Dual 1080ti in a Z620?

[NeuralEngineer]

Hi,

TL;DR: Is 2 x 1080ti too much for a 800W psu?  Can a 1080ti be run of a single 12V x 18A cable? Rationalizations below.

I'm wondering if I can power two 1080ti gpus in my z620.  It has a 800W psu .  I currently have one 1080ti and would like to add another for deep learning.  Would appreciate all opinions both positive or negative about the following.  

I've read the relevant related forum posts and studied the psu (link below).  I understand the psu supplies up to 800W of 12V.  Each cable can supply 12Vx18A = 216W.  Adding in the 75W pcie socke power yields  293W, enough for a single 1080ti. 

related forum post: https://h30434.www3.hp.com/t5/Business-PCs-Workstations-and-Point-of-Sale-Systems/HP-Z620-installing...

dual 970s: https://h30434.www3.hp.com/t5/Desktop-Hardware-and-Upgrade-Questions/Z620-with-Dual-GTX-970-Power-Co...

Connectors

Each gpu wants dual 8-pin.  For a single gpu, I currently use two 6 to 8 pin converters with two seperate cables.  I'm proposing to use an 8pin to dual 8pin adapter after the 6-8 adapter for each GPU.  Seems to be pushing it, I know.  I could then use EVGA's utilities to keep peak power to less than 250W per gpu.

Other power draw in the system: dual e5-2650 V2s, 128GB ram, and 2 SSDs. 

Adding up the power

GPU1, 250W

GPU2, 250W

CPU1, 115W

CPU2, 115W

8 sticks DDR3, 32W

SSD1, 5W

SSD2, 5W

total = 772W

I could remove a cpu to get lower total power.

Crazy idea: hack a Z820 1125@ psu into a Z620.  Both probably have the 18pin mobo connector, but they are a different shape...dremel time?

Reasonable idea: just buy a Z820 chassis (mobo & heatsinks) on ebay with a 1125W psu and swap the components in.  The z820 and espeically its psu is a beautiful piece of engineering.  They still fetch a considerable price however.  Despite what the above looks like, I've done this on a small budget $700+GPU(s), so ~$500 for a z820 chasis is a considerable added investment.  

Thanks for your thoughts, whether you think this is a good idea or bad one,

NE

Edit: added a TL;DR

[DGroves · Accepted solution]

your  pwr observations are missing  Fans,  motherboard pwr consumption, also any usb devices pluged in or used will also draw power, and the SSD's are 5 watt avrage with peeks over 7 watts,  second when a device starts up the power draw will be higher than when running

next we come to the thermal issue, the z620 is simply not able to handle the thermal load of two high end video cards and two cpu's and each pci-e socket also can draw 75 watts

just because you can do something does not mean you should.

last, you really haven't seen a z820 power supply have you? there is no way in the world that unit is going to fit into a z620 no matter what you try

i would not trust a system as you have described  as reliable for long term usage (or even short term)

[Brian1965]

Hi NeuralEngineer,

The first link in your post refers to installing a GTX 1080 (190W) + GTX 1060 (120W) giving a maximum power draw of 310W.

The second link refers to installing 2x GTX 970 cards (145W each) giving a total power draw of 290W.

As per the Z620 quickspecs, it clearly states the maximum power of all GPU cards cannot exceed 300W. Personally, I believe there is a little headroom in the PSU to exploit, hence why I'm now running a GTX 1080 Ti (250W) and a Quadro P2000 (75W) in my Z620, (325W total). I definitely would not recommend trying to squeeze 2x GTX 1080 Ti cards into a Z620 chassis, effectively trying to draw 500W of power! Also, in my GTX / Quadro set-up, I am only running a single CPU (E5-1680 v2), and both the CPU and GTX card are water cooled to eliminate any overheating / throttling issues due to heat build up. I am using both my auxillary PCI power cables to power the GTX since the P2000 does not require auxilliary power. I chose my current gpu configuration to maximize the available CUDA cores for rendering and computation use.  

If you definitely need to run 2x GTX 1080 Ti cards, then I would opt for a Z820 chassis with a 1250W supply. Alternatively, select a lower powered Pascal card (GTX or Quadro) as the second gpu if your sticking with the Z620 chassis.

[NeuralEngineer]

Thanks for your replies.  I'll be sticking with one 1080ti (and sleeping more comfortably).  I appreciate your sound advice.

[NeuralEngineer]

And update: I decided to buy a z820 with the 1125W power supply with the plan of putting 3 x gtx 1070s in there.