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In this paper, a novel concept, which is called mixed derating, is introduced for the maintenance of distribution transformers against the copper and core losses increase due to the nonlinear loads and unbalanced supply voltage, respectively. Hence, the traditional equation which is utilized for the derating of transformers under nonlinear loads is modified to determine the proper rated power of a transformer under nonsinusoidal conditions. Based on the proposed concept, the harmonic loss factor as an efficient feature for defining appropriate rated power of transformers under nonsinusoidal conditions is calculated and compared with IEEE standards. The time-stepping finite-element method is utilized to simulate the distribution transformer under different nonsinusoidal conditions and evaluates transformer performance precisely. In this modeling, winding distribution, geometrical, and physical characteristics of all segments of the transformer are taken into account for accurate determination of the necessary signals and parameters to scrutinize the transformer under nonsinusoidal conditions.