8950 psu (8 or 10 pin) is incompatible with 7090 (6 pin) best shot is to use XPS 8940 dell oem 500w psu which is definitely c/w 7090. 500w is sufficient for rtx 3070 using psu calculator.Load Wattage: 405 WRecommended PSU Wattage: 455 W for higher power psu use non-Dell SFX psu plus 24-6 pin adapter.
Hello Guys, Kindly I want to ask about upgradable my PSU for dell OptiPlex 7090, so I ask about 750W PSU for XPS 8950 it works on dell OptiPlex 7090 and it is suitable for case? if not suitable, any little modification for this PSU? Thanks in advance 
XPS 8950 750W PSU on Optiplex 7090