Just going to run through a quick example. ATX power uses 18 AWG copper cable. 18 AWG copper cable has 6.4 ohms of resistance per thousand feet. A 6 foot 18

Just going to run through a quick example. ATX power uses 18 AWG copper cable. 18 AWG copper cable has 6.4 ohms of resistance per thousand feet. A 6 foot 18 AWG cable would have .0384 ohms of resistance. Let’s say you has a pretty heavy load of 10A. This would be a 0.33 ohm load. There would be about 10% loss, or 3W. Not nothing, though at more typical loads the cable loss would be lower. 5V rails are more typically used and they would have close to half the loss.