A system for determining the current carrying capability of one or more overhead power transmission lines monitors one or more spans of each line on real-time basis and identifies the span having the lowest current carrying capacity which in turn establishes the maximum capacity of the entire line. The thermal state of each monitored line span is determined by measuring the conductor temperature, line current, solar radiation, ambient temperature, and in some cases wind speed and wind direction. These parameters are montiored by a sensor-transmitter unit that may be removably clamped on the line conductor which may range in size from one to several inches in diameter, and includes a radio transmitter for transmitting sensed data to a receiving substation. The data from the sensor-transmitter is multiplexed and transmitted by a telecommunications link to a computer which automatically determines line capacity using the real-time data and also calculates the time required for the "critical span" having the lowest current capacity to reach its maximum safe temperature based on any of a number of step changes in load demands. Each sensor-transmitter may include sensors for monitoring the current level, conductor temperature, solar radiation impinging on the conductor, ambient-temperature, wind direction and velocity and conductor sag.