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Crossfire and Amperage??

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June 7, 2010 4:07:50 PM

I have this power supply from my last build and after doing some research Ive discovered that ontop of wattage some videocards have Amperage requirments.

Cooler Master Real Power Pro RS-750-ACAA-A1 750W ATX12V



This is the powersupply, it says it has 19Amp per 12v. Im new to this and figured I should double check with experts :D 

Does this mean if my videocard uses 2 6-8 pins, that it would only be getting 38A from the two rails? Or am I doing the math wrong lol.

Im hoping to reuse this PSU with an I5 build with a 5850 or 5870, and possibly crossfiring them in the future.

from here at toms, sticky Power Requirments and Specs for Popular Videocards Guide

5850 requires - Recommended is a 500watt PSU with two 75 watt 6 pin PCI express connectors. 600 watt PSU with four 6 pin connectors to run crossfire is recommended but I suggest a 650 watt for some head room.
Recommended 42 Amps on the 12 volt rail for single card and 50 Amps for Crossfire.

5870 requires - Recommended is a 500 watt PSU with two 6 pin 75watt PCI Express connectors. If you plan to crossfire you will need a 750 watt PSU.
Recommended 42 Amps on the 12 volt rail for single card and 63 Amps for Crossfire.

I fear my powersupply just doesnt pack the amps needed for these new gen cards. I hope someone can confirm if this is correct.

It'll mean buying a new PSU if this one is insufficient. Recommendations would be great :D  Heard good things about the HX series.

More about : crossfire amperage

a c 139 ) Power supply
June 7, 2010 7:54:12 PM

Your CM PSU has (max power for all +12V rails of 648watts divided by 12volts) 54Ams of +12v. You're good to go for 2x 5850s.
And I suspect you'd even be OK for 2x 5870s - depending on what else you have sucking power off the PSU.

Video card take up to 75watts directly from the PCI-e x16 slot. Thats why lower power video cards don't have any PCI-e power connectors.
6 pin power connectors can each supply up to another 75watts of power. And 8pin connectors can supply up to 225watts of power

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a b ) Power supply
June 7, 2010 7:55:46 PM
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You are fine. Remember, Volts multiplied by amps equals power. So they are directly related since volts is a constant in this case at +12V. I have always found that the split rails of +12V are weird and do not add up in the manner you would think. It's not even constant across all brands. But let's take a look at your power supply.

648W total max draw on all +12V rails. So, basically, you have 648W available for the CPU and Video card. You have more than enough. Worried about amps? Well, the max you can draw from the +12V rails is 648W. ok, Amps are directly related. P in watts is equal to I, amps times V, volts. So Amps equals 648W/12V= 54A max total draw from the +12V rails.

"Recommended is a 500watt PSU with two 75 watt 6 pin PCI express connectors. 600 watt PSU with four 6 pin connectors to run crossfire is recommended but I suggest a 650 watt for some head room.
Recommended 42 Amps on the 12 volt rail for single card and 50 Amps for Crossfire. "


Will you take a look at that, 42x12=500W
Although, you will never be able to find a 500W power supply that will provide 500W on the +12V rails lol. You'd have to look at 600W units. But considering that a single 5870 draws no more than 188W and I do not know of a CPU that draws more than 140W and hard drives draw no more than 10W each, you'd be more than covered with their recommendations.

In short, your power supply is more than fine.
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June 7, 2010 8:00:28 PM

Best answer selected by ashkaji.
June 7, 2010 8:02:29 PM

Ty very much! The math kinda friend my brain for a moment but I think after some coffee I'll have it down!

And this will shave $120-150 off my next build! Very nice :D  lol. Only downside is I'll have to put a crappy generic 500w back into this pc, but it wont have much power draw at all (no videocard) So it should be fine :] .

Ty again!
!