The thermal resistance depends on the geometry of the cable and the thermal resistance ‘K’ of the insulate. For a single core cable the thermal resistance between the conductor of radius ‘r’ and sheath of overall radius ‘R’ is proportional to the thermal resistivity “K” and thickness of the cable insulates, and inversely proportional to the surface area of the sheath. It can be expressed as Rth = K/2π 〖log〗e R/r °C/watt . For a 1 cm length of cable (i.e. thermal ohms/cm).