Fire crews were dispatched at 5:08 p.m. Sunday to Mullinix Packages at 3511 Engle Road, near Ardmore Avenue.
About 100 employees safely evacuated the one-story building before the fire department arrived, the department said.
Firefighters found a fire in high-voltage electrical equipment, which ignited some material stored nearby.
They waited for an Indiana Michigan Power crew to arrive and shut down the power before extinguishing the fire.
The buildings sprinkler system kept the fire from spreading, the department said.
The blaze caused moderate damage, and no injuries were reported.