When California regulators in December adopted tough new rules to prevent power lines from starting wildfires, they based the rules on a map.

Years in development, the detailed map shows which parts of California face an elevated or extreme risk of wildfires. The higher the risk in a particular area, the tougher the rules that will apply to electrical utilities operating there.

Except the map isn’t quite finished.

The California Public Utilities Commission, which adopted the new fire safety rules in December, will vote Thursday on extending until July the deadline for completing the map, whose last draft was released by the commission in November. The current deadline is Sunday, Jan. 14.

By July, California’s next fire season will already have begun. But a commission spokeswoman said Tuesday that the regulatory agency expects to finish the map well before then, even if it can’t quite make the current deadline.

“We are likely just weeks away from completing the development and adoption of the (high fire threat) Map,” spokeswoman Terrie Prosper wrote in an email. “But since an extension is necessary, the typical extension is 6 months.”

The new rules govern how often utilities must inspect their equipment in the field, how far tree branches must be kept from electrical lines and how the companies prioritize safety-related repairs.

The delay in finishing the map, which the commission’s staff blames on the complexity of the task, is just the latest in the long effort to adopt and enforce new fire safety regulations for utilities.

That effort began after a devastating series of wind-driven fires, sparked by power lines, ravaged San Diego County and other parts of Southern California in 2007. The commission finally approved the rules 10 years later, after deadly blazes struck the Wine Country in October and Southern California in December. Investigators have not yet determined the cause of most of last fall’s fires, though they are looking into whether power poles and lines impacted by high winds played a role.

“The PUC continues to blow through deadlines,” said state Sen. Jerry Hill, D-San Mateo, who will hold a hearing in Santa Rosa on Jan. 26 to discuss fire safety measures for utilities. If the map isn’t adopted until July, “We’ll miss another fire season, and God knows what’ll happen then,” Hill said.

Pacific Gas and Electric Co. is already planning its tree-trimming and field maintenance operations using the current version of the map as a guide, said spokesman Paul Doherty. That work, to prepare for next fire season, will likely get under way in mid-February, he said.

If the map changes, altering the safety standards that apply in different places, the utility can revise its plans accordingly, Doherty said. PG&E on Jan. 5 filed with the commission a letter, backed by utility San Diego Gas & Electric Co., recommending that the current version of the map be adopted. Anyone with objections, PG&E recommended, should file them by Jan. 15.

“We’ll continue to work with the commission and adjust our plans if need be,” Doherty said.

SDG&E and Southern California Edison also plan to base their maintenance work on the latest version of the map, representatives for both companies said.

The map has been developed by staff members of both the commission and the California Department of Forestry and Fire Protection, or Cal Fire, working along with representatives of the utilities and independent technical experts. They have studied wind speeds, vegetation types, seasonal patterns and the way different plants burn under different conditions.

An early version of the map drew complaints because it didn’t mark as high-risk an area of the Sierra Nevada foothills that had already suffered a massive wildfire: the Butte Fire of 2015. Finalizing it has taken longer than expected. The previous schedule extension was granted on Oct. 6, just two days before the Wine Country fires began.

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

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