In any industrial installation power frequency electric systems generate extremely low frequency (ELF) magnetic fields. Field amplitude values must fulfill the limits related to health effects or to disturbances on electromagnetic devices and a shielding system could be necessary. The design of a magnetic field shielding apparatus requires the simulation of the sources and of the shields in order to evaluate the main shielding parameters like geometry, material and position with respect to sources. An alternative approach is proposed in this paper which requires to measure a spatial distribution of the magnetic field amplitude without any information on the time phase shifts among the components at each measurement point. Actual field sources are simulated by means of suitable simplified sources handled by analytical relations. An identification process of measured magnetic fields is performed using a genetic algorithm applied to the simplified source. The proposed simplified sources are a suitable combination of three-phase ones and it seems able to create a realistic magnetic field distribution both in space and in time. The paper presents the proposed method and its application to the design of a conductive shielding to reduce the leakage fluxes of a three-phase transformer without tank. The magnetic field to be shielded is evaluated with a simple 3D model of a transformer constituted by three windings in air with current values deduced from the equivalent magnetomotive force, which is considered the actual source. Unshielded fields on a plane region are used to identify the simplified source and the effects of a conducting aluminum shielding are compared using actual and simplified sources. The influences of some parameters of the simplified sources on the results are discussed. The case of harmonic fields with a negligible harmonic content is analyzed.