Transformers are normally designed and built for utilizing at rated frequency and prefect sinusoidal loads. Supplying non-linear loads by transformer leads to higher losses, early fatigue of insulation, and reduction of the useful life of transformer. To prevent these problems rated capacity of transformer supplying non-linear loads must be reduced. In this paper, at first, a useful model of transformer under harmonic condition is presented. The model in addition to ordinary parameters includes a potential difference defined as the second derivative of the load current that represents eddy current losses in windings and the other stray losses represented as a resistor in series with the leakage inductance and dc resistance. Finally, losses and capacity of 50 KVA transformer under harmonic load current is calculated. This model simulated in MATLAB/Simulink and then its results are compared with losses and capacity calculated by standard.