An evacuation order has been issued for an area home to more than 82,000 people, just east of Los Angeles.
The Blue Cut fire, as it has been named, has destroyed homes and disrupted transport links between California and Nevada.
Some people have been running for their lives just ahead of the flames.
The blaze first ignited on Tuesday in a drought-ravaged mountain pass and a day later it had spread across nearly 47 square miles.
The flames have advanced, out of control, despite the efforts of 1,300 firefighters.
Commander Mike Wakoski said he had never seen such extreme conditions in 40 years of service.