A method for calculating non-stationary thermal processes in wires for low-voltage overhead power transmission lines is proposed. The technique is based on the representation of a wire as a system of several homogeneous bodies and the solution of differential equations describing this system. The equations are solved by the method of electrothermal analogies based on the thermal substitution circuit and the Laplace operator transformation. Formulas are given for determining the power losses in the wire, thermal resistances and heat capacities of the wire. Special attention is paid to calculating the coefficient of heat transfer from the surface of the wire. Algorithms for calculating the temperatures of single-core and multi-core wires when influencing external factors change have been developed. It is shown that the temperature calculation must be performed in several iterations. The concept of heat exchange angle is introduced, characterizing the part of the wire surface through which heat exchange takes place. Experimental studies for different brands of wires at different currents have shown that the maximum absolute error of the calculated insulation temperature of the wire relative to the measured temperature is no more than 6 °C. For the long-term allowable currents in the wires of the SIP-4 brand (‘self-supporting insulated wires’), the values of the reduction coefficients are calculated depending on the number of cores. For example, for the SIP-4 4x16 wire, the permissible current should not be 100 A, as given in the directories, but 80 A at an ambient temperature of 25 °C. The calculation methods and algorithms presented in the article can be used to estimate the capacity of low-voltage electric networks, as well as at the design stage of power supply systems.