Pepco Holdings Inc. (POM) will delay the completion of a high-voltage transmission project by one year and scale back its length as power demand slumps.

Pepco's Mid-Atlantic Power Pathway, or MAPP, will go into service in June 2014 instead of 2013. The high-voltage line will run 150 miles from northern Virginia, across southern Maryland, to near Millsboro, Del. The announcement Tuesday follows a recommendation by the PJM Interconnection, which runs the power grid in the Mid-Atlantic states and parts of the Southeast and Midwest.

Pepco also said Tuesday it's reducing the size of the transmission project, eliminating a proposed section from near Millsboro to Salem, N.J. The estimated cost of the project will drop to $1.2 billion from $1.4 billion.

The MAPP project is the second major transmission project in PJM to face delays because of slumping demand. Last month, American Electric Power Co. (AEP) and Allegheny Energy Inc. (AYE) announced a one-year delay in their proposed transmission project from southwestern West Virginia to central Maryland.

-By Mark Peters, Dow Jones Newswires; 201-938-4604; mark.peters@dowjones.com