This week I had a request to build a rig that would support 3 x HD7950 cards. Before the client ordered the required components I wanted to run a test to ensure that the rig would run ok with 3 cards on the Gigabyte Motherboard I would like to use for this system.
I based this rig on the previous design I have used for 2 card rigs. The only additional component I am using is a 16x to 16x pcie extension cable that allows me to squeeze a third card on top of the case. The rig has 2 x Gigabyte HD7950 cards and you can see a third Sapphire HD7950 card mounted on top of the case using a pci-e extension cable.
Here are some photos of the end product. A rig running with 3 x HD7950 cards that will hash up to a maximum of 1800 kh/s
And here is a video of the machine
So a tested and working Spec for a 3 x HD7950 Card Rig is as follows:
1 x COOLMASTER XB CASE ( Case is a personal choice. You could also build in a Crate if you wanted to )
3 x 3G GigaByte HD7950 GV-R795WF3-3GD ( or any other HD7950 card)
1 x GigaByte GA-990FXA-UD5 MOTHERBOARD
1 x Sandisk 64GB SSD – Solid State Drive – SDSSDP-064G-G25
1 x 8Gb Corsair DDR3 Vengeance, PC3-12800 (1600),Non-ECC Unbuffered
1 x AMD Sempron 190 Dual Core Processor
1 x CORSAIR 850W PSU Power Supply
1 x 16x to 16x Powered PCIe Extension Cable for 3rd GPU
Currently I have the system hashing at around 1650 kh/s but I know I can improve this if I increase the intensity and powertune settings in cgminer. You could also buy 2 extra pcie extension cables if you wanted to so you could raise all three cards out of the case. The cards run cooler outside the case which allows you to crank up the power a bit more.