Which PSU for xfire 7970?

S

Stigz

n00b
Joined
Nov 23, 2011
Messages
26
Gonna build a 2600 I7 rig
Gonna start with one 7970 adding another one in 2-3 month
Would a quality 850W PSU be enough?

Thanks all
 
Your I7 system would probably draw 120 watts fully loaded , then allow minimum 200watts per 7970. quality 750-850w should be plenty, but i wouldnt go less for peace of mind.
 
An eight-hundred fifty watt P.S.U. of high quality should be enough, as Techpowerup's Furmark results for a single 7970 with its PowerTune at its maximum showed a maximum of two-hundred seventy watts, so doubling that would be five-hundred forty watts, so there is room to overclock the O.P.'s CPU and G.P.U.s, and the following linked article shows a difference of seventy-six watts from a reference 7970 at its stock speeds versus a PowerTune maximized, overclocked to 1.125GHz core and 1.575GHz VRAM at stock voltage XFX 7970 BEDD:
http://www.anandtech.com/show/5314/...ouble-dissipation-the-first-semicustom-7970/6

So, running two of those overclocked 7970s in Furmark, to stress them as much as possible, would consume about six-hundred eighty watts, so there is room to overclock the O.P.'s i7-2600, which I hope is a K version, too.

The following linked article shows a difference of two-hundred thirty watts from a single 7970 versus Crossfire of two 7970s, but I cannot determine what software that was run to load those cards:
http://www.pcper.com/reviews/Graphi...tion-and-CrossFire-Results/Overclocking-Power
 
Last edited:
If the OP were to overclock his 7970s in Crossfire to their CCC's maximums and overclock his 2600K to 4+GHz, would an X-750 be enough for OCCT's PSU test?
I dislike the idea of any situation being unstable.
 
Last edited:
No less then 750w psu and make sure its a quality brand. Seasonic is a good one to start with. Corsair is another.
 
well 1 7970 non overclocked with an i7 overclocked pulls around 430W (mine at least) so id say at least a 700w for 2 cards and overclocking headroom.
 
Above he showed the average draw for one 7970 is 194W-270W at stock. Hardocp's review showed 180W system idle, 490W system peak at stock under full load, 607W system peak with a Maximum OC (+117W) at +20% power tune and 75% fan speed.

Some varrying wattage numbers in there, but the power draw measured from the wall is 160W for an i7 920 OC system idle without a gpu, idle with gpu: +20-30W, Stock full load: +275W average to +330W peak per card, Maximum OC: +385W average to +440W peak per card. As someone mentioned in another thread you're unlikely to peak that high if you have vsync turned on. If you're buying a new power supply I'd get a 850W.
 
Last edited:
Those 270 watts were not from being overclocked; they were from a stock 7970 but with PowerTune at its maximum of +20%, so I am not sure of how much of a difference that the overclocked 7970 with +20% PowerTune versus the stock 7970 with +0% PowerTune was, so I added the 76 watt difference between the overclocked 7970 at +20% PowerTune and the stock 7970 at +0% PowerTune because that linked Anandtech article had stated, "So what’s the tradeoff for this overclock? It’s actually very little. Even with our increased PowerTune limit – which doesn’t increase the power consumption, just the potential for power consumption – power consumption under Metro only rises 26W, and at least some of this will be additional work on the CPU in response to the relaxed GPU bottleneck. OCCT looks worse, but again this has more to do with the PowerTune limit than the overclock."

With that stated, I had figured that the +0% PowerTune 7970 at stock speeds was not being limited too much by its default PowerTune, so I had added the 76 watt difference to what Techpowerup's Furmark test had shown for PowerTune at +20%, which were 270 watts, but later I had found that linked Techpowerup Crossfire of 2 7970s at stock speeds but each with +20% PowerTune, which were 525 watts, so if I add 152 watts (76x2 for 2 of CCC's maximum allowed overclocked 7970s at +20% PowerTune) to 525 watts, I get 677 watts, but that figure is not 100% accurate because of the difference that 2 7970s in Crossfire were versus what I had estimated, and I do not know of how to do that math.

It would be helpful if someone could find the power consumption of 2 7970s overclocked to CCC's maximums (including +20% PowerTune)!
 
Updated my post and I'm really surprised with the peak wattage. The average draw with a stock 7970 also matches up with EquaLiZr's post.
 
-PK-, how did you get 607 watts from a single 7970, please? I had not seen that figure in HardOCP's review anywhere...
 
That was from the wall including the 160 system watts, but it says excluding the speakers and monitor. Although the max OC was something I couldn't see anyone doing without watercooling. It's higher than what PowerTune +20% allows, and requires locking the gpu fan speed at 75%.

I separated it into +Watts per card in the 2nd paragraph since we're talking about xfire. Most people will be looking at their system idle w gpu (+20-30W), so if you're taking your own measurements the increase at full load would show up 20-30W less than what I listed.
 
Last edited:
What about a Seasonic M12II 750 for xfire 7970's OCed with my sig rig minus the gtx285.
 
Seasonic is a quality PSU but after reading all the replies I wouldn't go lower than a quality 850W PSU.
 
It seems like 750W would be fine most of the time but you'd want 850W, because it can peak too high if you stress both cards. If it scales back power to the gpu, it'll cut your fps in half. I'd like to see hardocp do a xfire review first before taking all of the numbers from their single card test.
 
-PK- said:
It seems like 750W would be fine most of the time but you'd want 850W, because it can peak too high if you stress both cards. If it scales back power to the gpu, it'll cut your fps in half. I'd like to see hardocp do a xfire review first before taking all of the numbers from their single card test.
Click to expand...
A PSU won't scale back the amount of power it supplies. It will either go past its rating, shut down due to OCP, or fail.
 
You will be all set with a quality 850 under any circumstance. With I7 (edit 950 )OC to 4011 MHZ and 7970 crossfire OC'd running furmark and prime 95 8 threads I pulled 950 watts from the wall. Getting close for an 850 but unrealistic load.
 
I don't get how speakers would cause power pulling from the wall to change. Don't speakers have their own power source?
 
I'm on a Corsair AX850 with my mildly OC'd 7970s and 4.5ghz 2600k. Even with my 850w PSU, I get worried for some reason, even though I should be fine. Some people's cards are pulling A LOT, like the [h] review. When I built this comp, I didn't have intentions of going CF 7970s so I only went with a 850w. I do have a lot of fans and such running as well.

I'd say if you are spending $1200 on video cards, do not cheap out on the PSU, that's the stupidest thing to do.
 
WiL11o6 said:
I'm on a Corsair AX850 with my mildly OC'd 7970s and 4.5ghz 2600k. Even with my 850w PSU, I get worried for some reason, even though I should be fine. Some people's cards are pulling A LOT, like the [h] review. When I built this comp, I didn't have intentions of going CF 7970s so I only went with a 850w. I do have a lot of fans and such running as well.

I'd say if you are spending $1200 on video cards, do not cheap out on the PSU, that's the stupidest thing to do.
Click to expand...

Buy a $20 Kill-a-Watt and measure how much you are pulling if you want to put your mind at ease :).
 
Using my setup whole system is using less than 400Watts (under load Gaming from the wall). idle 80W. Since my power suplly is ~90% Eff you can calculate the rest, actual power usage. Looks like 750 Watts is enough for Crossfire, you should have no problem running one card on a 600 Watts.

My current setup
2700K @4.6 1.385v
G.SKILL Ripjaws X Series 8GB @ DDR3 2133 1.6V
Sapphire HD 7970 @ 1125 Core/1575 Mem_Stock Volts
ASUS Maximus Gene Z
ASUS Xonar Essence STX
2 X OCZ 120GB Vertex 3 SSD
Deskstar 5K3000
Corsair 750AX
SilverStone TJ08-E

using 550 Watts if you add
Dell 3007FPW
M-Audio Studiophile BX8a
Power measurement taken using Blue planet energy meter
Hope that helps
 
You must log in or register to reply here.
Back
Top