This paper presents a numerical computation method for determining the magnetic field of high-current busducts of circular cross-section geometry, based on the subdivision of the busduct phase conductors and screens into the conductor filaments and the subsequent application of the mesh-current method, with the aid of the geometric mean distance method. The mathematical model takes into account the skin effect and the proximity effects, as well as the complete electromagnetic coupling between phase conductors and metal enclosures (i.e., screens) of the single-phase isolated busduct system (of circular cross-section geometry). This model could be readily applied to the computation of the magnetic field of the Gas Insulated Transmission Lines (GIL) as well.