I'm confused on how PSU rails are rated.

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Coolmanjack

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Jul 10, 2015
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If a PSU is rated for 25A on 1 12v rail and 25A on a second rail can they both together sustain 50A of current?

My PSU ratings says 12v1 35A, and 12V2 35A. Can it sustain 70A of current?

thanks for replies.
 
Solution
yeskay


Hello coolmanjack,

The Amps delivery on dual or multiple +12V rails are determined based on the max output or % of efficacy rated on the +12V rail. Consider this quality Antec PSU:

Antec HCG-750M - http://www.antec.com/product.php?id=706266&fid=5022035&lan=nz

In the "Features" section of this 750W PSU, it is clearly specified, "99.2% +12V - Output for maximum CPU & GPU support". So its like this:

750W x 99.2 / 100 = 744W

744W / 12V = 62Amps

This PSU has dual +12V rails, that is +12V1 40Amps +12V2 40Amps, which can't...
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Emerald

Emerald

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NO, 35A is the max each of the rails can provide; however, the total Amps is equal to the max Watts of the 12V rails divided by 12V.

Example: if your PSU can provide max 600W across the 12V rails then the max Amps the PSU can provide is a combined 50A.

600W/12V=50A

which mean if 12V1 is using 30A, 12V2 can only provide 20A.
 
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yeskay

yeskay

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Jul 12, 2014
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Hello coolmanjack,

The Amps delivery on dual or multiple +12V rails are determined based on the max output or % of efficacy rated on the +12V rail. Consider this quality Antec PSU:

Antec HCG-750M - http://www.antec.com/product.php?id=706266&fid=5022035&lan=nz

In the "Features" section of this 750W PSU, it is clearly specified, "99.2% +12V - Output for maximum CPU & GPU support". So its like this:

750W x 99.2 / 100 = 744W

744W / 12V = 62Amps

This PSU has dual +12V rails, that is +12V1 40Amps +12V2 40Amps, which can't be simple added together as 80Amps.

The 99.2% efficacy is for the two rails combined. Hence the actual Amps delivery is 62Amps, not 80Amps.

Update:

In the above mentioned model, the Amps delivery is 31 in each +12V rail.

Cheers!
 
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Coolmanjack

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Thank you guys for answering. One more question.

@yeskay
In your example you said the total output for the two rails was 62A, So could one rail output 40 and another 22? Or are they individually maxed at about 31?
 
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C12Friedman

C12Friedman

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Jan 25, 2012
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The design of rails dictates a maximum amperage, over current protection (OCP) will kick in when the rail exceeds a little more (in most cases) than the rail is rated for. So with a 31A rail, once it goes above ~35A, it'll cause the PSU to shut down - do consider though that 31A at 12V is providing 372W. It's a highly unlikely scenario outside of your graphics card being shorted that would cause it to overload.
 
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