The great impact of renewable energy sources, which are expected to dominate in the energy supply in a future as well as the need of the construction of new substations becomes an actual topic in last years, resulting in a significant problem – there is a limited throughput capacity of the transmission systems. Consequently, new electrical connections and an existing power grid expansion will be required both from a national and international perspective. There are several solutions for increasing the capacity of existing lines, for example, replacement of the traditional type conductor (ACSR) with the High-Temperature Low Sag (HTLS) conductor; replacing the existing conductors by ones with a larger cross-section; rebuilding of the overhead line for a higher nominal voltage; using the installation of series and shunt compensations; new line construction. However, an economically justified way would be to use existing infrastructure as far as possible led to the accommodation of new technologies into the existing overhead power line systems. Paper provides a methodology, which analyzes the mechanical and thermal limitations of transmission line based on the selected climate conditions with the main purpose – to determine the hidden capacity reserve of an examined transmission line. Therefore, the possible additional clearance was determined based on the presented conductor temperature and load current calculation algorithms, which help to evaluate the specific transmission line capacity reserve, while improving the accuracy of the mathematical model. The study is based on a specific Latvian power overhead line model, which is an important energy object both from a national and international perspective. Based on the selected line route, two estimation variants will be disscussed – before and after the reconstruction of a power line. Thus, based on the proposed variants a comparative analysis of a conductor temperature and maximum load current results of an overhead line was obtained. The obtained calculation results are presented in detail in a graphic form.