The blaze, in Lake County north of San Francisco, has destroyed 24 homes and displaced about 12,000 residents.
It is just one of 21 fires across the drought-ridden state being battled by about 9,000 firefighters.
California is in the fourth year of a severe drought which has left the state vulnerable to brushfires.
"This is a very fast-moving wildfire," said Daniel Berlant, a spokesman for the California Department of Forestry and Fire Prevention, or Cal Fire.
The rapid expansion of the fire over the weekend represented an "unprecedented growth in that short amount of time", Mr Berlant said.
The Lake County blaze now threatens a further 6,300 buildings and has forced the closure of two state highways, according to Cal Fire.
Some 2,700 firefighters - nearly a third of the state's total force - are tackling the fire but officials said only 5% of it had been contained.
"We're certainly stretching our resources," Cal Fire Chief Ken Pimlott told CNN on Sunday, adding that National Guard troops had been mobilised.
Thousands of lightning strikes in recent days contributed to the huge number of major fires across the state, Mr Pimlott said.
California Governor Jerry Brown on Sunday declared a state of emergency across the California.
A separate blaze that killed a US forest ranger on Thursday near the Oregon border has also expanded, but remains a fraction of the size of the Lake Country fire.
David Ruhl, 38, died in the Frog Fire raging through Modoc National Forest.
A wildfire raging through northern California more than doubled in size on its fifth day on Sunday, expanding to 84 square miles (220 sq km).