Rookie question

Ok I have a novice question. When a graphics board says:

Minimum 550 Watt for SLI mode system.
Minimum recommended power supply with +12 Volt current rating of 34 Amps

Does that mean 34 amps per a +12 rail? That seems pretty high. I'm asking because I run 2 7900GT's on a 450w PSU and believe it's causing poor graphics results in my games.

34 amps total on the combined 12v rail(s). And that is minimum for the two 7900Gts not including the rest of your system. Basically all the major hardware in your system uses the amperage from the 12v rail.

Well, your computers main hardware hard drives, RAM, video cards, etc. all draw from the amperage of your 12v rail(s). My statement is correct. I am speaking in general terms. The 34 amps needed is 'minimum', not a 'guideline'. Of course the cards will fluctuate with power usage under load.

OK: carver_g estimates 16 amps for video cards. I think that's a little high, but I am not certain enough to argue. Besides, you cannot get in trouble overestimating the power needed.

10 amps for the CPU, 15 amps for a really hot, late generation P4
Figure about 3 amps @ 12 volts and 3 amps @ 5 volts for the motherboard, 1.5 amps @ 3.3 volts for each GB of RAM (memory draws from the 3.3.volt rail).

Hard drives need about a amp each at 5 volts and 12 volts. DVD burners need about 2 amps @ 12 volts and 2.5 amps at 5 volts. Figure 1 amp @12 volts for every three or four fans.

For a PC with 2 GB of RAM and single hard and optical drives, that's about 33 amps (372 watts) at 12 volts and 42 watts for everything else. That's about 420 watts. So a good 450 w PSU with 34 - 35 amps on the 12 volt rails would work. I like to add 20% for Mr. Murphy. That takes you to 500 watts.

I would power a system like this with a 600 - 650 watt PSU. I don't like running a PSU at more than a 60 - 70% load - but that's my military background speaking.

So there you go. Several paths to the 34 amp estimate.

Technically, it's 'minimum recommended'.

The 34A is the minimum requirement stated by the manufacturer of the 7900GT, and it is not a 'guideline' it is a minimum requirement to run the two graphics cards like I said.

Their actual draw will fluctuate with their load. Their 'actual draw' can vary, but if OP has 34A on his 12v rail(s) the two video cards will operate correctly.

Few 450w PSU's supply 34A on the 12v. I believe, generally speaking, as jsc suggests, at least 500-550w PSU for 34A and 600-650w is recommended.

Thanks, carver_g. But the only extreme PSU calculator I used was the one in my head. It may not be easy, but it is fairly simple:

CPU power consumption - take the TDP and add 10% to allow for losses in the motherboard PWM power supply. Or for any C2D or C2Q CPU, figure 10 amps. Remember, I want conservative over estimates.

GPU's - documented on line mostly.

Drives - calculated from data on labels, or figure 20 watts for a HD and 30 watts for an optical drive.

Motherboard estimates were based on actual measurements on my EP35-DS3P. The power leads on my 650 watt Antec are not sleeved. I cut cable ties so I could separate the various power lines. Then I borrowed a lab calibrated clampon ammeter from work. I was pretty surprised at how little power passes through the mainpower connector. Then I rounded each measurement up to the next amp. For the RAM, for example, I clamped the meter around all of the orange wires (3.3 volts) on the main connector. I measured a little under 3 amps. That's about 10 watts for 2 GB.