Currently, the development trends of global medium voltage cable networks are aimed at the introduction of cables with cross-linked polyethylene insulation. Such cables have a higher operating temperature, which allows to increase their throughput, and increased stability when working in conditions of overload and short circuits. It is possible to determine the allowable heat and electrical loads only if the heat balance is observed. Fundamentally important for solving the problems of heat balance is the significant dependence of thermal conductivity and heat capacity of polyethylene insulation on temperature. It is established that the capacity of the cable, determined by regulatory models and average environmental parameters, provides only preliminary information about its heating. For certain requirements and for innovative cable designs, a necessary condition for determining the load capacity in real stationary modes of operation is the use of the heat balance model. A system of mathematical equations is proposed, which due to the combination of structural and thermophysical parameters of the cable with the conditions of the external environment allows to determine the parameters of the stationary thermal regime of the cable at any load current.