The transport problem is solved and a new equation is derived for calculation of the limiting current of copper deposition on a plane vertical cathode in CuSO4 + H2SO4 (in excess) solution under the conditions of natural convection. In contrast to the earlier published studies, no ionic transport numbers were used for consideration of the contribution of migration transfer. The derived equation allows us to determine quantitatively the effect on the limiting current not only of the mass coefficient of supporting electrolyte, but also of the diffusion coefficients of its ions.