The finite volume discretization of massive conductors for the calculation of eddy-current distribution causes fictitious surface charges at the boundary between adjacent volume elements having different current values. In this paper, the effects of these surface charges on the numerical solution of three-dimensional eddy-current problems are evidenced. Furthermore, comparing solutions of eddy-current problems in the presence and absence of surface charges, an empirical criterion is derived for the mesh refinements of the current inside massive volumes.