Mac Pro Power Supply Unit

[Jabbamk1]

I know what you mean, half of the pc manufacturers (inc ) dont post the psu on the purchase page. I bought my iMax last year thinking i could upgrade later (check sig) it supports up to 4GB ram. I was going to buy a 9600GT but realized it only has a 350 Watt PSU. Dell is the only company i know of who actually put higher quality PSU's in their pc's.

 

[nanofrog]

I know what you mean, half of the pc manufacturers (inc ) dont post the psu on the purchase page. I bought my iMax last year thinking i could upgrade later (check sig) it supports up to 4GB ram. I was going to buy a 9600GT but realized it only has a 350 Watt PSU. Dell is the only company i know of who actually put higher quality PSU's in their pc's.

Unless you can get confirmation, those values should be taken as peak. Even boxed PSU's have this issue. The number used to classify it likely isn't the nominal value.

To get the nominal value, you just do an RMS calculation. Take the peak rating and divide by the square root of 2 (approx. 1.414). So 350W/1.414 = 247.5 or ~250W nominal.

If buying a boxed PSU, you can do some research, and can quickly tell what value that number really represents (if nominal, it will say it, otherwise, it's the peak value). Independent reviews can also help, as they may offer additional information (sometimes a surprise, but rare). But with a boxed system, it's hard to tell, so presuming the values as peak is a really good idea, unless you can find a specification to the contrary.

So it's possible to be looking at two different vendors using the same PSU, but present different ratings in the web sites. One would list 700W, while the other lists 1kW. It can get confusing, and if not realized, you can be led to believe you've more power available than the system can actually provide under full load.

 

[hyram]

The issue can be even more problematic when you look at the combined rail info. For example, I took this from a common PSU:

AC INPUT 115V/230V - 60/50Hz
DC OUTPUT +3.3V @ 24A, +5V @ 30A, +12V @ 60A, -12V @ -.8A, +5VSB @ 3A
750W Max Combined

If you add up all the power for each rail then you find that it reaches about 950W, which you obviously can't do on a 750W PSU. You've got to look at each device load for each rail and add up the delivered power. Nanofrog is correct about the peak vs RMS power. I always shoot for a factor of 2 overrating on the current draw from all rails, this usually keeps things at a reasonable level.

hyram