Thermal analysis of cables is an important area in network design. By cabling the network, the excess heat of the cable is dissipated differently than network free in the air. The critical points from the point of view of thermal analysis are the sealed passages in the walls. To this end, the field of thermal analysis of cables has been analysed, existing documentation has been reviewed and a program has been developed to calculate thermal conditions for different situations. The program is based on the finite element method and was developed in the open source FreeFEM++ programming environment. The sealed cable passage through the concrete wall was analysed in more detail, as it is assumed that this is one of the most critical points of the thermal analysis. The thermal conditions for a single cable passing through the sealed transition are calculated analytically and numerically - using the finite element method. The results of each method are compared with each other. Finally, the thermal conditions using the finite element method are calculated in 3D space for a case that can be encountered in practice - the crossing of a three-phase system through a sealed passage of a concrete wall. The results confirm the assumption that the crossing of the concrete wall through the sealed passage is the critical point in terms of thermal conditions. In fact, it is found that the waste heat is dissipated worst there. Therefore, the temperature is the highest there.
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