The present guide provides a general method for calculating the thermal rating of overhead lines. It is intended for updating and expanding the Cigré Technical Brochure 207, which only covered the thermal behaviour of overhead conductors at low current densities (<1.5 A/mm2) and low temperatures (<100ºC), and did not consider variations in weather conditions or current with time. In the recent years, various modeling improvements have been developed that take account of these time variations and