How would I get enough power for a 1080 ti in a HP Z620

I have a msi 1080ti and the HP Z620  only has 2x 6 pin cables and my 1080 ti needs two 8 pin connectors. If I am correct the power from the PCIE Slot is 75 watts and each 6 pin power connector will supply 75 watts giving the gpu 225 watts of power. If I were to get a 6 pin to 8 pin connector the power wouldn't be enough for a 1080 ti, so I was wondering how I could supply enough power to the card.