it the 12v plus rail of the power supply. say it 30a on the label. so 30 amps times 12v is 360w. so that the max wattage for that 12v line. what you have to do is look at the wattage of the 12v line then look at your cpu. you have to remove the cpu voltage first from the 12v line and what left up to about 80 percent of that number would be left for video card. why i say 80 percent is you have to leave some wattage so you dont burn out the power supply and two you have to add in the hard drives and any extra card.