This shall be determined for single-way duct(s) not embedded in concrete in the same way as for cable, using the appropriate formulae from 2.2.1, 2.2.2, 2.2.3 or 2.2.4, and the external radius of the duct or pipe including any protective covering thereon, replacing the external radius of the cable. When the ducts are embedded in concrete, the calculation of the thermal resistance outside the ducts is first of all made assuming a uniform medium outside the ducts having a thermal resistivity equal to the concrete. A correction is then added algebraically to take account of the difference, if any, between the thermal resistivities of concrete and soil for that part of the thermal circuit exterior to the duct bank