SAN FRANCISCO — Facing down 600 wildfires in the past three days alone, emergency workers rushed to evacuate tens of thousands of people in Southern California on Thursday as a state utility said one of its major transmission lines broke near the source of the out-of-control Kincade blaze in Northern California.

The Kincade fire, the largest this week, tore through steep canyons in the wine country of northern Sonoma County, racing across 16,000 acres within hours of igniting. Wind gusts pushed the fire through forests like blow torches, leaving firefighters with little opportunity to stop or slow down the walls of flames tromping across wild lands and across highways overnight.

And north of Los Angeles, 50,000 people were evacuated as strong winds swept fires into the canyons of Santa Clarita, threatening many homes.