Debunking Power Supply Myths
by Christoph Katzer on September 22, 2008 3:00 AM EST- Posted in
- Cases/Cooling/PSUs
Connectors and the 12V Rail Issue, Cont'd
As a second example, let's take our high-end system. This requires more power, making it a little more difficult to find an appropriate power supply -- especially if you want to run 3-way SLI. NVIDIA initially only certified 1200W power supplies for that sort of setup, but Corsair was eventually able to receive this certification for a 1000W unit. To run a 3-way SLI setup, we need at least six 6-pin PEG connectors, so let's start by looking at the number of connectors available on our high-end PSUs.
| PSU PEG Connectors | |||
| Manufacturer and Name | # of 6-pin PEG Connectors | # of 6/8-pin PEG Connectors | Total |
| Antec NeoPower Blue | 1 | 1 | 2 |
| Antec Signature | 2 | 2 | 4 |
| Silverstone Decathlon | 4 | 0 | 4 |
| Zalman ZM850HP | 2 | 2 | 4 |
| Enermax Pro82+ | 0 | 4 | 4 |
As you can see, none of the power supplies we listed are able to run more than two GPUs, since none of them have the available six PEG connectors. We still don't recommend 3-way SLI, has the advantages over two-way SLI are sketchy at best. Regular SLI also has the advantage of only requiring four 6-pin PEG connectors even for the highest-end GPUs. All of the listed power supplies can meet this requirement, except for the Antec NeoPower Blue.
For the second 12V rail issue, here's a look at the specific power requirements of our graphics card, the GeForce 8800 Ultra:
| GPU Power Requirements by Connector | |||||
| Vendor and Chip | Through 6-pin Jack |
Through PCI-E Slot |
Total Power | ||
| NVIDIA GeForce 8800 Ultra | 9.4A | 112.8W | 6.1A | 72.2W | 186W |
If you plan to run multiple cards, you can just multiply 186W by the number of cards in the system. We found during testing that the cards don't consume equal amounts of power. The first GPU always uses the same amount of power, whether in a single or SLI configuration. The second and third cards however require much less power, which also means that they are doing less work. In testing a variety of benchmarks and games, we saw that the second and third graphics cards only use 40% to 50% of the power of a single GPU.
One thing is obvious when looking at the power consumption of the single GeForce 8800 Ultra: the maximum power draw is only 15.5A, which you can get from a single 12V rail. Most higher-end power supplies have multiple 12V rails, which should make distributing power to your GPUs even easier. As long as the 24-pin ATX connector and the different 6-pin PEG connectors run on different 12V rails, none of the rails should end up with an excessive load. That takes care of the 300W power requirements mentioned by GPU vendors.
If you take the high-end system (with an optional third graphics card), you are looking at one of the most demanding systems available. Of course, you could always overclock the processor and graphics cards, which might increase power requirements by another 30%, but we'll leave that topic for another day. We were able to run this test system with a normal 850W power supply, and even with a reasonable load the PSU fan didn't make that much noise. Most of the high-end PSUs we've chosen for this article could also run a triple-SLI setup, provided they have sufficient connectors.
At the beginning of the year, we requested a special AnandTech Edition of PC Power & Cooling's Turbo Cool 860W power supply with six PEG connectors, specifically for running a triple-SLI setup. We have been using the power supply since then with three GeForce 8800 Ultra cards, an Intel Core 2 Extreme QX6850 CPU, and 12 Western Digital hard drives without any problems. Clearly, quality of construction and number of connectors are far more important than a silly wattage rating.
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Christoph Katzer - Monday, September 22, 2008 - link
Yes I guess that was the problem. Thanks for pointing that out!ViRGE - Monday, September 22, 2008 - link
Christoph, thanks for the excellent article. There is one thing I noticed however that has me a bit confused. In a few of your video card tests, you have the cards pulling upwards of 100 watts from the PCIe slot. It has been my understanding, and is mentioned in AT's X38 chipset article*, that the limit for a PCIe slot is 75W. Your results put this value in contention, so I'm not sure what to believe at this point.Is it that I am mistaken that the limit is 75W, or are your video cards pulling too much power from the slot, or is there a problem with the data?
Christoph Katzer - Monday, September 22, 2008 - link
Thanks. The data is correct since the cards draw power through the PCIE slot and the additional PEG connectors (6-pin or 8-pin). Towards the end of the article you will find a list which card drew how much power from the PCIE slot and the PEG connector. 75 watts is however correct for the older PCIE slots, PCIE2.0 can draw up to 150 watts.mobutu - Monday, September 22, 2008 - link
Where is AMD 4850?Also almost all the graph cards used are kind of old, previous generation.
Dobs - Monday, September 22, 2008 - link
Here is a link to a comprehensive list of all graphics cards.Dobs - Monday, September 22, 2008 - link
damn... link didn't workCopy and paste :)
http://archive.atomicmpc.com.au/forums.asp?s=2&...">http://archive.atomicmpc.com.au/forums.asp?s=2&...
Clauzii - Monday, September 22, 2008 - link
Nice lists, thanks :)Christoph Katzer - Monday, September 22, 2008 - link
As stated I didn't have all of the new stuff here and this article anyway focuses primarily on power supplies and how to choose them and not the power consumption of the components... but it might come soon, who knows.LTG - Monday, September 22, 2008 - link
>>our labs don't have the latest GPUs available for testingIsn't this kind of a bad thing for a site with so many enthusiast readers?
First two caveats:
1) There are a lot of article bashers out there who nit-pick, give rude feedback, or are just plain wrong, I don't support these guys or want to be one.
2) The article written was very good and helpful, thank you.
However it doesn't matter that the article "primarily focused on how to choose power supplies".
- Leaving out a GTX 280 was a big omission. Even better to have an stock OC'd product.
- Leaving out very basic overclock numbers was a big omission. For example it's very common for readers to have an Intel QC clocked at 3.6Ghz since it's so doable.
Having the last two data points would be very helpful for those building top systems, and would even be interesting for those who are not.
7Enigma - Monday, September 22, 2008 - link
As mentioned, the article is about the myth that we all need 1k PSU's for the "best experience". Bottom line this article shows that the vast majority of people do not need more than a 500-600w psu. How is leaving out any stock product an omission? Just look at the TDP of the product and go from there. All stock products will fall within a narrow range.Once you get to OC'ing I also think this is a difficult thing to do. Individual chips, even from the same batch can have very different properties and require different voltages to reach the same speeds. I'm sure if the author had put that his Intel quad drew 120w at 3.6GHz, someone would have complained that their's took 135w, and someone else only 110w...Once you go above stock, it is not cut and dry and as the article states you should have a 20-30% extra buffer if you wish to OC.
I feel the scope of the article just doesn't require these extra requests. Sure it would be nice to see just how crazy of a system you can make (if I remember correctly they did that a while ago to actually use the >1000w supplies), but this article was to show what a normal base, mid-grade, and high-end setup would require.
I've complained in the past about articles, but this one I don't see any serious faults.