Introduction. The problem of estimating power losses in grounding wires with built-in fiber optic cable for overhead power lines of voltage class 330-750 kV is relevant, while it is obvious that the amount of losses depends on the chosen brand of wire. Problem. In the article, an analysis of the influence of the parameters of grounding wires on the amount of losses that occur in them in the normal mode of operation of the overhead power lines is carried out. Goal. The purpose of the work is to determine the criterion for the selection of grounding wires with a built-in optical fiber cable under the condition of increasing the energy efficiency of electricity transmission. Methodology. To calculate power losses in grounding wires, the methods of electromagnetic field theory were used, while taking into account the location of phase conductors on various types of towers of operating 330-750 kV overhead power lines and the possible current load of such lines. Results. The paper analyzed the dependence of losses in the grounding wires of the overhead power lines on the ratio of its active and reactive resistances, determined in which range of this ratio the losses will be close to the maximum. It is shown that the amount of specific power losses in the grounding wires of 330-750 kV overhead power lines in its normal operating modes can range from 1.6 kW/km for the 750 kV lines to hundreds of W/km for the 330 kV power lines. Originality. For the first time, it is recommended to use grounding wires with built-in fiber optic cable with running active resistance in the range of no more than 0.25 Ohm/km, which will minimize power losses and increase the energy efficiency of the 330-750 kV overhead power lines. Practical value. The obtained results can be applied at the stage of designing new or modernizing existing overhead power lines in order to reduce losses and increase the energy efficiency of lines.