PJM, which operates the biggest power grid in the US, has approved $1.25bn in transmission upgrades including rebuilding heavily-used transmission lines from West Virginia to Maryland.

The $320 to $370m rebuild of the 500KV line between the Mt. Storm substation in West Virginia and the Doubs substation in Maryland will increase the capacity of the line by more than 60% by improving electrical clearance and adding conductors.

In addition, the 277-mile, 765kV Potomac-Appalachian Transmission Highline (PATH) between West Virginia and Frederick, Maryland, has a required in-service date of 1 June 2015 for $2.1bn.

PJM senior vice president operations Michael Kormos said that after reviewing several options, the PJM board reaffirmed the need to move ahead with the PATH project.

The approval for upgrades and addition is part of PJM’s 15-year Regional Transmission Expansion Plan amounting to a total of $3.92bn.