In this paper, among the causes of electrical fires, the chemical properties due to thermal degradation of the wire coating, which is a polymer material, and the risk of ignition through temperature change by equivalent lifespan were analyzed. Insulated wires (HIV, 2.5SQ, single wire) used in small buildings under 500V were accelerated to deteriorate over time to produce samples for 0, 10, 20, 30, 40 years. Qualitative and quantitative analysis of insulators was performed using equipment such as strength, SEM, XRD, IR, TGA, and DSC. In addition, the temperature change by step above the allowable current was analyzed with an overcurrent tester, and through this, it was possible to confirm the risk of fire due to deterioration of insulation of old wires used in buildings. Through this study, it is possible to present the rationale for systematically introducing the replacement and repair period for old wires, and it is judged that it can be used as basic data for investigators to verify the cause of ignition due to insulation deterioration at the fire site.