When cables pass through thermal insulation (like in building walls), the heat generated by the cable is trapped by the insulation material. This significantly reduces the cable's ability to cool, necessitating a substantial derating factor.
Derating is necessary when the cable’s ability to is compromised or when the ambient temperature is higher than the standard. If these factors are ignored, the cable temperature may exceed the insulation's thermal rating (e.g., 90°C for XLPE or 70°C for PVC), leading to: derating factor of cable