A dynamic model of a core-type single-phase transformer is presented in this work, which takes into account not only the non-linearity of the core material but also the geometry of the core. The model is based on the decoupling and the direct solution of the electric and the magnetic circuits. Simulation results with a non-uniform cross-section of the magnetic core are presented in comparison with simulation results with a uniform cross-section of the magnetic core. The results show a strong influence on the electrical quantities in the case of a non-uniform cross-section of the magnetic core.