https://www.userbenchmark.com/System/Dell-Precision-5820-Tower-X-Series/96082 

I found one instance of a RTX 3090 Ti being used in a 5820 in the link above.  As long as you have a 950w PSU, it might not be a problem.  I say "might not" since the 3090 Ti requires a 16-pin connector.  I don't know off-hand if your PSU has that.  If it has one 8-pin connection, it cannot be split into 2 without frying wires.

Naturally, there's more instances of the 3090 being used, which is likely a safer bet.

The Dell 5820 certainly does not provide the new 12 or 16 pin GPU power connectors. But the EVGA 3090 TI FTW3 has a 16 pin to 3 x 8 pin cable. The 5820 only has 2 8 (6+2) power cables. I can't recall if one or both of those have more than 1 8 pin connectors on the end? Either way, not sure if just those two cables would supply enough power even if they had the connectors to fill all three connections. BUT, there is a cable available to go from 12V 10 pin CPU power to a 8 pin GPU 12V power. The 5820 power daughterboard seems to have two 10 pin CPU power connectors (same board is used in the dual CPU capable 7820)  and since the 5820 only has one CPU, seems that second 10 pin is spare and would work nicely for the third video card power connection. 

But hard to say how much power would come from each cable and the 10 pin connection? Anyways, it would be pushing closer to limits than is ideal I think? But it might work???

Here's that cable to go from 10 pin to 8 or 6 pin GPU power. Says it only works with the 5820 or 7820, but I think that's a standard 12V 10 pin CPU/mobo connector? But maybe the Dells have something special going on with that connector?

Manufacturer link.

https://www.comeap.com/COMeap-10-Pin-to-8-Pin-6-2-6-Pin-PCIe-GPU-Power-Adapter-Sleeved-Cable-for-Dell-Precision-5820-7820-21-inch-53cm-p372790.html

Amazon link. Newegg has it too, but it's over $30 there!

https://www.amazon.com/COMeap-Adapter-Sleeved-Precision-21-inch/dp/B07XMGDRBP

Hi @speedstep,

In a Precision 5820, does the 950w PSU have enough power through 2 extra 10-pin CPU connections for an RTX 3090 Ti which requires 16-pin connection?

I don't know yet of a diagram or list that simply says what rails have how much power.  As I said earlier in this thread, one instance of a 3090 Ti being used in a 5820 on userbenchmark, but I don't know they did it.

@bradthetechnut 

You   could use PCI-E 4.0 riser cable and Dual power supply outside the case but the 3090 is not physically fitting in the case with the cover closed.

Modding the case side would not be optional if you somehow got the card to physically go into the correct slot.

Linus tech tips tried a 1000W power supply with a system he build and found that Only the 1600W corsair power supply was enough to handle the spikes in power when they streessed the unit with Furmark and Prime95 running to max it out.

1500W is likely enough for a single card but I dont know about 3090 SLI.

Here is a guy who put it in an XPS 400 and used external power supply because stock Dell unit does not have enough power. 6 Dollar CPU with 700 Dollar GPU is severe bottleneck but it does "work"  aka XPS 400 Pentium D 915 to 960 with GTX 1080 TI.  I tested this and it does work abeit not well.

https://www.youtube.com/watch?v=qa9b0TkTsLM

RTX 3090 in a Precision 5820?

Is that the same size as the 3090 to?

Yeah, there's 3090s that will fit in a 5820 for sure. Dell sells a 3090 that's a pretty ordinary looking/sized card. The Dell 3090 in fact just has 2 8 pin power connectors and looks like it'd be pretty perfect for a 5820. 

Now, the 3090 TI is a bit different. EVGA's power supply calculator does indicate an 850w PSU would be fine. Granted that doesn't have the C422 chipset or W2100 series CPUs so I picked something kind of close. 

I did order that 10 pin CPU to 8 pin GPU cable, so in theory I should be able to support 3 8 pin connectors on my 5820. 

There's pretty tempting deals on 3090 TIs going around. Granted there could be a 4080s out soon for even less money and pretty similar performance? But that's a few months down the line for availability. Buying any high end card now is a leap of faith, but I just need more oomph than this Quadro P2000 NOW. 

Need to look at the specs, but I think the EVGA 3090 TI FTW3 will fit? But it is HUGE card. An XC3 version would surely fit, but they don't make the 3090 TI in that trim, only the FTW3 for the 3090TI.

Well, the EVGA 3090 TI is 3.5 slots, so I'd have to basically give up slot's 2 to 5. And my Thunderbolt card is in slot 5 and I think it is supposed to be in slot 5. I guess I could try it in slot 1? But I think the other dimensions should be fine. I can't power the machine now to open it up, but just putting a ruler on that back, looks like a 5.5" high card would fit and 11.81"/300mm is standard full length PCIe slot size I think, so I'd just have to remove that spacer the 5820 has.  I think a 3090TI is workable with the 950w PSU, that extra 10 pin to 8 pin power cable. The question is could I keep Thundbolt 3? Eventually I'll be getting a smaller card for the 5820, so I could put the Thunderbolt back in then. And a 10GBe card will probably happen in the future, so the 3090 TI wouldn't live in this machine forever. Heck, may be worth trying the Thunderbolt AIC in slot 1 just to see what would happen? 

EVGA size specs for their 3090 TI.

• Height: 5.38 in - 136.75mm
• Length: 11.81 in - 300mm
• Width: 3.5 Slots

Which 3090 is that in that 5820? Looks like it could be the Dell 3090? Even shorter than I guessed. Looks downright ordinary.