CALIFORNIA’S biggest wildfire on record is expected to burn for the rest of the month, as hot and windy conditions challenged thousands of fire crews battling eight major blazes burning out of control across the state.

The blaze known as the Mendocino Complex grew to span 117,639 hectares by Tuesday, with barely a third of it contained since two wildfires merged at the southern tip of the Mendocino National Forest, the California Department of Forestry and Fire Protection (Cal Fire) said.

It is the largest of the major fires burning out of control across California, prompting US President Donald Trump to declare a "major disaster" in the state.

The size of the blaze has surpassed that of last year's Thomas Fire, which burned 114,078ha and destroyed more than 1000 structures.

Nearly half of the 10 largest California wildfires on record occurred in the past decade.

The Mendocino Complex has burnt 75 homes and forced the evacuation of thousands of people.

Fire officials had hoped to extinguish the fire by mid-August, but pushed that date to early September on Tuesday.

Temperatures could reach 43 degrees Celsius in northern California over the next few days, with gusty winds fanning the flames of the fire, a National Weather Service meteorologist said.

About 3900 people battling the Mendocino Complex were focusing on keeping flames from breaking through fire lines on a ridge above four foothill communities, said Tricia Austin, a spokeswoman for Cal Fire.

Elsewhere in California, evacuations were ordered for some cabins in Orange County on Monday afternoon, after a blaze broke out to quickly engulf 283ha.

The Carr Fire, which has torched 67,582ha in the scenic Shasta-Trinity region north of Sacramento since breaking out on July 23, was 47 per cent contained.

The Carr Fire has been blamed for seven deaths, including that of 21-year-old gas company employee Jay Ayeta.

He was killed in a vehicle crash as he worked with crews in dangerous terrain.

Environmental activists and some politicians say the intensity of the state's wildfire season could be linked in part to climate change.