In this paper we describe the steps taken to solve electrothermal problems on distributed configurations. For that purpose, we computed the 3D temperature distribution in a dry type transformer, by solving the Poisson equation with source terms specific to the topology under scope, that is heat generation by Joule losses and Eddy currents, as well as natural cooling on the outside of the metal casing. Our approach is based on the assumption that the heat transfer in the electrical device is diffusion predominant. The solver used for this problem is the well-know BICCG (Block Incomplete Choleski Conjugate Gradient).