/r/buildapc - Planning on building a computer but need some advice? This is the place to ask!

Hi, I recently built a small form factor PC and am having some trouble getting the PC to stay powered up when I connect my new graphics card (3090) to it. It works fine with my old card (780 ti), and it works fine without any GPU connected.

The issue is when I connect the 3090, the computer will power up, the fans on the GPU will spin for 10-30 seconds then the computer mostly powers down (the case fan still spins until I turn it off). No signal is displayed on my monitor, and the keyboard and mouse do not receive power either. If I remove the 3090, then everything works.

I've run through a few different troubleshooting ideas already:

• I tested the 3090 in a different PC and it worked.

• I placed in my old 780 ti and my new PC worked.

• I can use integrated graphics without a problem (only when the card is not connected).

• I have swapped out power supplies (750w to 850w) and still have the same problem.

I am suspecting it's just not enough power despite an 850w PSU being enough on paper. Is there something else that could be causing it or any suggestions/advice? I'm kind of stumped on this one.

My parts list is below: