The intensity of heating of the insulation material of loaded electric wires was investigated. Calculations were made for insulation materials made of polyvinyl chloride plastic and rubber, and copper and aluminum wires. The dependences of the heating temperature of the wires on the time of their operation at load currents smaller, close to and larger than the maximum allowable ones are plotted. The time during which you spend heated to critical temperatures, is individual for each brand-size. For wires with a copper core, heating occurs more slowly compared to wires with an aluminum core. Wires with polyvinyl chloride core insulation heat up more slowly than wires with rubber core insulation. In all cases, addiction temperature of the wires from the time of operation at different load currents has a logarithmic form.