[IMAGE]If you use basic online psu calculator you may find that Dell 500w psu is able to support RTX 3000 series card.  Also there is user benchmark evidence that xps 8940 supports RTX 3090 and many many RTX 3070 etc. check out [URL] (do not think all these users updated psu to non-Dell >500w) but if you still want to use an external psu to separately power the 3000 series card in 8940 case, it is theoretically possible.  It is as if you are running two PCs each having its own psu, but one psu sends gpu power to the Dell 8940.  In the end it is just 12V power. basic psu calculator Load Wattage: 452 WRecommended PSU Wattage: 502 W Dell 500w psu is just about right to support RTX 3070 + i7-11700k    
I have a 8940 with a 500W PSU and a Asus GTX 1060 6gb GPU. Was looking at upgrading the GPU to a RTX 3 series but I quickly realized the 500W Dell PSU will not keep up. The RTX 2060 is the max I calculated I could upgrade to with a 500W PSU, since the 2060 only gives me 43% more speed than the 1060 6gb, I don't see it as worth the trouble. I DO NOT want to upgrade the PSU in the 8940 (size, 6-pins MB connector ...), this solution is ruled out. So I was thinking ... can I buy a mini ITX case with a 650W PSU to power a 3 series RTX which would be plugged to the MB in the 8940 ??? It seems to be possible when I look at posts on internet but the issue is to power the external PSU (in the mini ITX case) at the right moment. Anybody here has experience with that ??? What does it imply ??? I realize I would have to run a power cable from the mini ITX to the rear of the DELL desktop case. Again : I DO NOT want to upgrade the PSU in the 8940 (size, 6-pins MB connector ...), this solution is ruled out.
XPS 8940, adding external PSU to power high wattage GPU