The branch of the Bank of Costa Rica located in the Carmen district, San Jose, has anobsolete electrical installation that does not meet the regulations of the NationalElectrical Code.Among the anomalies found no breach with the color code, wrong selected protections,no GFCI protection. So, with the development of this project it proposes appropriatechanges to get an adequate electrical installation.A study of short circuit was developed. A single-phase fault (most commoncontingency) and a three-phase fault (the one with higher currents) was analyzed.Resulting in a maximum asymmetrical current 13 500 A in the secondary of thetransformer.Part of the study includes a diagnostic of the quality of energy. Interesting results wereobtained in harmonic distortion. No significant voltage distortion, while the currentvalues are very high (reaching a current of 24.80 A in the total harmonic distortion).Finally, a cost estimate was made for the Maintenance Office of the Bank of Costa Ricato consider a budget of approximately 15 million of colons only for materials.