How to calculate amps on 12v?

Mez

Mez

Active Member
Hey guys I was wondering, how do you calculate how many amps you have on a power supply with more than 1 12v rail. Do you just combine the two or more together to get the overall amps?
 
On the PSU label, does it tell you how many watts are dedicated to the rails? I believe you take that watt amount and divide it by 12. No, you don't simply add them together.

Looks like 504 watts, so if you divide that by 12, you get 42.
 
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It depends if it is an expensive true double rail or the cheaper ATX standard double rail fed from a single rail and current limited to 240 watts each (20 amps)

If it is the second one then one rail (12v1) is dedicated to the CPU and even if the CPU only draws 8 amps then the remaining 12 amps on that rail is unusable. That leaves only 20 amps for everything else on the 12v2 rail.

The combined wattage is the key figure here. Here's a plate showing some info on a 4 rail PSU, notice how the watts don't add up.



See where it says 18 amps each for 4 rails, that's 12v x 18a=216 watts, multiply that 4 times and you get 864 watts yet the maximum is 580 watts combined, then allow for inefficiencies and it's even less, no wonder people get confused.

Plus, if this example is not a true independent multi-rail then if one rail pulls more than 240 watts the PSU will shut down.

So no, as said, you can't add them together and if I'm wrong on anything I'm sure someone will point it out :D
 
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Here's the specs taken from a review at overclockers club...
 

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mep916 said:
On the PSU label, does it tell you how many watts are dedicated to the rails? I believe you take that watt amount and divide it by 12. No, you don't simply add them together.

Looks like 504 watts, so if you divide that by 12, you get 42.
Click to expand...

Yeah 504 watts dedicated on the rails.
 
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