The transmission capacity (Ampacity) of OHL is determined by two key factors. The first is the maximum permanently permitted conductor temperature which is set at 80 °C for ACSR. The second is minimal allowed safety clearance from the conductor to the ground or object beneath which is defined by the safety standards and depends on the type of object and voltage level of the OHL. The first factor affects the plastic deformation of the conductor which is why, after a certain number of thermal cycles, it can lead to the rupture or slippage of the conductor from terminal clamps. The second factor has a considerable influence on the safety of the people and the environment from voltage flashovers.
Both factors can be affected by a number of physical quantities which can change instantly. Solar radiation, air temperature, speed and wind direction have a direct impact on the heating and cooling of the conductor. Considering that these physical quantities are very difficult to predict or even estimate, the methods for determining the maximum allowable transmission capacity of the OHL are very conservative.
