The fire started when a dry branch from a eucalyptus tree was flung nine metres (30 feet) by high winds into a city Department of Water and Power line and caused sparks, authorities said.
So far, the winds have stayed away from the section of Los Angeles that burned Monday in the Getty fire, where there's concern that smoldering embers could be carried away by gusts and start new fires.
"Our focus was taking full advantage of those conditions overnight", said Margaret Stewart, a Los Angeles Fire Department spokeswoman.
The size of the Getty fire's evacuation zone was reduced by roughly 3,000 homes on Tuesday, but residents of about 7,000 dwellings remained displaced, fire officials said.
"The fire department has been doing a tremendous job with firefighters on the scene and water-dropping aircraft to keep our campus completely safe from the fires", the spokesperson said.
An "extreme red flag warning" is in effect Wednesday morning for much of Los Angeles County as firefighters worked to increase containment lines around the Getty Fire burn area in the Sepulveda Pass.
The findings, based on evidence from the site where flames erupted early Monday, came as fire managers and utility operators braced for another onslaught of fierce, dry desert winds expected to begin howling across much of California on Tuesday night.
An army of some 1,100 firefighters battled the Getty fire Tuesday in a narrow window of slower winds.
The library, which holds the presidential archives and whose grounds include the graves of Reagan and his wife, Nancy, was well-equipped when flames surrounded it.
The danger posed by the high winds was made painfully clear again Wednesday morning when a fast-moving brush fire broke out in Simi Valley along Tierra Rejada Road, south of the 118 Freeway and near the Ronald Reagan Presidential Library. "Many times, it depends upon where the ember lands", he said, The Los Angeles Times reported.
The wildfires spreading through the state come as California is still reeling from the aftermath of the most destructive wildfire in state history - the Camp Fire, which destroyed the town of Paradise and killed 86 people past year. The power lines remained intact, but likely arced and sparked brush.
"When you have cold air, it tends to sink, and when you have winds coming down the mountain, it causes it to accelerate", said Kristen Stewart, a meteorologist with the National Weather Service in Oxnard. By early Wednesday, crews had managed to contain about 15 percent of the blaze.
Electrical equipment has been linked to several of California's most devastating recent fires.
Winds topped out at 70 miles per hour (112 kph) north of San Francisco Bay and began to ease early Wednesday, but forecasters said the fire danger would remain high because of continuing breezes and dry air.
"Man these LA fires aren't no joke", he said.
Governor Gavin Newsom has accused utilities of failing to adequately modernize and safely maintain their power systems.
North of the San Francisco Bay, the week-old Kincade Fire - the state's largest active wildfire - has destroyed almost 77,000 acres across Sonoma County and more than 180 structures, including 86 single-family homes, officials said.