The blaze, dubbed the Narrow Fire, was reported off state Route 78 near Highway S2 about 5:05 p.m. Wednesday, Cal Fire spokesman Mike Mohler said.
The fire posed no immediate threat to structures. Full containment was expected by noon today, according to Mohler.
Two tanker planes and two helicopters made drops on the fire until sundown Wednesday, while an estimated 400 firefighters worked to contain the blaze on the ground, Mohler said.
Staffing was reduced to about 200 firefighters overnight, Mohler said.
Inmate crews were among those called in to work on the steep ground where the active fire was burning, Mohler said.