Percy B. Manfield, 84, a retired electrical engineer for the federal government who helped design the Pentagon, died of congestive heart failure Sunday at his home in Falls Church.

Mr. Manfield, who used his initials "P.B." as his nickname, moved to Arlington from his native Jamestown, N.Y., in 1939 after joining the Navy Bureau of Yards and Docks. He retired in 1961 after a variety of assignments that took him to Korea, Italy and the Hudson Bay region of Canada. He moved to Falls Church last year.

After the Korean War, Mr. Mansfield was the manager of the Seoul Electric Co., which provided electric power and transit service to that nation's capital. During his two years there, he helped rebuild Seoul's badly damaged streetcar system.

After graduating from Mechanics Institute of the University of Rochester, he joined the General Electric Co. locomotive works in Erie, Pa., where he helped develop the first diesel-electric engines. Before coming to the Washington area he operated a factory motor design and installation business in Jamestown.

During his government career, he helped design engineering features for the Pentagon, spent the winter of 1946 working on the shore of Hudson Bay on Project Frosty, which tested the winter hardiness of Army mechanical equipment, and helped create the electric power supply system for an installation in Brindisi, Italy, that was set up to track Russian satellites.

Mr. Mansfield was a charter member of the National Society of Professional Engineers and past president of its Northern Virginia chapter, and was a member of the Society of Military Engineers.

Survivors include his wife, Ethel R., of Falls Church; a son, James R., of Windsor, Conn.; a daughter, Katherine Folk of Rockville; nine grandchildren and six great grandchildren.