I currently have 1 x 6970 & 1 x 7970 in my mining rig. I'm mining terracoin & litecoin, not bitcoin.
I want to add a 2nd 7970 gpu for helping me mine litecoin. I was looking at the power requirements for adding this 3rd gpu, since my psu is an Antec Quattro 1000w, and all existing 12v rails have been used. I was thinking of splitting the 12v rails for each rail to feed both pcie connections (although I've read that this is supposed to be a bad idea).
OK, so I found out what my existing 12v rails are rated at 18amp each. I began to realise that whatever I did to power my gpus, it all depended on 1) what power usage my gpus require 2) how many amps the 12v rails can handle.
The amperage specs on my psu are here:
http://www.overclock3d.net/reviews/power_supply/antec_truepower_quattro_1000w_atx_psu/1However it occurred to me that I needed to know how many watts my gpus were using. Several internet listings of various gpu watt usage differed, so I decided to put a watt meter on my PCs power cable, and check the watt usage etc.
What I found really interested me. My 6970 gpu has Arctic's Extreme III cooler on it. My 7970 still has its stock cooler (single fan on back of case (which apparently means its a reference design?)). I mine terracoin with my 6970, Litecoin with the 7970. I first noted the watts used without any gpu mining (peaking about 120 watts). Then I mined using each of the gpus in turn. 6970 really surprised me. When mining using cgminer, the watts only increased by 20 watts!!?? When I mined using the 7970 only, watts increased by approx 260 watts (less than I expected). I decided to test the 6970 further, and loaded the PC after uninstalling the 6970 (riser disconnected, pcie power cables disconnected). The PC usage watts (without any mining) was about 100 watts, meaning that the 6970 was potentially only using only 40 watts!! (still surprising).
This exercise taught me that the fans on these gpus really eat a lot of juice! The stock fans seem to eat far more juice than the aftermarket fans (certainly the one I'm using!). I need to change the fan on the other gpu (and on any new gpu). This also taught me that IF the gpus are using so little juice, I CAN probably split the 12v rails and easily stay within the amp rating of the rails (which is cool).
If upgrading the coolers reduce the watt usage so much, I guess one would be able to have much better usage/mileage out of the PSUs!
