The rating of an overhead line is limited by the maximum temperature it is permitted to reach. This temperature is chosen at the design stage along with the size of conductor, the height and spacing of the towers and overall optimisation of the design to meet the required balance between capital and revenue costs. Outside the control of the designer is the complex and variable cooling effect of the weather on the conductor. Fortunately the three basic parameters of wind, sun and air temperature do not each reach their extreme values for minimum cooling simultaneously. Hitherto, the degree of noncoincidence of these extreme values has been difficult to quantify owing to a lack of data on wind speeds below 1 m/s. Research work at CERL has provided comprehensive data on the incidence of low wind speeds, together with simultaneous values of wind direction, insolation and ambient air temperature. The temperature of a typical transmission conductor (400 mm2ACSR, Zebra) carrying full-load current was also recorded. The variations in this conductor temperature reflected the varying cooling effect of the weather parameters. The range over which the conductor temperature varied was divided into 5 deg C intervals, and the percentage of time that the conductor temperature was in each interval was calculated. The data was divided into three seasons for the purpose of the analysis. A relationship was then established between the time that the conductor exceeded any given design temperature and the ratio of the heat input under test to the heat loss that would occur under design conditions. This relationship was found to provide a correlation of the data for the three seasons, which was then used with existing heat transfer data to predict the rating of any conductor for any chosen design temperature and selected percentage of total time that the conductor may exceed that temperature. In practice, system design and security standards restrict actual continuous loadings, and lines are generally operated at a fraction of their full thermal rating. This permits the selection of higher continuous ratings on a statistical basis. Applying the same statistical basis to the conditions prior to and during a fault condition permits an improvement in the overload rating. The overload approach permits the selection of continuous and overload ratings for overhead lines on a statistical basis, which is determined by the various system design and security standards and the cooling effect of the weather.