More than 2,800 fire crews are fighting the Rim Fire in Yosemite National Park, which is threatening three hydroelectric power stations and the communities south of Highway 120 in Mariposa County.
The power stations serve San Francisco and two of the three have been taken off line and 12 miles of power lines were cut.
Governor Jerry Brown is warning residents that supplies of water and electricity may suffer as a result of the fire.
Crews have established "spike camps" deep in the forest, which allow them to deploy resources to areas needing them quicker and also provides protection using containment lines.
Efforts are also underway to save two groves of giant sequoias and the Hetch Hetchy Reservoir, which delivers 85% of San Francisco's water.
The fire is only 7% contained has cost nearly $8 million to fight so far.