nVidia say that each power connector should be connected to plugs on completely separate wires, and recommend a 480 Watt power supply. We decided to test this, to see if their recommendations were founded.

A multimeter was used to measure the current the card consumed whilst running benchmarks. This is a very risky thing to do, so don't even think about trying it yourself. The card consumes power on both the 5 Volt and 12 Volt lines. The figures shown include some measurements taken from a Radeon 9800XT for comparison, and all the measurements are peak current draw, not an average.

It is almost impossible to measure how much current the cards draw through the AGP slot, but from the AGP specifications we know the absolute maximum the slot can supply is a total of 46 Watts, at various voltages. This means the overall power consumption of the GeForce 6800 Ultra must be somewhere between 77.5 Watts, and 123.5 Watts.

You will notice that the 6800U draws no more current on the 5 volt line than the 9800 XT. This means that the 12 volt rating of your power supply is the only specification you have to worry about. The 400watt PSU that I use in our Pentium 4 test bed can supply 18 amps on the 12 volt line and it coped fine without running too hot or having any reliability problems. I also tested the card in an Athlon XP machine, which has a 350watt PSU that supplies 13 amps at 12 volts. This also ran fine, perhaps in part due to the fact that the slightly elderly Athlon draws less current on the 12 volt line than a Pentium 4.

When you are shopping for a power supply, the current supply at 12 volts is often unspecified, even when you look at the manufacturers web site. If they quote a wattage for the 3.3v and 5v lines combined, then subtract this number from the total wattage and divide that number by 12. This will give you a fairly generous estimate of the current supply at 12 volts. If in doubt, buy a power supply which supplies 5 amps more than your present one on the 12 volt line, and you can be sure it will work fine with the 6800 ultra.

Now we must ask; are the dual connectors justified, and do you really need to connect them to entirely different cables as suggested? The 5 volt wire is well within spec for the current it carries. On the 12volt line, the peak draw is 5 amps, and the average draw whilst gaming is closer to 4 amps. On a good quality power supply, a single wire is sufficiently thick to reliably supply 5 amps continuously and I had no problems running both connectors off a power splitter, with either power supply I used. Very cheap power supplies often use thinner wires and will not be able to supply this current reliably without using separate wires. In practice this will not be much of a problem, because generally, if you are in the market for a £500 graphics card, chances are you are not going to be running it with a £10 power supply. The power requirements may cause problems with OEM systems, which often use the cheapest power supply possible.