Nonlinear loads produce harmonic currents, which induce additional losses in transformers and cause temperature rise, especially in the windings. The estimation of winding eddycurrent loss, in the presence of harmonics, is based on the knowledge of winding eddy-current loss at power frequency. This paper deals with the transformer winding loss estimation and gives a simplified winding resistance expression that takes winding eddy-current into account and allows to estimate associated losses. The impact of transformer winding characteristics, as conductor size and layer number, on the resistance variation with frequency is shown.