Nowadays, air is the popular use for insulation, for example in power distribution system, power cable, arrester, because of the cheapest insulation. The drawback of this insulator is the humidity, heating, chemical behavior between air and conductor. Another important, the insulator must be design for the rated voltage, good cooling system. The characteristic of air in the normal condition, it can defined the permittivity value as the constant value (r = 1), on the other hand (wet condition) it defined to r = 0.8 or lower which depends on dielectric loss. It can calculate the breakdown voltage of air insulation as equation (1)