$100 \text Amps / 0.72 = 139 \text Amps$
Where:
When multiple cables are run close together (e.g., in a tray, conduit, or ladder), they radiate heat onto one another. This mutual heating prevents individual cables from cooling down efficiently. The more cables there are in a group, and the closer they are spaced, the lower the derating factor. derating factor of cable
To select the correct cable size for a required load, the calculation is typically performed in reverse: $100 \text Amps / 0
This is the story of the : the engineering reality check that bridges the gap between laboratory perfection and the messy, hot real world. The Theory: The "Invisible Tax" in a tray