Negotiators announced tentative agreement on a new contract for employes of Potomac Electric Power Co. yesterday and talks are to begin today for a new labor agreement at the Woodward & Lothrop department stores.

At Pepco, a spokesman said the company agrees to terms of a threeyear contract with Local 1900 of the International Bortherhood of Electrical Workers, which represents about 3,400 utility company employes.

The Pepco pact would give employes a 7 percent wage increase retroactive to June 3, another 7 percent raise next June and a 6 percent increase the following year. A cost-of-living allowance could result in a large increase in the third year of the contract.

Under terms of the agreement, Pepco workers would no longer have to contribute to the company's retirement program and would get improved health and life insurance benefits.

IBEW members are expected to vote next week on whether to accept the contract proposal. A company spokesman said the total package did not exceed the Carter adminstration's wage guidelines.

Negotiations for the new Pepco contract began May 15 after the IBEW, in a National Labor Relations Board election, ousted the independent Electric Utility Employees Union, which had represented Pepco workers for 42 years.

The replacement of another local independent union with an affiliate of the AFL-CIO earlier this year set the stage for the talks that are to begin today between Woodward & Lothrop and Local 400 of the Retail Store Employes Union.

Local 400 in June defeated the 40 year-old Independent Union of Woodward & Lothrop Employes in a bitterly contested election. Woodies management had urged workers to reject both unions.

A union official said wages, fringe benefits and a demand for a union shop -- in which Woodies' workers would be required to join Local 400 -- will be key issues in those talks.

The bargaining at Woodward & Lothrop will be critical for the union, which says Woodies is its first target in a drive to organize Washington's general merchandise retailers.

The union reportedly is beginning organizing efforts at the Hecht Co. as the next step in its drive among local department stores.

Woodies is the largest private employer in the Washington are, and the union election in June was the biggest organizing drive ever held here.

Although the AFL-CIO electrical workers negotiated their initial contract at Pepco in only three months, the talks for the first Local 400 contract at Woodies are expected to take longer. The negotiations probably will extend into the heavy fall and Christmas selling seasons, when retail business is strongest.