The California Highway Patrol responded to a report of downed lines on the highway, also known as Jameson Canyon Road, just east of state Highway 29, at about 2 p.m., CHP Officer Peter Van Eckhardt said.
PG&E spokesman Jason King said it appears a third-party construction crew working on a Highway 12 widening project accidentally clipped a power line with one of its machines, causing the line to fall.
The highway was closed from Kirkland Ranch Road in Napa County to Red Top Road in Solano County, according to Caltrans.
As of 4:41 p.m., the roads were reopened, Van Eckhardt said.
PG&E crews have de-energized the power line before removing it from the road, King said.