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The paper develops an efficient computational method for establishing equivalent characteristics of magnetic joints of transformer cores, with special emphasis on step-lap design. By introducing an equivalent material, the method allows the real three-dimensional structure of the laminated thin sheets to be treated computationally as a two-dimensional problem and enables comparative analysis of designs. The characteristics of the equivalent material are established by minimizing the magnetic energy of the system. To verify the proposed approach, a series of experiments have been conducted. First, the anisotropic characteristics of the step-lap were established, and then space components of the flux density at specified positions measured. This enabled detailed analysis of the flux distribution in the step-lap region, in particular the way in which the flux travels between the laminations close to the air-gap steps. Encouraging correlation between the homogenized 2-D model and experiment has been observed.