graphic card amperage requirement.

so i've been reviewing my crappy physics, and here's something that i've noticed:

1 watt = 1 V * 1 A

since the PCI-E slot provides 75W, and each card power connector provides 75W as well, a modern graphics card with two power connectors should draw a maximum of 3*75W = 225W

now, going with the above equation, the amperage requirement on the 12V rail should be:

225W / 12V = 18.75A

similarly, a card with only one power connector will draw a max of 2*75 = 150W, and:

150W / 12V = 12.5A

anyways, are my calculations+assumptions correct?

I feel a lot happier thinking of it as three separate 6.25A rather than one lump of 18.75A. Can't get a fuse bigger than 13A for the bugger anyways

Moriz, you are forgetting full power mode, and 12v rails being split into other sections of the PC.


Also, the laws of inherent particle loss to chaos (entropy) Amperage is not a set value, because of chaotic source. Electricity is not a fun thing to calculate for.

head = explode

too much physics is too much.

alrighty, my E7200 draws 20.8W maximum, so that amounts to 1.733333333A maximum on my CPU, assuming that it draws from the 12V rail as well. i'm pretty sure the other components (ram, harddrive, DVD drive) all draws from the 5V or 3.3V rails...

according to this:
http://www.atomicmpc.com.au/forums.asp?s=2&c=7&t=9354

the HD4870 draws 157W maximum, or 13.08A. if this is all true, it looks like my power supply can take it after all...

Yes, it can. We discussed this though, right? Besides, you have combination rails, being split for dual card configs, or tri/quad.

34+ amps on the full rail is preferred for maximum acceleration and gate control in optimized mode.

i dug up some more info on my particular brand of PSU.

it's one 34A 12V rail split into two, each rail capable of supporting up to 22A. HOWEVER, both rails together can go up to only 408W. this means, that if both are maxed out, they can only run at 17A maximum.

i'm assuming a HD4870 will be drawing from both rails simultaneously, in which case there shouldn't be a problem. but then again, i'm not sure if it does or not. this power supply thing is a lot more complicated that i thought, and i can't seem to find any good info.

sorry about the endless barrage of questions. i really want to make absolutely sure before spending $300+ on my already constrained budget. the last thing i want is a $300+ card that's choked by my PSU, or an unstable machine.

PCI-e connectors for graphics cards draw power from their own +12v rail.
But yeah, that's the issue with multiple +12v rails, they never "addup".