I need to power a cluster of 8 Raspberry Pis 4 and I have a 480W PC PSU with a maximum load of 28A on 5V+. My question is really simple and dumb (sorry for that) but I did not found a precise answer, but when I attach them to the 24pin motherboard connector should I use all the five 5V+ cables and 5 ground respectively? PSU cables are 20awg, and I estimated 3A for the Raspberry Pis so a total of 24A.
Yes, you should use all the 5V conductors and at least as many GND to reduce voltage drop over the cable; using extra grounds would only help. With 5 supply and 5 return wires, each conductor will be carrying almost 5A. Even this starts to push the limit of what you would want going over 20AWG.