Transformers are one the most important facilities in electrical power systems where their operations are the main role in provides reliability of the systems themselves. Because of the cost of transformers and their permanent connections to the transmission and distribution systems, any improvement is needed to extend the life span of the transformers. Insulation life of the transformer and loading capabilities depend on several parameters i.e. cooling mood, ambient temperature, oil viscosity and, etc. Hot spot temperature has played the most effective factor on the insulation life of the...