There are about 3,000 customers without power, most in the Clements area of Limestone County, according to Athens Utilities.
The utility is asking customers to help conserve power by only using essential heat and light.
The Clements substation went down when an insulator malfunctioned because of so much current running through the substation.
Athens Utilities General Manager Gary Scroggins said the system set another record peak for demand at around 320 megawatts. One megawatt can power about 200 homes in Limestone County.