The 6-pin PCI-E power connector specification includes two 12V wires and three ground wires and has a rated maximum power output of 75W. The 8-pin includes three 12V wires and 5 ground wires and has a rated power output of 150W.
So if you plug a 6+2 pin + a 6 pin that's 225 Watts + the 75 Watts of the PCIe slot = 300 Watts. The RTX 3080 can draw a maximum of 350 Watts.
You need a PSU that can handle 2 PCIe 6+2 pin cable.
Furmark power reached 370W on the founder edition.
What is your PSU brand and model?