Electric power consumption has been growing continuously, especially it has been observed over the last years. In this way, the implementation of new advanced technologies such as overhead line thermal monitoring systems is one of the topical solutions for improving the existing transmission line network with the purpose of increasing the throughput capacity of the lines and the reliability of power supply. In general, these real-time monitoring systems are designed on the basis of the existing methodologies by using the limiting conditions for determining of the ampacity of high voltage overhead lines. Therefore, the evaluation of commonly used thermal rating estimation methods, which include computation of the conductor temperature as well as the conductor sag, is quite a topical issue that must be considered. The paper focuses on a comparative analysis of the measured and calculated steady-state conductor temperatures and line sagging, on the basis of which the testing of thermal rating methods was conducted. Furthermore, it is worth noting that the experimental measurements were conducted by using special monitoring equipment for examining three main cases. The experimental and computational results of this study are presented in a paper based on the existing line model of the Latvian transmission grid.