look at the bottom left of your z820 motherboard almost at the edge, by the board mounting screw you will see the white 4/3 pin PWM fan header it's the last white connector on the bottom left use this to power the raid cards fan which will then be shown in the bios fan speeds or the HP monitoring software (if installed)
if free, i recommend the pci-e slot numbered 21 (which is just above the PCI slot) for the raid card as it's inline with the front cooling fan which allows the cards small fan to have a steady flow of cool air to push onto the raid cards heatsink
if you want "PM" me for a custom win 10 or 11 z820 install recovery key method i've created
my day to day system is now a lenovo p520 which is close hardware wise to the HP G4 systems, the z820's are used for multi cpu based projects nowadays