The design of transmission lines (TL) is usually based on conservative aspects, in order to meet with safety standards and avoid the deterioration of line construction materials. With the increasing demand for energy, the idea of improving the transmission capacity of already existing lines, designed in a high safety manner, is attractive; instead of duplicating or constructing new ones. A piece of equipment for monitoring current and temperature in 138 kV transmission lines using photonic power and data communication is presented; providing information for a sag-current relation, and consequently a safer ampacity sizing, while a full operation of the line is being carried out. The power supply for the sensors and data processing circuits in high voltage area is provided by photonic energy, transmitted through an optical fiber. Likewise, the measured results are sent to the low voltage area using another optical fiber, achieving good electrical safety.