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Can i power 2x 7850's with a 650 short term

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March 2, 2013 8:39:26 PM

ok im wondering if i could power 2x radeon 7850's in crossfire maybe for 6months (my psu is 3-4yrs old 5yr warranty) the extreame wattage calculator recommends 652watts minimum required says 607watts

iv read that those cards can run with pcie 1 x6 pin power connector mine takes two(factory overclocked) and the power supply only has two but i could easily use an adapter for the second 6x plug on each card (1reguler on each 1 adapter) ((wich im guessing is best since mine has 3x 12v 19amp rails prolly each 6pin has a 19amp line and the last 12v rail for all other components wich ill tax with adapters.

was wondering if this would run ok it wont be permanent maybe 6months, think this would work? heres my specs

power supply cooler master real power pro 650watts
http://www.coolermaster.com/product.php?product_id=2551

ok my system is a

core i7 920 mildly overclocked (actualy using slightly lower voltage then stock)
2x4gb ddr3 ram
ex58-ds4 gigabyte board
sound blaster creative labs quad core
pci x4 sata3 card
2x mechanical HDD
1ssd

now with all the technical stuff out of the way would this work or get over-taxed and go poof or it not turn on? i know most 7850's only need 1x 6pin but what do you think?
a b ) Power supply
March 2, 2013 8:59:51 PM

You only have 540W on the 12V rails combined so I wouldn't do it. It will probably run if you try it though, but may be unstable.
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a c 152 ) Power supply
March 2, 2013 9:02:08 PM

Each HD 7850 will draw almost 11A off the +12V rail(s) when maxed out. Your PSU only has 2 19A rails. Your CPU is cabable of drawing another 10 amp at max. If you knew which rail fed which Molex/PCIe power cables, you might sneak by. But the CM line of PSUs isn't know for their dependability or quality. So I would say use at your own risk. If you notice shutdowns or re-boots, it is probably the PSU not being able to keep up. If you had a better 650W PSU, I would say more than likely no problem.
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March 2, 2013 10:22:03 PM

it actualy has 3x 19amp rails but im wonderin it has overvolt and sorts standered protection featers was wondering worst case scenario it wont be a risk to go poof and take parts with it will it
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a c 152 ) Power supply
March 3, 2013 1:11:10 AM

WINTERLORD said:
it actualy has 3x 19amp rails but im wonderin it has overvolt and sorts standered protection featers was wondering worst case scenario it wont be a risk to go poof and take parts with it will it

Ahh... you're right. It does have 3 +12V rails. I must have mis-read it the first time. There's always a chance of any PSU failing and damaging other components. The lower quality PSUs would be the most likely. The CM falls somewhere in the middle. The PSU will run the 2 cards fine. It's just when they max out their voltage demand that you may have issues. Normally, the system just shuts down or re-boots when that happens.

If you decide to try it with Molex to PCIe adapters, be sure to grab a separate Molex from different Daisey chains for each adapter. That will keep the wire size within safe specs for each load.
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a c 144 ) Power supply
March 3, 2013 8:15:52 AM

You have a PSU which has three 12 volt rails, each of which can provide a max of 19 amps. The total 12 volt power is 540 watts which is 45 amps, not 57 amps. This makes it a mediocre power supply. A good single rail, 650 watt PSU should be able to produce 50 - 52 amps.

Having said that, you should be OK. Two 7850's together will need about 250 watts (24 amps) during peak loads. The CPU will need around 10 amps. That is about 30 amps from the 12 volt rails. The rest of the components shouldn't need more than 150 - 200 watts with less than 2 amps from the 12 volt rails. An X58 motherboard is not particularly efficient.

The extreme wattage calculator usually overestimates the needed power.
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