Dealing with heat transfer in electric cables, there are several methods which reduce a full transient three dimensional problem to a more simple one. For large times, a common method is to reduce the time dependent problem to a stationary one. These useful reduction methods are often applied without knowledge of resulting errors. In this book, we develop a heat transfer study where the particular reductions are treated via an asymptotic analysis of the associated parameters, time and length among others. Within the reduced model, we use nonlinear boundary integral methods applied to multiply connected domains. An iterative procedure computes the relevant temperatures on the boundary of the cross-sectional domain.