Thermal Resistance (Cables)

A cable's current-carrying capacity is ultimately determined by how effectively it can dissipate the heat generated by I²R losses in the conductor. The thermal circuit from conductor to ambient consists of several resistances in series: the insulation thermal resistance (T1), the bedding or filler thermal resistance (T2), the outer serving or sheath thermal resistance (T3), and the external medium thermal resistance (T4). IEC 60287 provides detailed calculation methods for each component. The external medium resistance T4 depends heavily on the installation method — cables in free air benefit from convective cooling, while directly buried cables rely on conduction through soil. Soil thermal resistivity varies enormously: moist clay may be 0.7 K·m/W, while dry sand can exceed 3.0 K·m/W. Higher thermal resistance means the conductor reaches its maximum temperature at a lower current, directly reducing the cable's ampacity. For critical installations or unusual conditions, IEC 60287 calculations provide more accurate cable ratings than the simplified tabulated values in wiring standards, which assume standard soil and installation conditions.