In the following diagram, each square represents a device(PCB) and the devices are daisy-chained by a 3C cable and everything is in DC. The 24V comes from a power supply. The 3C cable current rating will be the largest among the 3 lines.
Current in each line:
Line A: 120W/24V = 5A
Line B: 12W/5V = 2.4A
Line C: The ground is being shared by 24V and 5V, what current rating cable should this line be using?