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How to use two HD 6970 + HD 6970 with 850W PSU?

ms-tech 850w psu

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    Resident One Post Wonder

  • Joined: 02-May 13

Posted 02 May 2013 - 22:17

MS-Tech 850W PSU MS N850VAL 12cm Ver. 2.3 retail
Rail one is 12V1 max 324 Watt, 28 Ampers
Rail two is 12V2 max 324 Watt, 28 Ampers
PSU has one 6 pin PCIE cable and one 8(6+2) pin PCIE cable
Several SATAs + several Molex
One 6970 takes ~370 Watt fully loaded and idle state takes ~120 Watt
Can I install second GPU 6970?
Where are first and second rails?
What amper load is currently on first and second rail?
How to evenly distribute Amps current load between two rails?
Should I use sata to 6 pin pcie converter cable and molex to 8 pin pcie converter cable to feed second 6970 GPU?
I am so confused right now…

#2 +PeterUK


    Neowinian Senior

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Posted 03 May 2013 - 10:12

MS-Tech 850W PSU MS N850VAL 12cm Ver. 2.3 retail
PSU has one 6 pin PCIE cable and one 8(6+2) pin PCIE cable

Your going to need a new PSU if it only has one of each and you need two of each.

#3 Phouchg


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Posted 03 May 2013 - 10:33

The price alone of that box full of crr... err, PSU is a dead giveaway that you probably should not attempt it regardless of ratings.

Now to the rails. It's a distribution system. Each rail has its own overcurrent protection circuitry, which basically means this - if a short-circuit happens, only the stuff connected to that rail will end up in smoke, the rest will usually live.

Basically, if the manufacturer doesn't specify (as in this case), it's a guesswork. But in this case it's also easy - it doesn't provide the necessary cables. First, that is a clear indicator that the manufacturer is not confident at all about using it in any kind of multiple card configuration (therefore it's vastly overrated). Second - most likely processor, ATX and peripherals are on the first rail and both PCIE are on the second rail. If it hasn't crashed yet, I'm likely right. And there's not enough power left on any of the two rails to handle another graphics card of 6970 levels.

I might add, though, that one 6970 does *not* ever eat 370W. First, it'd be well outside PCI-Sig High Power ratings. Second, it's TDP is 250W, which means it would burn where it stands in seconds, if such power input would be sustained. You are likely confusing it with total system wattage. Actual numbers are about 210W, with 240W peaks.

Depending on the processor (if it's a low rated one, say 70W) and if you have just one HDD and nothing else, you *may try* with Molexes and hope that it holds and doesn't crash. But I wouldn't recommend at all.

I'd strongly suggest investing in a reputable PSU. And if you think about two graphics cards like 6970, then >50 amps on single rail configuration (starting from 700W) or >35 amps on *all* of the rails of multiple configuration is the minimum (also starting from 700W, not at all coincidentially, but very much in contraty to this overrated MS-Tech garbage can).

#4 Semtex


    Neowinian Senior

  • Joined: 20-February 11
  • Location: Europa/Poland

Posted 03 May 2013 - 10:38

850W in this PSU, good joke :shiftyninja: In this construction 500 is way maximum, buy some 750W but a good branch.
First main rule in building new rig, do not try earn some money on PSU, better get one GPU with good stable PSU, less risk to loose whole rig in one sunny day ;)