This paper describes the problem of calculating the electromagnetic field of overhead power lines. From Maxwell’s equations, equations for calculating the magnetic field strength, magnetic induction and electric field strength due to currents in the phase conductors of a straight section of an overhead line were derived. Based on the developed theoretical basis, a computer program was created for calculating the electromagnetic field of an overhead line section. A total of three selected examples were solved using the computer program and the calculation results were analyzed. In the second chapter, the equations of the harmonic electromagnetic field, which follow from Maxwell’s equations for the harmonic field, are defined and described. In the third chapter, the numerical approximation of the longitudinal currents and potentials of the phase conductors of the considered overhead line section is described. In the fourth chapter, the equations of the potential of a straight section of an overhead line are derived. In the fifth chapter, the imaging of a point source of harmonic current located in the first layer of a two-layer medium is considered, where the first layer is air and the second layer is a homogeneous soil of known complex electrical conductivity. In the sixth chapter, the calculation of the magnetic field strength and magnetic induction of an overhead power line section using the Biot-Savart law is presented in detail. The field is calculated in the symmetrical plane of the overhead power line section. In the seventh chapter, the algorithm for calculating the electric field strength of an overhead power line section is described. In the eighth chapter, a test example of a single-conductor section is solved, as well as selected examples of overhead 110 kV overhead power lines of the type of girder without a protective wire and with a protective wire. The obtained results are graphically presented and compared with the prescribed limit values of magnetic induction and electric field strength in air.