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A new and simple formulation of equivalent conductivity in the direction normal to the laminations is derived by analyzing and comparing the electromagnetic fields and the eddy current losses in a laminated iron core and a single lamination. Then the anisotropic conductivity tensor is updated. The conductivity formulation can make it easier to calculate numerically three-dimensional eddy current fields in laminations with good accuracy. The validity of the formulation is verified by comparing the eddy current losses, computed with the finite element method using the proposed conductivity formulation and the long known formula, in an analyzed model.