If the surrounding air or soil is hotter than the reference temperature, the cable starts "hotter." It has less capacity to absorb heat before reaching its maximum operating limit (e.g., 90°C for XLPE insulation).
The NEC uses a formula based on conductor temperature rating.
Note: Altitude affects air cooling but NOT buried cables. cable rating and derating factor
At the heart of cable selection lies two critical concepts: (Ampacity) and Derating Factors .
Manufacturers publish current rating tables based on . For example, under IEC 60287, the base conditions are typically: If the surrounding air or soil is hotter
Overloaded cables are a leading cause of electrical fires.
Proper consideration of cable rating and derating factor is crucial to ensure: under IEC 60287
$$I_actual = I_base \times K_1 \times K_2 \times K_3 \times K_4 \times K_5$$