Dual rail power supplies explained.

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number69

[H]ard|Gawd
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Hi, I have a question about dual rail power supplies. Basically if you have a PSU with two 12v rails that supply 18 amps on each rail that technically means that the PSU supplies a combined total of 36 amps...right? Basically if you have a video card that requires say 24 amps on the 12v line will a PSU that has two 12v rails with 18a on each be in the clear? From what I understand it will. It'll just pull a combined total of 24a off both rails. It just can't pull more than 18a off one rail correct?

From what I read most power supplies that advertise as having two 12v rails don't have true separate rails.



Thanks.
 
Even when a PSU is stated to have 2 rails at 18A each, often its just marketing and safety related.
Read up on the PSU you have to find if it really does have 2 rails or just the one.

Yes the PSU will supply a total of 36A regardless of how the rails are split.
Each 'rail' may be able to supply only 18A or the full 36A depending on how the PSU is made.

No video card will need 24A on 12V, thats a total of 288W.
That figure is most likely a complete systems power usage, not just the graphics card.


Worst case scenario for your 36A total available power on 12V:
If it is split evenly over 2 rails, one rail will be for the CPU only.
The rest of the system including your graphics card will be running off the other rail.

Lets say you have a 65W CPU, approximately 5.5A so your total remaining power needed is 18.5A.
As you can see this is over the rails limit and will be pushing the PSU too hard.
Also you shouldnt really push an independent rail (or a PSU for that matter) beyond 75% of its max for sustained periods of time as the PSU wear rate will increase.
(I prefer to keep max power use nearer 50% of the PSU's max but thats just me :))
That leaves you with 13.5A total on the rail for your 18.5A requirement!

This is hypothetical anyway as I dont know anything about your machine or your PSU
Your PSU may have a single 12V rail in which case it should be fine as long as the PSU is in good condition.
ie the PSU can supply 36A, you need 24A so the PSU will max at 66% usage approx.
Does your PC actually draw 24A peak?

To be clear, some PSU's have genuinely split rails, others dont.
 
Should be ok. That PSU has 29A on the +12V rail, meaning 348W available on the +12V rail. You system won't draw more than ~200W from the wall even under heavy load. Don't worry about your power supply not being able to handle your PC.

Basically if you have a PSU with two 12v rails that supply 18 amps on each rail that technically means that the PSU supplies a combined total of 36 amps...right?
Click to expand...

No. You determine the combined total amperage by dividing the total wattage available on the +12V rail by 12. Most of the times the PSU label will tell you how much wattage is available on the +12 rail. So in your case, the AX450-PN has 348W on the +12V rail. Divide that by 12 and you get 29A on the +12V rail.

Read this FAQ on PSUs:
http://www.jonnyguru.com/forums/showthread.php?t=1036
 
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