Sonoma County plans to sue Pacific Gas and Electric Co. over last fall’s deadly Wine Country wildfires, as the utility battles dozens of suits seeking to hold it responsible for the blazes.

The county’s Board of Supervisors voted 5-0 in closed session Monday to pursue a suit that could demand tens of millions of dollars from PG&E to cover cleanup costs, lost tax revenue and damage to public parks.

The vote comes even though the California Department of Forestry and Fire Protection has not yet named a cause for any of the fires that broke out across Northern California on Oct. 8, killing 45 people.

Cal Fire’s chief said last week that determination could come within months, although the department has no set deadline. But Cal Fire officials have listed power lines, blowing in a fierce windstorm, as one of the possible causes under investigation.

“The due diligence we’ve done, that the attorneys have done, has given us reasonable cause to believe that the fires did originate with PG&E equipment,” said Sonoma County Counsel Bruce Goldstein. “If something comes out conclusively the other way, we’d have to re-evaluate, of course.”

The county plans to work with California Fire Lawyers, a coalition of law firms.

“There has been no determination on the causes of the fires, and we remain focused on doing everything we can to help Sonoma County recover and rebuild,” said PG&E spokeswoman Ari Vanrenen. “Our thoughts are with everyone impacted by these devastating wildfires.”

James Gore, chairman of the Sonoma County Board of Supervisors, said his colleagues did not take this step lightly, and he called PG&E “great partners” for the county.

“We’re not trying to jump on some ambulance-chasing bandwagon,” Gore said. “At the same time, you have to protect your rights. And there’s a lot of evidence that does tie to power lines.”

PG&E reported to state regulators finding damaged equipment at the suspected ignition points for all four of the largest Wine Country fires. And a Chronicle review of emergency dispatch logs from the first night of the fires found that the utility was notified of sparking or falling equipment in at least 54 locations in Sonoma County alone.

Goldstein said the county could face up to $25 million in cleanup costs, money the county will seek from PG&E. Gore said the board also wants PG&E to pay for moving many power lines underground, to help prevent fires.

PG&E faces more than 50 lawsuits from North Bay residents blaming the company for the fires, which caused an estimated $10 billion in damage. It considers the financial threat significant enough that in December, it suspended dividend payments to shareholders for the first time since the utility declared bankruptcy in 2001.

David R. Baker is a San Francisco Chronicle staff writer. Email: dbaker@sfchronicle.com Twitter: @DavidBakerSF