Hello,
I bought a couple of RX 580's pretty cheap and will probably sell one off them.
I switched from a Gigabyte GTX 970 G1 so I got everything working with cables...
In terms of power (1 x 8-pin) I thought it would be okay to plug both the cards in and see if I could run both.
My bet is that I will get a shutdown with prober benchmark software. I just ran a Unigine Heaven test in osx and got the same score as with one card.
Is it because of Heaven, osx instead of windows or maybe drivers that number two card doesn't pitch in?
Any ideas?
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If I read your picture correctly, your config is 100% sure will hit the shutdown protection if you can fully utilise both GPU.
It seems you have a dual mini 6pin to single 8pin cable. And then use a 8pin female to dual 6+2 cable to power two Nitro+ RX580.
Only plug in the 8pin is OK for this card as per Sapphire's manual, and many users confirm this already. So this is not a problem.
However, the Nitro+ will draw really close to 150W from the 8pin when under real high stress. And many apps can easily make it draw ~120W from the 8pin.
So, what you are doing is combining two mini 6pins together. And make it a single 240W power source for two Nitro+ RX580.
When you running Ungine Heaven, it's an OpenGL benchmark, and no Crossfire support in macOS. So, only the RX580 that connected to the monitor is working. This is why your multi GPU benchmark is same as single GPU benchmark. Because that benchmark software cannot utilise multi GPU.
In this case, the RX580 that is working hard may draw ~120W from the 8pin, and the other RX580 may only draw ~10W. So, 130W in total for both mini 6pin. Easy job, no problem at all.
But if you try to run something like Luxmark. Both GPU will work together, and each of them will draw really close to 120W from the 8pin. In that case, the power draw may be already high enough to trigger the shutdown protection.
And if you connect two monitors, one monitor for each card. And then run two Furmark (one on each monitor). This will make both cards draw 150W each from the 8pin, and 100% guarantee will shut down your cMP.
Also, if you run Unigine Heaven in Windows with Crossfire enabled, that may also good enough to trigger the shutdown protection.
Anyway, what you plan to do with this setup? If just something like FCPX, most likely will be OK.