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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?
There is one segment among the 30 devices where the ground wire must carry \$\frac{9}{10}\$ × 5 A + 2.4 A, for a total of 6.9 A. In all of the other segments, the ground current is less.
