Electric customers in Maryland and District of Columbia will have to subsidize consumers served by the disabled Three Mile Island nuclear power plant in Pennsylvania, federal energy regulators ruled yesterday.

Setting aside long-established principles governing the price utility companies charge each other for power, the Federal Energy Regulatory Commission decided that other companies can sell power to the owners of the Three Mile Island reactor at reduced rates.

The decision will cost customers of Potomac Electric Power Co. and Baltimore Gas & Electric Co. about $8 million a year, FERC said. Pepco told the D.C. Public Service Commission last January that the discount would cost each customer about $2 to $4 a year. l

District of Columbia People's Counsel Brian Lederer, who raised the only objections to the plan, said the FERC decision sets a precedent for making consumers pay, for the problems of power companies hundreds of miles away.

"It means we're going to socialize the cost of the Three Mile Island accident throughout the power pool" that serves Pennsylvania, New Jersey, Maryland and the District , Lederer complained.

Lederer also criticized the federal agency for accepting a plan drawn up in private by a group of power companies without public hearings.

The unanimous ruling of the five-member commission, said, however, that the decision "will not establish principles or precedent or control future proceedings." FERC said, in effect, that there was no need for public hearings because the utilities had settled the dispute among themselves.

The issue arose when General Public Utilities asked for a discount on the price of the electricity it has been forced to buy from other companies since the accident at its Three Mile Island reactor in March 1979.

GPU is a member of the Pennsylvania-New Jersey-Maryland (PJM) power pool, along with Pepco, Baltimore Gas and two Pennsylvania utilities. pThe power pool members routinely buy and sell electricity among themselves.

The system is meant to save money for the companies and their consumers. For example, it's cheaper for Pepco to buy a small amount of power from the pool than to start up a another generator when only a fraction of the generator's output is needed.

Usually the companies charge each other for power on what is called a "split-the-savings" basis. The price is halfway between what it would cost the seller to generate the power and what it would cost the buyer to generate the power.

The cost of power purchased from the pool is passed directly to consumers through the fuel-adjustment clause on electric bills. Any profits made on pool sales are also passed back to customers.

Usually pool members buy about as much electricity from the pool as they sell to it. But since the TMI accident, General Public Utilities has been forced to buy huge amounts of electricity.

To save money, GPU asked the other utility pool members to give it a discount and sell their power for 10 percent more that it costs to generate. The discount amounted to $18 million a year.

The $18 million savings normally would have been passed on to GPU's customers. But under yesterday's ruling, the money will come out of the pockets of the customers of companies selling the power, who otherwise would have paid lower electric bills.

Without the discount, Pepco customers would benefit from a $6-million-a-year profit on that company's sales to GPU, and Baltimore Gas BG&E'S consumer bills would have been $2 million a year smaller.

General Public Utilities last March asked FERC -- which regulates wholesale power rates -- to approve the change in the power pool pricing. The other members of the pool then filed a "settlement agreement," accepting the request.

The FERC decision yesterday accepted the settlement.

D.C. People's Counsel Lederer had urged FERC to look deeper into the issue. He said yesterday he may try to overturn the decision in court, or ask the D.C. Public Service Commission to challenge it.