Employes at the Potomac Electric & Power Co. have voted overwhelmingly to throw out their independent union in favor of the International Brotherhood of Electrical Workers.

Preliminary election results, which must be certified by the National Labor Relations Board, show Pepco workers voted by a better than 3-to-1 margin to be represented by the AFL-CIO union. The 3,600 Pepco workers involved in the election are currently represented by the Independent Electric Utility Employees Union.

The vote count, announced yesterday, gave the IBEW 1,611 ballots, the EUEU 938, and the Utility Workers Union of America (UWUA) 93. Another 24 workers voted for no union at all.

The Pepco vote is the biggest bargaining unit election won by an AFL-CIO union in Washington, said Michael Lucas, director of organizing for the IBEW.

An even bigger contest between an independent union and an AFL-CIO backed rival is under way at the Woodward & Lothrop department stores where the Retail Clerks International Union is seeking to replace the Independent Union of Woodward & Lothrop Employees.

Pepco's contract with the independent union expires in May, but negotiations have been held up because the rival unions sought to represent the company's employes.

In granting Pepco a $14.1 million dollar rate increase yesterday, the Maryland Public Service Commission assumed Pepco workers would get a 6 percent increase in wages and benefits.

Lucas said wages were one of three major issues in the Pepco vote. "Washington has the highest cost of living" in the area served by utilities in the Pennsylvania-New Jersey-Maryland power pool and Pepco pays "the lowest wages and the lowest fringe benefits" of any company in the pool, he added.

But Lucas claimed any wage increases for Pepco workers would have "substantially no effect" on electrical rates paid by Pepco customers. Compared to the cost of power plants and fuel, labor is a small factor in utility bills he added.

The IBEW has more than 900,000 members nationwide and represents all but two companies in the power pool, a group of interconnected utilities.

He said the other deciding factors in the vote were the ineffectiveness of present grievance procedures for Pepco workers and the relative abilities of the three unions to represent workers at the Washington power company.

The union election covered about 970 weekly-paid office and clerical workers and about 2,400 hourly paid workers in Pepco plants and power line crews.

Pepco yesterday reported its profits for the first three months fell substantially. Net income dropped to $12.5 million from $15 million, and earnings per share of common stock fell to 23 cents from 31 cents.

Chairman W. Reid Thompson predicted the company's earnings for the remainder of the year "will be favorably affected" by the Maryland rate increase announced earlier in the week, which is expected to add about $1.50 a month to electric bills in that state.

Thompson complained, however, that the company has been waiting for more than two years for the District of Columbia Public Service Commission to act on a rate increase request.

The company said its operating revenue for the first three months of the year totaled $173 million, up from $161 million a year earlier.

Pepco's directors also declared the company's regular quarterly dividend of 33 1/2 cents per share payable June 29 to stockholders as of May 30.