Photos: Battling the Electra fire in Northern California.

A wildfire burning 50 miles southeast of Sacramento in the Sierra Nevada foothills had exploded by Wednesday to 3,900 acres, prompting evacuation orders for thousands of people and injuring an emergency medical worker.

Named the Electra fire, the blaze started on Monday near the Mokelumne River and is burning through an area that was also scorched in 2015, said Chris Vestal, a spokesman for the California Department of Forestry and Fire Protection, which had contained 10 percent of the blaze by Wednesday afternoon.

Smoke from the fire is expected to spread across parts of Northern California and beyond Reno, Nev., according to the National Weather Service, which warned that air quality could deteriorate.

The fire slowed overnight on Tuesday, but, Mr. Vestal said, “There is still very much a threat.”