PSU amperages- confused!

Gundog48 · 20 Apr 2011 at 21:22

I am making a dummy load for testing PSUs and have been somewhat confused by the amount of amperes I am dealing with. For example, I have a PSU with 4x12V rails running at 25A each. So to work out the wattage for one rail I do 12*25= 300W. There are 4 rails so I do 300*4= 1200W? It is a 750W PSU but by the calculations I know it says that the 12V rails alone make 1200W!

I want this dummy load to handle up to 1200W PSUs which means that the load needs to draw up to 100A be it on a single rail, or spread across multiple.

Reason I am checking is that I am going to have to get some serious wiring to deal with 100A of power and I need to find out if I am making a silly mistake!

Reaper 392 · 20 Apr 2011 at 23:33

from what i can work out, the limiting factor in that PSU is one of the parts before all the power is split up onto its rails.

while the 12V rails could technically handle 1200W, there is a part further back in the PSU, before the power is split up into 12V, 5V and whatever, that can only handle 750W, which is limiting the entire PSU

so for your testing, you could draw all 750W from the 12V lines, but any more could be very dangerous

Aretenon · 22 Apr 2011 at 17:19

Sounds like the Antec Truepower New TPN-750 to me. On the label somewhere it will say the maximum combined 12V load.

Gundog48 · 22 Apr 2011 at 17:29

You're right! I am not trying to work out the wattage of the 12V rails on that PSU though, I am trying to work out why my maths don't add up!

Marvin · 22 Apr 2011 at 18:01

The specs simply mean that each rail (of which there are 4) can handle up to 25A, but no more, and combined, due to other components, the four rails can provide 62.5A between them. So that could be 25A on rail 1 and rail 2, and then 6.5A on rail 3 and 6A on rail 4. Or, it could be 15.6A per rail.

Basically, it's stating a maximum that each rail can individually handle, in case someone has the bright idea of putting most of a system on one rail.

IE: The PSU rating is less than the sum of its parts.

Gundog48 · 22 Apr 2011 at 19:06

So although each 12V rail says it can handle 25A, they wouldn't be able to do so if each rail had a full 25A draw on them? So looks like I'm going to have to build something pretty beefy to handle 1200W then as it would need to handle 100A without the wires burning. Also means I may have to use better switches and terminal blocks