Line overheating:

A severe mismatch between load and transmission line can cause another problem: physical destruction of line!

A feed line should be able to handle a kilowatt (1 kW) of the power when it is perfectly matched. But if the mismatch exists and you feed 1 kW into the line, the extra current at current loops can heat the conductors to the point where the dielectric material melts and line shorts out. It is possible for the voltage at voltage loops to cause arcing between line conductors. This perforates and burns dielectric, ruining the line.

When a line should be used with a mismatch, derating functions are to determine how much the power line can handle safely. Manufacturers of the prefabricated lines can supply you with the information.