High current conductors of transformers cause to produce losses and thermal problems in their tank cover. Finite Difference (FD) Method (FDM) magnetic analysis is used to find out an exact estimation of the magnetic field and the losses near the bushings in the transient solution, considering the non-linear magnetic permeability of the cover plate, because FDM is more flexible to deal with the nonlinear constitutive law and easy-to-be implemented especially in the case of simple geometry. Finite Element Method (FEM) thermal analysis is used to determine the plate temperature based on magnetic FD analysis, taking account non-linear heat flux boundary condition. A calibration procedure is used between the analyses to ensure the precision of assumptions. The reliability of the technique, confirmed by experimental and FEM results.