The construction of the transmission lines, in the middle of the cities, are always a big challenge for the designers, industry, government and electric utilities. Components of the project, in the electrical, mechanical and civil works are designed for a high reliability, however, the underground cable design according to international standards have some discrepancies, due to this challenge, a new proposal for the new design and engineering failure analysis. Our findings included a case study of a real underground transmission lines with important contribution with models and mathematical calculation for ampacity calculation as function of depth, temperature, screen connection, materials and thermal analysis for transitory and normal operation of the underground cable in high voltage. This novel proposal generates a recommendation for an update of the international standards IEEE, IEC in the next version.