With their contract expiring next Monday, Potomac Electric Power Co. and Local 1900 of the International Brotherhood of Electrical Workers are far from agreement over a new contract covering 3,500 workers.

Union officials say Pepco is demanding major give-backs, including eliminating annual cost-of-living adjustments from the new contract and requiring employes to take their first six days of sick leave without pay.

Neither side is predicting a strike, but Pepco has contingency plans to keep the utility running with supervisory employes if the union workers walk off the job.

The union, however, has yet to call for a strike vote, said John W. Mitchell, international representive of the IBEW and the union's chief negotiator.

"We haven't decided when to call for a vote," Mitchell said. "It depends on the progress made." He added that he was not optimistic about the negotiations but indicated a willingness to "stay most of the night."

Pepco has moved house trailers into some of its power plants to house employes in the event of a strike. Because most power-generating equipment is automated, it can be run by supervisory employes. The company, however, might have trouble restoring power if spring thunderstorms knocked out power lines.

Representatives for labor and for management said yesterday there has been little or no progress in contract negotiations, which have gone on for more than three months.

Mitchell said the union had requested the aid of a mediator, but the company had refused twice. Pepco representative Nancy Moses insisted Pepco is "willing to sit down with a mediator at any time."

The dispute involves 3,500 of Pepco's 5,400 workers. The average hourly wage of these employes is $9.76.

Pepco representatives said the union has not yet made its wage demands known, but negotiations are hung on other issues.

The IBEW said only eight minor items out of 160 proposals had been settled. Responded Pepco: "We're not keeping score."

Moses refused to comment on specific issues, but Mitchell said the union opposes Pepco proposals for elimination of the overtime list by which overtime is rotated among employes; elimination of the first six days of paid sick leave, ending of the "right of past practice," which makes prevailing working conditions a part of the contract work rules; abolition of the cost-of-living allowance for the next three years and elimination of all job descriptions.

Pepco had a company union until three years ago when IBEW organized its workers. Mitchell, who negotiated the 1979 contract, accused the company of "stonewalling" on the contract renewal talks.