This paper describes a mathematical model for calculating the magnetic flux density of a cable system, which can take into account any number and arrangement of single-core cable systems arranged in a triangular bundle or laid in a plane at any distance from each other. A matrix of reduction coefficients is used, which takes into account the complete electromagnetic coupling between the associated cable lines. The application of this model allows the calculation of the distribution of currents flowing through metal screens in a cable system, and then the magnetic flux density at arbitrarily selected points in the air, depending on the geometric arrangement and number of single-core cable systems, which is shown in the selected numerical example. In addition to the well-known factors that significantly affect the amount of magnetic flux density, such as the amount of current and the depth of laying, the results obtained also indicate a significant influence of the method of laying and arrangement of phase conductors, which can be used in the planning and design of new cable lines.