I think you may be talking about the PC43I3503 which is listed in various places as a 350 or 400 watt power supply. I have found reference to a redesign of this PSU so that my explain the discrepancy.
The 400W version has the following specs
[code:1:24d08787e5]
+12Vsb A | 17A
+12Vsb B | 17A
+12V | 10.5A
+5V | 3.5A
+3.3V | 3.5A[/code:1:24d08787e5]
maximum wattage for the combined 3.3V line and 5V line is 18W
total output wattage is 400 W
peak output wattage is 450 W
see pic of PSU
http://www.barebonecenter.org/images/stories/User2/Shuttle/sd37p2/800/SD37P2%20042.jpg
For the 350 W version
[code:1:24d08787e5]
+12Vsb A | 17A
+12Vsb B | 17A
+12V | 3.5A
+5V | 3.5A
+3.3V | 3.5A[/code:1:24d08787e5]
maximum wattage for the combined 3.3V line and 5V line is 18W
total output wattage 350W