Gas-insulated high-voltage and extra high-voltage transmission lines are a new technology for transmitting high-power ratings over long distances. A previous paper was devoted to predict the current distribution along with the external flux density in this multiconductor system, highlighting its proximity effect. It served as the vehicle for the present further research taking into account the thermal problems coupled to the electromagnetic field. The aim of the present paper, therefore, is to show the concordance between literature analytical formulas and the finite-element method (FEM) approach considering the concrete tunnel installations. The FEM model is able to study the precise thermal distribution inside the phase and enclosure thickness both in the steady-state regime and transient condition due to the phase-to-ground short circuit.