Multi PSU Rig - System on one psu or do I need to cut risers? (Sanity check)

[hyperionx]

Hello fellow miners!

I could use your help with a sanity check here.
Right now I'm expanding my rig a little and I'm debating whether or not to cut the pcie risers.

     Here's the deal:
     Athlon II x2 240 2,8Ghz
     Gigabyte GA970-Something
     2*4GB XMS3 ddr3
     5 x Sapphire 280x Dual-x
     And the big PSU mess: 1*HX850, 1*CS750M, 1xCX400 (All corsair)

I know I could have bought a single larger psu but im on a budget and this is the stuff I had lying around so I do not need any snarky remarks ^^


My two options:
      Run the entire system including risers and one 280x on the hx850 and then 3 280x on the cs750m and one on the cx400.
      OR:
      Cut the risers and run 2x280x plus system on hx850, 2x280x on cs750m and 1x280x on cx400.

I must say I would prefer the first way and not cut the risers if my psu wattage allows it.




Here ar my calcs for the first option:

     powered risers 75w*5= 375w (I used 75w since its the specified max for x16 pcie, my risers are x1-x16 though, no idea if that matters)

     system = 150w (looked up tests for my cpu and they all seemed to be around 150-200 at full load even with a gpu so figured 150 would be safe)

     gpu (8pin+6pin) = 225w (150w max for 8pin and 75w max for 6-pin)




     HX850: 375w+150w+225w=750w

     CS750M: 225w*3=675w

     CX400: 225w



So what do you think, could this be feasable or did I miss something in my calculations?
The gpus will be undervolted too so I'm hoping they'll be way below 250w each instead of the 300w I calculated with for safety.

I realize that cutting the risers would allow me to shift a little bit of the wattage to the cx400 and ease off the other two a bit but like I said, I'd prefer to run it like this if it would work.

Thanks alot for your thoughts on this!

[Gator-hex]

Each motherboard slot or powered riser is 75W (the power riser is to help out un-powered 1x PCI-E slot motherboards)

Each 6pin PCI-E is 75W

Each 8pin PCI-E is 150W

add 100W for your CPU and other bits, if you're CPU TDP is higher add that instead.

You need to know the +12v power rail for each power supply. If they are single rail it makes everything so much easier. Don't overload any rail.

6pin+8pin graphics card in a 16x motherboard slot is 75W+150W+75W = 300W

6pin+8pin graphics card in a 1x slot riser is 75W+150W+75W = 300W

[hyperionx]

Thanks for your response.

All three supplies are single rail ones with 840w, 744w and 360w max on 12v rail.

I added 150w for cpu etc., I'm guessing youre saying 100w would be enough? It probably would since the cpu is 65w tdp.

Other than that my calculations are the same as what youre saying. So i guess we're thinking I might be able to pull it off without cutting the risers.

[Gazza1]

You should run the mobo, cpu, and all powered risers on the same PSU