A mathematical model for calculating steady non-sinusoidal mode of a complex electrical network is proposed, taking into account the distribution of overhead transmission lines parameters, suspension geometry and brand wires, availability of overhead ground-wire cable and the way of their grounding along the route of the line. Based on the calculation of complex network steady mode the fundamental losses are accounted which were caused by positive-sequence currents of the fundamental frequency, and the total losses caused by the actual currents of the fundamental frequency and high harmonic currents in wires and ground wires. The paper introduces the example of the steady mode calculation and the results of study of the effect on the loss of electric power suspension geometry of wires and overhead ground-wire cable.