psu for 3 6870 cards

[jonnynogood]

i want to get these graphics cards out of my main machine into a mining rig and put this together will an 850w PSU be ok?

i have these already
3x  6870 xfx GPU

this is what i was going to buy
ASRock 870 EXTREME3
OCZ ZX Series 850W Fully-Modular 80PLUS Gold
2x     Kingston 1GB 240-Pin DDR3 SDRAM DDR3 1066
AMD Sempron 140 Sargas 2.7GHz
Seagate Barracuda 500GB

putting it all in a ghetto aluminum rack, ordered 2 pci extenders so i can space the cards apart and keep them cool

what kind of PSU would i need if i were to add a 4th card in the same range as the 6870's that i have already?

[1714190034]

1714190034

[1714190034]

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[1714190034]

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[hypnosquid]

Not sure, but here is some additional info from what I just tried.

I bought a 6870 and ran it in a machine with a 580w PSU. I had another slot, so I bought another 6870 and crammed it in there. The cards worked, and I could mine on one, but when I fired up the miner on the second card, the entire machine just shut down.

So, I dug around in the closet, found an old 480w psu. I plugged it in outside the cpu, and ran power to the second card from that. Fired up the miners and it worked without a problem. Judging by this, you might consider a slightly more powerful PSU.

[jonnynogood]

thanks what kind of processor did you have on that machine?  any idea on the efficiency of the 580w PSU?

from searching around it looks like these cards might pull 250 watts at full load but i am not sure about that.

the rest of the components i have are about as low wattage as i could find i think.  I am probably going to ditch the hard drive and use a USB thumb drive

[Crs]

http://extreme.outervision.com/psucalculatorlite.jsp

[jonnynogood]

that calculator tells me that i need a 363 watt power supply with a dual card setup

i would be scared to turn that machine on

[saadtariq30]

one 6870 takes around what...~150watts?...that power supply will be enough for 3

[jonnynogood]

i have read everything up to 254 watts at full load.

this says 254watts
http://www.maximumpc.com/article/reviews/xfx_radeon_hd_6870_review

this says 247watts
http://www.bit-tech.net/hardware/graphics/2010/10/22/ati-radeon-hd-6870-review/9

this says 163watts
http://www.techpowerup.com/reviews/HIS/Radeon_HD_6870/27.html

hence the confusion

any harm in plugging them all in and trying it out?

[smoki]

Of those reviews, only the techpowerup one lists the power consumption of just the card, the other two list numbers for the whole system. And even those numbers are measured at the socket, so they are 15-20% higher then what the components are pulling from the psu.

Also, those 163W in the techpowerup review are the worst-case scenario, probably running furmark. Mining should use less power as some parts of the gpu are not used at all. And if you downclock the memory (which you should), you save something like 10-20W per card. I'd be surprised if a 6870 at stock gpu clock and downclocked memory uses more than 120W while mining. Overclocking the gpu, without raising the voltage, can push that to maybe 140W. You can reach ~1GHz on the gpu without overvolting.

So the three of them should use some 400W, most of it from the 12V rail. Add max 100W for the rest of the system, so you need a PSU that can output ~40A on the 12V rail. Any quality >=600W PSU should be able to do that, even some 500W might work.