I am building a system with an HD 5770 graphics card, and I was prepared to order a 600W power supply...until I started reading into choosing the 12V rails. I am confused.
Assuming I am buying a power supply with a single rail...does the total amps at which a Power supply run always correlate to the total wattage? For example, will a 600W power supply always have about 40 amps in its 12v rail and will a 750W power supply always have about 50 amps in its 12V rail?
How many watts and how many amps on the 12V rails should my power supply have if I am getting an HD 5770 card?
Nope, wattage ratings on the sides of the PSUs dont mean a thing.
This diabletek unit is a great example of how little rated wattage means, go to the pictures and find the image of the sticker, you will see its only rated for 530W on the 12V rail, or about 44A, but its also listed as a 750W unit. http://www.newegg.com/Product/Product.aspx?Item=N82E168...
There is a lot of variation in quality between PSU manufacturers and between models in a manufacturers lineup unfortunately. When shopping for PSUs always look for the max load table, on newegg its usually one of the images. If its not skip that PSU and move on.
As a general rule of thumb a good 500-550W unit with ~40A on the 12V rail will run any single GPU card, a good 700-750W unit with ~60A on the 12V rail will run most crossfire or SLI setups. The 5770 is low power so you could safely crossfire it on a quality 650W unit.
450 Watt or greater power supply with one 75W 6-pin PCI Express® power connectors recommended (600 Watt and two 6-pin connectors for ATI CrossFireX™ technology in dual mode)
The amp requirement fir the 5770 is often overstated.
If you aren't overclocking, then the above will work.....if doing serious OC's, the sweet spot will be 650 watts for a single GPU setup and 850 for twin GFX cards....the latter also leaves room for a dedicated PhysX card. The reason I says weet spot is that quite often a PSU of 500 watts will be almost the same price as the 650 watter. For example:
For comparison purposes , the list below contains performance ratings (10 scale) at jonnyguru.com / prices as per newegg on 02/21/2010 and ones w/ double asterisks (**) made it onto silentpcreview.com's Editor's Choice List meaning they excelled at both electronic and acoustic performance: