Transmission of the electric power is accompanied with generation of low –frequency electromagnetic fields. Electromagnetic compatibility studies require that the fields from sources of electric power be well known. Unfortunately, many of these sources are not defined to the desired degree of accuracy. This applies e.g. to the case of the twisted-wire pair used in telephone communication; already practiced is twisting of insulated high-voltage three phase power cables and single-phase distribution cables as well. The paper presents a theoretical study of the calculation of magnetic fields in vicinity of conductors having helical structure. For the helical conductor with finite length the method is based on the Biot-Savart law. Since the lay-out of the cables is much more similar to a broken line than to strait line, in the paper the magnetic flux densities produced by helical conductor of complex geometry are also derived. The analytical formulas for calculating the 3D magnetic field can be used by a software tool to model the magnetic fields generated by e.g. twisted wires, helical coils, etc.