A comment was given in the introduction concerning the state of contemporary transmission grids in the sense that the society is very reluctant to build new overhead lines while showing no intention to decrease the consumption of energy. Ways and means to overcome this paradox have been revised and the logic behind DLR has been described. Indirect measurement of temperature by measuring sag has been dubbed plausible and it is precisely this method that is to be analyzed and modeled. Using IEEE 738 standard, thermodynamic equilibrium governing the temperature of the conductor during the flow of electrical energy through the conductor has been explained. Based on this mathematical model a computer program in C has been written. It has the ability to calculate steady state temperature if the steady state current is known, as well as transient change of temperature. Overhead line is situated in space and modeled as a catenary. Change of temperature results as a change of length, tension and ultimately sag. This relationship has been described in CIGRE-s brochure 324. By implementing the described methods three versions of a fortran90 computer program have been written. The reason behind three versions is a superior computer resource management. In the end a skewed span has been exposed to three different currents and a plot was made vividly visualizing the shape of different catenaries. Key words: current rating, maximum temperature, high voltage line sag and tension calculation