Between 1967 and 1974, the dam was expanded to add the Third Powerplant, with architectural design by Marcel Breuer. Beginning in , this involved demolishing the northeast side of the dam and building a new fore-bay section. The excavation of of dirt and rock had been completed before the new long section of dam was built. The addition made the original dam almost a mile long. Original designs for the powerhouse had twelve smaller units but were altered to incorporate six of the largest generators available. To supply them with water, six diameter penstocks were installed. Of the new turbines and generators, three 600 MW units were built by Westinghouse and three 700 MW units by General Electric. The first new generator was commissioned in 1975 and the final one in 1980. The three 700 MW units were later upgraded to 805 MW by Siemens.