Personal PSU Controversy - What the f---?

[Sovereign]

So I have a PC Power and Cooling 850SSI, which was basically the biggest, most badass power supply ever when I bought it for my gaming system last May. However, I am having a large number of issues that some are tracing to the power supply. Not that the power supply is faulty, but rather that there's simply not enough power to go around. By sheer wattage it seems OK, but I've been told that watts aren't everything.

The PSU has four 12v rails rated at 17A each. If my understanding of power supplies is correct, if one has multiple rails, and power is not utilized in one rail, it becomes trapped in that rail and the rest of the system can't touch it. Also, I have been told that dual 7900GTX SLI requires 2x22A rails (separate or on one big rail). This power supply has four 12v rails rated at 17A each and two PCI-E six pin power connectors. Each of these feeds a 7900GTX graphics board. Three SATA cables feed three hard drives, and two molex connectors drive two optical drives and some fans (there are eight molex power connectors total on two strings of four each). All mobo connectors are correctly attached, including the extra four-pin that plugs in for SLI systems!

The system configuration is as follows:
CPU: Opteron 170 at stock speed, 2.0GHz
RAM: 4GB DDR500 @ 400
MOBO: Asus A8N32-SLI Deluxe
HDD1: 150GB Raptor
HDD2 & 3: 250GB Caviar
GPU: Dual 7900GTX SLI
PSU: TurboCool 850SSI
OS: Windows Vista Ultimate x64

I have been told by a system integrator friend that my power supply is not enough. His logic is that each card by nVidia spec needs 22A, and I only have dual 17A, which starves my cards. I have further found this Thermaltake 250 watt dedicated graphics power supply which supplies 21A (max of 25A). nVidia certifies it for dual 7900GTX SLI as long as I have another power supply off their "All other SLI Configurations" list. The 850 certainly outstrips those units.

The solution based on these assumptions is to purchase the following:
- A Thermaltake 250w dedicated graphics unit
- A 2x PCI-E to 1x PCI-E joiner OR determine whether the four pin molexs are on different rails and if so, use a molex-to-PCI-E to combine the two

All I have to say to all this is WTF mate?

Note that none of these problems ever occurred under Windows XP with only 2GB RAM installed. I had a K8N Diamond Plus die on me, the PCI slot failed and I don't know if this was a manufacturing defect or a result of not enough power. Please enlighten me.

[Zertz]

I suggest you don't get that TT PSU. If you really want to replace you PSU, the Corsair HX620 is great one. Only 620W you say? Take a look at this!

But since your PSU looks like a good one, first I'd do what you suggested to yourself; find out what is on what rail and split the power.

[camaroguy1998]

From personal experience I wouldnt recommend any Tt PSU!
I had a 450w Tt that the 5v rail had a meltdown and wasted my Mobo(melted the plastic ATX connector)

I had issues before this happened and all issues where gone once I replaced the mobo with an identical one and a new Antec PSU!

Go with the Corsair that Zerts recommended or one with higher watts(same series)

[Apoptosis]

Last I heard a GeForce 7900GTX had the following power requirements: and single card being 450W w/ 12V @ 22A and dual cards needing 550W w/ 12V @ 30A

As you can see you can't just double the rating like one would think... Since your PSU was designed in ~2004 it had Four +12V rails @ 17A each, which is something PC Power & Cooling got away from and could be leading to some issues for SLI.... You should have got the Free Ultra 500W PSU for a backup ;) It has 28A on a single +12V rail and has done 7900GTX SLI before... hehe

Anyway there should be a solution for you without getting a new power supply. If you could get the four pin molex to six pin PCIe adapter you should be able to join two rails per card instead of just the one.

[Sovereign]

I am thinking the problem still traces to the OS/drivers now that I have the power requirements cleared up. Hopefully if Vista SP1 ever shows up, the net rumors say that the pack addresses "compatibility problems" with high end cards and "improves performance of certain high end graphics units in certain situations" or something like that.

[dicecca112]

This clear shows my issue with PSUs. People don't do enough research. Retailers are out touting Ws when they should be touting Amps. Always look to see what components uses the most amps (usually graphics) and tailor the PSU to that.

That being said, It very well could be Vista screwing up. I don't think its a psu issue, but I would go with Nate's suggestion. Its a cheap fix and if it doesn't work it didn't break your wallet

[Sovereign]

Now I'm getting this message during install of display drivers: Incompatible display adapter has been disabled. One or more display adapters are not compatible with these drivers, disabling blah blah blah except the problem is I have two 7900GTX cards so there can't be issues with one being compatible and one not, I tested both cards and they both work solo.