Question

why do telephone wires sag in summer??

Answer 1

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.Which causes them to sag.

Answer 2

In physics there is a coefficient of thermal expansion that describes how a conductor will change with a rise (or reduction) with temperature.
The "sag" that is seen in the summer is factored in as a part of the design. If this was not designed the Copper Wire would "snap" (break) in the Winter.

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