Otter Tail Corporation is an energy company based in Fergus Falls, Minnesota. Its main subsidiary is the Otter Tail Power Company. As of 2007, Otter Tail Power Company serves at least 423 towns at retail and delivers power to about 14 municipal utilities. The company currently has a workforce of over 750 employees, a generating capacity of 660 megawatts, and owns over 5,000 miles (8,000 km) of electrical power transmission lines (the majority of which are operated at 41.6kV). The company serves 128,500 customers in North Dakota, Minnesota, and South Dakota. The company was incorporated in 1907 when funds were secured to begin construction of the Dayton Hollow Dam southwest of Fergus Falls. Once the dam came online in April 1909, the company transmitted power at 22kV over a 25-mile (40 km) line to serve the customers of the Northern Light Electric Company at Wahpeton, North Dakota.