Unbalanced supply voltage and non-linear loads have become significant consideration in the designing transformers. This condition results in devastating effects such as high loss, early damage of insulation and premature failure of the transformers. For proper operation of transformers, capacity of transformer should be reduced. This paper presents a novel concept for mixed derating of distribution transformers under unbalance supply voltage in the presence of harmonic load conditions. The three-dimensional time stepping finite element method (TSFEM) based on Ansoft Maxwell is utilized as an instrument for calculation of losses and viewing magnetic flux density on core and windings of the distribution transformers. The results of simulation show that IEEE standard is a conservative method for transformer derating under unbalance voltage in the presence of high total harmonic distortion load current and TSFEM is a convenient and more precise method for transformer derating in this condition.