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Hi
which PSU should i buy to run 3 x 7970 , how many watt ?

I'm running four with a Corsair 1200AX, but you could get away with less. I'd recommend the Corsair AX860i; it's 860 watts and super efficient. Mine is powering two 7950s and never draws more than 415 watts... should be plenty of headroom for three 7970s, especially if you undervolt them a bit.

850 watts minimum, if you do go with an 850 watt it better be a quality one with some nice beefy rails.

At least 1500W because of this http://www.amd.com/us/products/desktop/graphics/7000/7970/Pages/radeon-7970.aspx#2

Whoa there buddy 1500w is excessive even a 1000w power supply is more than enough.

There's absolutely no need for a 1500 watt PSU to run three 7970s. Around 850 will work, but power supplies are a great example of getting what (watt?) you pay for. Just because a PSU draws 850 watts from the wall doesn't mean it converts all of that into usable DC electricity. A cheap one may only only work at 70% efficiency, which leaves you with ~595 watts; a better (read: more expensive) 850 watt PSU should work around 90% efficiency, and give you a clean 760 watts, which is plenty to power three 7970s.

thanks all  ;D
i will go for a good 850 watts one  :P

Any news on 850W PSU with 3 gpu-s? Is it working? Stable...?

Hi, about these 415 watts how did you achieve that? It seems very good, considering each 7950 as 200W. Is this from wall - do you use some kill-a-watt? Or is this number from Corsair software provided with AX860i?

800-900 watts is perfect.

PSUs are most efficient at the 80-90% range. So you'll want a 800-850W PSU.

Yes I agree

Nice!

GL

any one of them will do the trick

yes anyone of those will do the trick

800W of 12v min for sha, and ~900+ of 12v for scrypt

I own a 800W, works like a charm

AMD publishes PSU requirements like this because if they said PSU must be able to deliver ~20A on +12V rail nobody would understand what they mean ; )
Also because PSU manufacturers often lie about their PSU specs.

I'm running 3x7950 + 2x6950 mining LTC with Enermax 1250W works fine. One of 7950 was even using ~320W until I've noticed it and lowered voltage.

PSU = Power Supply Unit ; )

Power supply unit

why does scrypt requires more power than sha-256?  because we need more ram for scrypt?  thank you.   ???

I guess the figures are accurate.
And yes, more ram should be the reason for different power requirements.

850-1000W :D

Around 1000w should be OK. Preferably Gold certified.

850 is enough and i would use corsair just  preference   ::)

You could also just get a lower wattage Gold (which are usually cheaper) and buy a PSU adapter so both power supplies turn on at the same time. Else for 3x 7970s I'd suggest at least a 1400W Gold to give you some headroom since most PSUs are not rated for their 100% capacity for more than a few seconds.

1200W should be safe.

Corsair has great products