Stray loss reduction methods for losses outside the windings were analysed in this work. The work focused on the localisation of the loss. A program based on the finite element method was used in this work. The comercial name of the programme is MagNet. Tank and clamping losses were analysed in the first part of the work. Impact of construction changes on the loss were analysed. Impact of magnetic tank shields height and wideness, and distance between the shields (tank) and coils, on the tank loss were analysed. A cost-benefit analysis of the optimal tank shielding was made on three transformers of substantialy different nominal power. The analisys was made based on the capitalised cost of the transformer. An analisys of the bracket's size, position and material on the clamping loss was made. And finally matematical models for fast and easy calculation of loss in the shielded tank and clampings was made. The model is based on analical calculation and statistical data. Impact of the helical winding on the tank loss and eddy current loss in the high-voltage windings was analysed in the second part of the work. The Impact of the configuration of the high – current conductors was also analysed. Advantages and disadvantages of different configurations were explored. On the end of the work few comparisons of the measurements and calculations are given. Comparisons of loss measurements on commercial transformers and magnetic induction measurements on a research transformer with calculations from MagNet were done.