SAN FRANCISCO -- Construction of a 53-mile, $500 million power line that snakes under the San Francisco Bay into the city here was completed yesterday, providing electricity for the first time from cleaner sources of energy to the north.

The Trans Bay Cable, which extends from Pittsburg, Calif., on the east side of the bay under the Bay Bridge, is capable of meeting as much as 40 percent of this city's peak power demand. The line provides access for the first time to wind power projects that until now have been inaccessible.

The 400-megawatt high-voltage cable, built quickly on a five-year timetable, also means the city will be able to shut down its sole remaining power plant, the Potrero Generating Station owned by Atlanta-based Mirant Corp. The plant, fired by natural gas and diesel, is among the dirtiest in the state and has long been considered an unfortunate irony among urban residents who consider themselves among the greenest in the state.

The project was started in September 2005 with the idea that the Potrero plant could finally be taken offline following years of public outcry. A spokeswoman for the California Independent System Operator said that vision remains in place, though Mirant still has to file the necessary paperwork to close down the facility while the ISO continues to monitor Trans Bay to make sure it is working properly.

"We are watching the operational success of Trans Bay," said Stephanie McCorkle, director of communications at the ISO.

McCorkle explained that Potrero, which discharges water directly into the bay, is still on the ISO's list of "reliability must-run" (RMR) plants that are able to ramp up at a moment's notice if the power is needed. But she expects the agency to drop the RMR designation once Mirant moves to terminate its contracts with utility regulators, a process that could be finalized as soon as February.

"The ISO will likely remove Potrero from our list but that has not been done as of yet," she said.

As for the power flowing from the north, McCorkle described it as "substantially cleaner" even if the state's power grid is really one big mass of swirling electrons, much of it powered by natural gas in the region. Officials at Pattern Energy, the company that built the line, noted that wind energy projects in the Altamont Pass and other spots to the north and east of the city will bring cleaner energy and improve congestion.

"San Francisco is on a peninsula, so until now it had to create its own power or get it from the south," said Matt Dallas, a spokesman at Pattern Energy. "Now we have opened up a new avenue for the first time."

The cable itself was buried under the seafloor with a hydro plow on what Dallas described as a "phenomenal" timeline considering the extensive environmental permits needed to expedite the project. That the project was able to move so fast testifies to the political consensus behind removing the Potrero plant and ending the city's relative isolation from the state's power grid.

The line is owned and operated by SteelRiver Infrastructure Partners, which hired Pattern Energy to manage construction. Michael Cyrus, chief operating officer of SteelRiver, said the undersea line should help lower energy costs, relieve congestion and improve reliability through the Bay Area.