Thousands of people ordered to evacuate in the face of the growing Fairview Fire, which has now spread to 7,700 hectares, officials say.
A wildfire burning outside Los Angeles has doubled in size in less than 24 hours, firefighters said as they endured yet another day of blistering heat in the western United States.
"Active fire behaviour occurred in the east, south, and north portions of the incident," Cal Fire said on Thursday.
"Fire will become active with primary movement to the east. Steep drainages will aid the spread with the up canyon/upslope winds and heavy fuels."
Two people are known to have died in the blaze, which erupted on Monday, the midpoint of a heat wave that has lasted more than a week and seen temperatures repeatedly exceed 43 Celsius in parts of California, Nevada and Arizona.
Thousands of people have been told to evacuate in the face of the growing Fairview Fire, which has now spread to 7,700 hectares.
The searing heat has put enormous strain on California's electrical grid, as households crank up the air conditioning.
That has led to daily calls for consumers to conserve power to avoid blackouts, including text alerts sent to mobile phones.
The grid operator has hailed public cooperation which it said has helped keep the lights on throughout the state all week.
Another "Flex Alert" was issued on Thursday, asking for thermostats to be raised, and major appliances to be switched off during peak hours.
Thursday's call was of longer duration than previous alerts, with a request to conserve power between 3:00 pm and 10:00 pm -- two hours longer.
"The state and much of the West is enduring a historically long and record-breaking heatwave, straining the grid from high electricity use," California Independent Service Operator said.
"The Flex Alert is in effect for extended hours because of projected supply deficiencies during that time."
Forecasters say the heatwave will begin to dissipate over the coming days but warned California was not out of the woods yet.
"Dangerous heat also continues to impact the Golden State as high temperatures are once again forecast to reach well into the triple digits both today and Friday, particularly over interior valley regions," the National Weather Service said.