An overhead power line is a structure used in the electric power system to transmit electrical energy. The performance of overhead power lines depends on their parameters. An important parameter of the power line in the power system is its thermal limit. This article deals with the temperature calculation of overhead power line ACSR conductors according to the methodology stated in CIGRE Technical Brochure 601: Guide for thermal rating calculations of overhead lines. The calculated temperature is also compared with the measurement under laboratory conditions and also on a real power line in the Slovakia power system. At the end of the article, the dynamic thermal rating of ACSR conductors depending on climatic conditions is also calculated. Obtained results are compared with ampacity limits for summer and winter season used by Slovak transmission system operators. The article also compares the CIGRE Technical Brochure 601 with its older version CIGRE Technical Brochure 207: Thermal behavior of overhead conductors.