts of power.

[hojnikb]

Don't forget, that you cannot combine current capability of all rails, as there are combine limits as well.

[Ralf]

6 hours ago, Phas3L0ck said:

Should be perfectly safe. Most new power supplies only have a single 12v rail anyhow, and the wires/cables are just a breakout method.

If your PSU's nameplate/label has one single spec for how much current can travel across 12v for your model, it's all good.

This model says that it has 4 rails.

6 hours ago, OrionFOTL said:

It's not only "possible", that's how you're supposed to use them. That's the whole point of multi-rail PSUs. Connecting devices using multiple rails, so each individual cable is protected from short circuits while still allowing the device to use large amounts of power.

How are you supposed to use two gpu rails with one 8-pin without a custom cable?

1 hour ago, hojnikb said:

Don't forget, that you cannot combine current capability of all rails, as there are combine limits as well.

Let's say my other components(cpu, mobo, ram, ssd , fan) draw 150W and gpu rail 3 can output 264W. This would downgrade the 650W psu to a 490W(150+264+75), because the Rail 4 would just sit there unused. Now if I could combine them with a cable, I could throw another 150W through the 8-pin.

TLDR: One gpu rail=339W, two gpu rails=603W before we hope that OCP doesnt trigger on the rails or OPP kicks in.

[seon123]

2 hours ago, Ralf said:

Let's say my other components(cpu, mobo, ram, ssd , fan) draw 150W and gpu rail 3 can output 264W. This would downgrade the 650W psu to a 490W(150+264+75), because the Rail 4 would just sit there unused. Now if I could combine them with a cable, I could throw another 150W through the 8-pin.

TLDR: One gpu rail=339W, two gpu rails=603W before we hope that OCP doesnt trigger on the rails or OPP kicks in.

An 8 pin PCIe connector is 150W per spec, so I don't see why 264W (+ 75W through the motherboard) would be an issue?

[Ralf]

2 hours ago, seon123 said:

An 8 pin PCIe connector is 150W per spec, so I don't see why 264W (+ 75W through the motherboard) would be an issue?

Power spikes?

[Phas3L0ck]

On 1/13/2021 at 12:41 PM, Ralf said:

Power spikes?

I had the same thing in mind while planning the cable layout for my system.

According to spec, 6/8-pin should carry at least 150W. HOWEVER; when you consider the physical/electrical tolerances of common 18-16AWG wire, (and the fact that PCIe 8-pin is basically 6-pin with extra signals for negative but don't provide any additional power) you can actually send up to 250W (more or less) over a 6/8-pin PCIe power cable, in the form of 7A per physical wire pair, (1 positive and 1 negative) at 3 pairs in a 6-pin connector, all from the same solder joint inside the PSU. So if you can take at least 21A from a 12V rail (and I see your 12V4 line has 22A) I believe it will work perfectly.

 

Also, I was wondering, what GPU model are you looking to power, exactly?