TUOLUMNE CITY, Calif. — Crews are finally gaining ground on a massive wildfire burning near Yosemite National Park as fire officials expressed optimism even as the blaze grew larger while containment jumped to 20 percent.

As flames lapped at the edge of the main reservoir that supplies San Francisco, fears the inferno could disrupt water or power to the city diminished.

“It looks great out there. No concerns,” Glen Stratton, an operations section chief on the blaze, said of the Hetch Hetchy Reservoir.

Nearly 3,700 firefighters battled the roughly 252-square-mile fire, the biggest wildfire on record in California’s Sierra Nevada.

Weather conditions forecast for Wednesday may bring challenges in the morning as heavy smoke settles low to the ground, limiting visibility, but higher humidity was expected in the afternoon which could help dampen the flames, said Matt Mehle, a National Weather Service meteorologist assigned to the fire.

Crews remained confident they could protect hydroelectric transmission lines and other utility facilities at the reservoir, the chief source of San Francisco’s famously pure drinking water.

“I don’t foresee any problems,” Stratton said.

Utility officials monitored the basin’s clarity and used a massive new $4.6 billion gravity-operated pipeline system to move water quickly to reservoirs closer to the city.

So far the ash that has been raining onto the reservoir has not sunk as far as the intake valves, which are about halfway down the 300-foot O’Shaughnessy Dam. Utility officials said the ash is non-toxic.

Power generation there was shut down last week so firefighters would not be imperiled by live wires. San Francisco is buying replacement power from other sources to run City Hall and municipal buildings.