Determine Amps Of Server

I have always found this odd and do not understand enough to figure it out.

Any knowledgeable people would be cool.

My datacenter has given me 2amp circuit for my computer. It seems to run fine on it.

Looking at the back of my power supply it says the following

520Watt max

Input rating
100-240v~,50-60Hz
7A-3A

Output rating
+3.3V & +5V combined 150W max
12V combined current 39A max
(and a lot of other stuff on amps and volts for output).

Now, Watts = Volts * Amps.
So if I have 520watts and 12 volts would be 43 amps
If it means 520 watts and 3.3volts it would be 157 amps
However it seems to max at 39amps on the back for output.

I imagine input is all that matters here and at 520 watts and 100 to 240volts that would be
5.2 amps to 2.16 amps

And the thing says 7amps to 3 amps on the back for input.

So....I imagine that is the max it can pull before dying.

However, I have 4 drives, 4 ram sticks, and 2 quad core cpus all going and I would figure I am at least halfway or more of the power use.

And the power says it starts at 3amps anyway.

But I only get 2amps.