Mac Pro 5,1 PCI GPU power 6/8 pin confusion I have a Sapphire HD 7950 from eBay I got to go in a Mac Pro 5,1. It has dual 8-pin sockets. Being second hand it has no instructions/data sheet.
The one I already own [different Mac Pro] has dual 6-pin sockets, which was the perfect fit for the motherboard's 6-pin PCI power. Plug &amp; go.
Google gives far too many opinions over far too many models for me to know what the solution is. There seems to be cable adapters for dual 6-pin to single 8-pin, which really doesn't seem optimal; single 6-pin to dual 6 &amp; 6+2, which sounds a bit optimistic.
Simplest start point… can I use my two existing 6-pin cables in the 8-pin sockets, or will something go boom?
I can't seem to find a single 6 to 8-pin cable, of which I could buy two.
In terms of power requirements/stress, this Mac is never going to be used for anything more than video playback. I just want to get the Mac to run Mojave, rather than be stuck on High Sierra [Metal2 requirement].
The 2 pins more in the in the 8 pin plug are 1x ground and Sense1. Sense1 is only to inform the card whether a 6 or 8 pin plug is plugged in.
I think the big difference in your usage is that 8-pin plugs are able to pull 150 W, where 6-pin plugs only provide 75 W at max.
As far as I see the Sapphire HD 7950 needs 80 W at idle.
So I see 3 Possibilities:

Buy a 6 to 8 Pin Cable and risk your power supply (amazon link) (worst idea)
Use the card with only 150 W max (75W from PCI Power Supply + 75 W directly of the mainboard)
Get another Power Supply