Why do power lines sag on hot days?
In hot weather, power lines can overheat just as people and animals do. The lines are often heavily loaded because of increased power consumption, and the conductors, which are generally made of copper or aluminum, expand when heated. That expansion increases the slack between transmission line structures, causing them to sag.
Transmission lines are designed to meet the requirements of state electrical codes. State codes provide minimum distances between wires, poles, the ground and buildings. Industry standards are often more strict and are incorporated in transmission line design, construction and maintenance. As a precaution, no one should be on an object or in contact with an object taller than 15 to 17 feet while under a high-voltage line.