When a fault occurs, the cable temperature rises rapidly. If the cable size is too small, the conductor will melt or the insulation will ignite before the breaker trips. The minimum cable size ($S_min$) is calculated using the adiabatic equation:
Electrical cable calculation is a structured, multi-step process that balances thermal limits, voltage regulation, and fault tolerance. It is not about picking the fattest cable or the cheapest – it is about engineering the optimal conductor for the specific installation conditions.
These tools automate derating factors and short-circuit checks, but the engineer must still verify assumptions.
Where:
A cable’s tabulated current rating (I_t) is given for standard conditions (e.g., 30°C ambient, free air or buried). Real installations rarely meet these ideal conditions. You must derate the cable using factors from standards like or NEC Table 310.15(B)(2)(a) .