Negotiators for Potomac Electric Power Co. and Local 1900 of the International Brotherhood of Electrical Workers yesterday reached tentative agreement on a new three-year contract.

The agreement will give "moderate" basic wage increases to Pepco's 3,500 unionized employes and will soften some union work rules opposed by the company, sources familiar with the settlement said.

Details of the tentative settlement, including the overall cost to Pepco and the effect--if any--on local electric rates, are being withheld until union members have voted on the new contract.

Local 1900's bargaining committee yesterday urged rank-and-file members to accept the agreement, reached at 12:30 a.m. Sunday after 14 hours of nonstop bargaining.

No date has been set for the vote on the agreement, but the union expects the ballot to be completed by the end of the week.

The union has been working without a contract since June 10, when Local 1900 officials voided an emergency provison that had extended the old agreement on a daily basis since it expired May 31.

Both union and company officials yesterday said they were satisfied with the outcome of the intermittent, often heated contract talks that began Feb. 15.

"We are extremely pleased to have a tentative agreement, which was reached without disruption of customer service," said Pepco spokesman William H. Jones Jr. He said the agreement was achieved "through the total commitment of both sides to the collective bargaining process."

Local 1900 officials refused to discuss details of the settlement, saying simply, "We have reached an agreement and are recommending it to our members."

The lengthy talks went nowhere until five weeks ago when both sides asked the Federal Mediation and Conciliation Service to aid in the bargaining. And the two sides had agreed on a series of "non-cash" items, including grievance procedures and health and safety measures at Pepco plants, but were still arguing over other points when the old contract expired.

The company was pushing for changes in job classifications, work rules, and sick leave provisions, saying they were needed to increase operating efficiencies. The union accused the company of attempting to take away previous gains and sought a wage increase "substantially higher" than the 20 percent raise plus cost-of-living allowance provided in the old three-year contract.

At the termination of the old agreement, unionized Pepco workers were earning an average hourly wage of $9.76.

Asked if the union accomplished its goal, FMCS mediator Brian Flores said yesterday: "The fact that the bargaining committee is recommending approval of the contract means that they think the settlement is adequate in terms of what they could get right now."

After a six-week strike, IBEW workers at Public Service Electric and Gas Co. of New Jersey earlier this month won a nearly 20 percent increase in wages and benefits in a new two-year agreement.

Pepco had made extensive and costly strike preparations, including assigning more than 500 supervisors to work in critical jobs after Local 1900 formally terminated its contract. Many of the Pepco supervisors spent 13 days living in mobile homes at power plant sites when talks were stalled earlier this month.

Pepco serves 525,204 customers in the District of Columbia, Montgomery and Prince George's counties in Maryland and a small part of Arlington County in Virginia.