Edward P. Johnson, 90, a retired chairman and chief executive officer of the E.C. Ernst Co., an electrical contracting company whose projects included work on the U.S. Capitol and other federal buildings, died of pneumonia May 22 at the Baptist Hospital in Miami.

Mr. Johnson, a resident of the Washington area for more than 40 years, had lived at the Ocean Reef Club in Key Largo, Fla. He moved to Florida in 1972.

A native of Ellenville, N.Y., Mr. Johnson began his career as an electrician, and helped found the International Brotherhood of Electrical Workers. About 1929 he moved to the Washington area, and one of his first jobs here was that of foreman on construction of the Shoreham Hotel.

About 1931, he joined the Ernst Co., and eventually made it one of the largest electrical contractors in the country. In addition to working on the Capitol, it performed work on the House and Senate office buildings and Walter Reed Army Medical Center.

The company also did work for the National Aeronautics and Space Administration at Cape Kennedy, Houston and other sites in the United States. Mr. Johnson received a commendation from President Nixon for the firm's contributions to the space program. In addition, the Ernst company had contracts in Saudi Arabia and other countries.

Mr. Johnson was a principal owner of the business and at different times served as secretary-treasurer, president and chairman and chief executive officer. He retired from the latter two positions in 1978.

Mr. Johnson was a former president of the National Electrical Contractors Association, and was a trustee of the IBEW.

He was a Mason and a Shriner of the Almas Temple and a member of the Congressional Country Club, the Georgetown Club and the Atlantic City Country Club in Atlantic City.

His first wife, the former Grace A. Atkinson, died in 1962. His second wife, Helen T. Johnson, died in 1968.

Survivors include his wife, Nancy Johnson of Key Largo; a son by his first marriage, Edward P. Johnson Jr. of Potomac, and three grandchildren.