In power cable, thermal factor must be considered where heat generated in power cable must be limited and lower than thermal rating of insulation materials. Overheating on cable insulation material will cause the electrical insulation aging is accelerated. This paper is present the result of thermal incremental test of the medium voltage XLPE cable type N2XEBY with three core and analysis when time constant come. From the test, temperature rise data is obtained transient condition to steady-state condition and time constant. A cable loaded with constant alternating current in one of its core, and measured the temperature of conductor (Tc), insulation material(Ti) and the outer surface of the cable (Tp). Base on the test, Conductor is faster than insulation material and surface of cable to reach to thermal time constant, but They have same typical of graph. When current is raised from 90 A to 300 A, time constant is decrease and increase depend of current value and typical for all material. In this paper shown predict corelation of the steady-state temperature and time constant, then empiris equation are developed by step function.