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4x18amp rails & "40 amps recommended" for GTX 285? Is my PSU good?

Last response: in Graphics & Displays
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January 15, 2009 5:10:16 PM

OK, I have an eVGA 65nm GTX 260 Core 216 that I paid $330 for in mid October. Today is the last day of my step up program, and the 55nm GTX 285 is out, so I'd love to upgrade (for only $55!) to this new card. However I have a question on my PSU.

The 65nm GTX 260 Core 216 card says it recommended a 500w PSU minimum with 36amp rating. Well, my OCZ StealthXstream 600w PSU has 4x18amp rails, for a total of 72amps. HOWEVER - it is split between 4 rails, and if I am correct, the PCI power connectors are on two seperate, dedicated rails - delivering a total of 36amps to the graphics card (18amps each).

Now, the GTX 285 says: Minimum of a 550 Watt power supply (Minimum recommended power supply with +12 Volt current rating of 40 Amp Amps.)

Will my PSU (http://www.newegg.com/Product/Product.aspx?Item=N82E168...) work safely/properly with this new card (http://www.evga.com/products/moreInfo.asp?pn=01G-P3-128...)?

My rig would be the following:

GTX 285 55nm
Intel Q9450 Quad-Core 2.66ghz 45nm
8gb DDR2 800 RAM
2x SATA HDDs
1x SATA Optical
Creative X-Fi Fatal1ty PCI-E
OCZ 600w PSU (4x18amp rails, 2 dedicated rails for PCI-E 6-pin, total 72amps)


I just want to be sure I have clean, safe, stable power that is proper for my rig. Hopefully the GTX 285 will work great, as the new power consumption figures are great - it uses less than the GTX 280, and ditches the 8-pin connector, now just need 2x6 pin connectors.
January 15, 2009 6:53:00 PM

and PS, please help soon... because my 90 day step ends today!
January 15, 2009 7:33:29 PM

Should be fine. The recommended PSU from Nvidia is for a complete system, not just the GPU.

Step-up (you lucky dog).
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January 15, 2009 9:05:29 PM


So the amperage issue shouldn't be a problem? Even though the card is getting amps delivered from two rails (instead of one) that total to 36amps, and it says that the minimum recommended is a 12v+ rail with 40 amps?

(the GTX 260 says 36amps, which is exactly what I have delivered now, I am just worried that if it says 40amps now my 2x18a rails won't cut it)
a c 130 U Graphics card
January 15, 2009 9:21:40 PM

As has been said the numbers you are looking at are for the whole system not just the card. As long as you have 2x 6 pin plugs you will be fine.

Mactronix
January 15, 2009 9:56:17 PM

Awesome, that is great news! And thank your the clarification and confirmation.

As a related question, I am curious - could you explain how the amperage comes into play on a whole system basis? I am a little confused. It makes perfect sense to me on a single rail PSU, but on a PSU that splits the 72amps into 4 rails... how does that work when they rate the whole system @ 40amp amps?

Also, what would be a reasonable estimate as to how many amps need to be supplied to JUST the GTX 285?


Thanks a lot for the help! I placed my order for the step-up and I'm stoked! :) 
a c 130 U Graphics card
January 16, 2009 6:25:23 AM

The PSU splits the rails for stability, some would say a single rail is better, personally i dont really know if it is or not but thats what a lot of manufacturers do these days. It gives you a definate Wattage/Amperage dedicated to the GPU as well so it kinda takes the guess work out a bit as far as what the rest of the system is using from your +12v rail.
Things are a lot better these days as far as trying to get to the bottom of what you actually need from your PSU to power your card. It used to be that all the info you got was the size of the PSU needed in Wattage.
As you know its the Amps that really matter and these days they do at least tell you the max power draw of the card or what is required in terms of power connectors, we used to have to find reviews to get that kind of info.

If you look under the specifications tab on this link http://www.nvidia.com/object/product_geforce_gtx_285_us...
You will see that the card is listed as having a max draw of 183 Watts. To find the Amperage you divide the Watts by the Rail voltage of 12 =15.25 Amps
The card can draw up to 75 Watts from the PCIE slot and each 6pin connector can deliver 75 Watts also so that gives an available power draw of 225 watts or 18.75 Amps.
Also alowing for the fact that the companies overestimate the power requirements to factor in a percentage to cover their backs id say you easily have the power to run the card.

Mactronix
January 17, 2009 4:03:24 AM

Wow - great explanation. Thank you very much.

So for total system power, I saw on this review of the GTX 285 (the highly OCed edition, on an OCed powerhouse system):

http://www.hardwarecanucks.com/forum/hardware-canucks-r...

Those power consumption figures show a MAX of 347 watts total system power draw from a stress test on that hardware - well below the "minimum recommended 550w"

Are PSU power requirements really that overstated? Could a 350 watt PSU actually power that system? (Now, I know it would be far from ideal for efficiency, heat, wear & tear, a safety net, power spiking on boot up, etc.) - but is that really on the wattage it takes?

If so, 550 watt PSU is WAY on the safe side, huh?

January 17, 2009 7:44:06 PM

You'd need a 350 watt PSU that can SUSTAIN 350 watts, many PSU's are rated at maximum power, not sustained. Also, PSU's put off several different voltages, so the number of amps needed and the number of amps supplied on each rail would need to match near-perfectly.

I suppose, in theory, you could have a 350 watt PSU specifically designed to run that system run it, but once you factor in a number of real-life things like capacitor aging, heat, and not custom designed amperage for a PSU, no, a 350watt wouldn't run it.
January 18, 2009 2:05:19 AM

Cool thanks guys.

So with a max draw of around 18.75 amps on the card, according to math done on nvidia's max power draw & the amount of watts possibly supplied... they recommend a "40 amp PSU"...

Does that mean the average system requires just over 20 amps to be powered, not including the Graphics Card?
January 18, 2009 2:12:06 AM

No, it means a build with several hard drives, dvd/blu-ray drives, an add-in sound card, and a quad-core uses 15 amps and they already counted on capacitor aging.
January 26, 2009 4:57:11 AM

Great job answering the thread Guy's! Great job!
!