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I have a Coresair AX750 and an ASRocks 970 Extreme5 mobo.

I want to put in 3 - 7970's.

I have two 7970's on the mobo. The Coresair came with two PCIe cables and has 2 big slots for those cables.

I need to power the 3rd CARD.

The only power slots open on the AX750 are 6 pin slots. Can I / should I create a custom cable to my 3rd 7970?

How normally would you do this?

Get some PCI-E power splitters from newegg. They're cheap and great. They split a 6 pin PCI-E into 2 x 6+2 pin's. I got 16 of them and they work great. http://www.newegg.com/Product/Product.aspx?Item=N82E16812198018

It looks perfect! Thanks. And thanks for throwing in the link!

That 750 watt PSU will be hardpressed to run 3 7970's.

I have one OC'ed to 1075 core & use 370 watts.My system @ idle uses 150 watts (quad core CPU,8 gig RAM,3 HDD's,4 80mm fans).

http://www.newegg.com/Product/Product.aspx?Item=N82E16814129264

So the 7970 is using 220 watts mining (which @ 610 mh/s @68c w/fan@ 65% is fantastic ).220x3=660+150 for system=810 watts.

Good luck  ;)

That is why underclocking and/or undervolting pays off.  One of my 7970's consumes 146w by lowering the voltage to 1.0v and having it slightly overclocked.

Bear in mind that's after inefficiencies. Assuming 90% efficiency, 810 watts at the wall is something like 730 watts which is within the capabilities of the power supply.

I always count inefficiencies toward the total wattage. If your PSU is being run 24/7, you want to keep it below 90% of its max draw. Assuming 90% efficiency, this means keeping your at-the-wall draw lower than the total output.

Just having two 7970's on that PSU is pushing it. If your gonna go for 3 cards i wouldn't have anything less than a 1250W Gold.

Uggghhh! So how do I know? I have a watt meter I can use. Does that mean I should see 750 x .9 on the meter to be safe? Or is it something else?

Rule of thumb - if it's a 750W PSU, try to keep it below 750W using a kill-a-watt or something similar.

Isn't 90% a bit generous? It might get that on a single card. My understanding of 80Plus Gold is it's rated for 87%. I wouldn't trust those numbers if you want your hardware to last. 80% is a much more realistic estimate. I personally wouldn't run more than one 7970 on a 750W. If you do some extreme tweak then maybe you'll be ok.

scifimike12 on post #5 says he got the card to draw only 150W. So 3xcards 450W + system 150W = 600W. If you could make that happen you should still want a 1000W PSU which you shouldn't expect more than 800W which is a nice buffer to have.

Realistically these cards pull 220W-260W depending on how hard you clock them. So a more conservative estimate is 3Xcards 750W + system 150W = 900W. You can figure out the rest for the recommended PSU.

Just for some practice reference - I'm pulling 600 Watts at the plug. Without cgminer running I'm at 150 Watts.

That's 450 / cards or 225 watts per card at the plug - I think I'm going to use an old PSU to run the 3rd card.

An interesting piece of info i came across on the new 7990 card. The minimum recommended PSU is 850W. I'm guessing that card uses less power than two 7970's.