A simplified mathematical model is suggested that describes the harmonic electromagnetic field distribution in high-voltage steel-core twisted cables with a single-layer winding at an industrial frequency. As a physical idealization, the twisted strands are represented in the form of an anisotropically conducting layer. When twisting is taken into consideration, an axial magnetic field arises, which substantially influences losses in the cables. Using the model, the dependences of the internal impedance on the core’s magnetic permeability are derived for AS-70 cable at three winding angles of the external strands. These dependences are compared with results obtained in terms of a hollow cylinder model and with calculation data carried out in the complete statement of the problem with the finite element method.