Powering 2nd GPU via a riser (connections & safety advice)

[jayuk20]

I have an RX 480 8GB Nitro in my machine at the moment and wanted to add an R9 390x 8GB to it. I have number dyslexia pretty bad so I keep muddling up numbers and it's difficult to get them correct so hopefully someone can help.

I have an 850w PSU powering an Am1m mobo and Sempron 2600+ with 4GB ram, a USB wifi device and a USB flash drive, 3 140mm fans and mouse and keyboard. I've calculated an approximate load of 583w but a second opinion would be good.

Also in terms of the powering of the riser, I've 1 spare 6+2 pin and one 4+4 pin connectors left. I need to connect the 6 pin direct to the riser and the other to the GPU, I assume this is correct? I also don't have the cable length to take it outside the case so I am thinking. Could I use a PCIe splitter to plug into the GPU and riser and then the other single end into the 4+2 coming from the PSU?

 

[Jay85]

More important question is, what are you doing with the 2nd card? Are you mining?

 

[jayuk20]

I am, not doing more than two cards at the moment. Just waiting until I can get some Nebras

 

[Jay85]

Will both cards be used at the same at 100% load?

 

[jayuk20]

Yes I'd say 100% over 12+hrs

 

[gpuerrilla]

Share what exactly the PSU is. Make, model, modular etc.

 

[Ch3m1c4L]

Simple tl;dr
Ideally, if you're 850w is a branded psu:
1: use a molex or sata cable for the pci-e riser board (should come with adapter if not native).
2: use 6+2 pin or 8 pin pci-e cable from psu (4+4 is not the same, that's eps 12v, you could use an adapter on that though). One psu cable per gpu, an 850w should have at least two sets of pcie connectors.


Detailed:
If you're running the GPUs at ~half power for mining, technically you could have multiple GPUs per pcie cable from psu, but while you learn about power loads and only have two GPUs, may as well use individual cables to be safe.

If its a decent psu the molex and sata cables should be good for about 120w power per psu cable, not connector. (riser slot uses a max of 75w).
An 8pin pcie is 150w, I forget what a 6 pin is,maybe 75w?
Eps 8 pin (your 4+4) is at least 150w (might be more) so is fine to adapt to anything else.

The physical wires should be rated for a bit higher than those numbers. You can work it out from whatever the wire gauge used is, but cheap Chinese psu's might lie about the gauge they use. Higher loads, from using too many splitters or adapting the wrong ones etc, can literally melt cables, but it's kindly to be an issue for only two GPUs unless you run them at full power.

 

[jayuk20]

Much appreciated, Chemical!