A simplified model of transformer hysteresis curve utilized for calculation of core losses is presented. It incorporates eddy current losses and hysteresis losses into the same hysteresis curve thus simplifying a set of equations used for solving the magnetic circuit. The parameters of the hysteresis curve model are determined by evolutionary optimization which minimizes the difference between measured and calculated current waveforms from open-circuit test of a three- phase, three-leg transformer rated 400 V, 20.8 kVA. The core losses calculated from the surface area of hysteresis curves calculated for all three legs with supply voltage variation from zero to 10% above rated show a good match with measured losses.