Potomac Electric Power Co. asked the federal courts yesterday to overturn a ruling by the Environmental Protection Agency that Pepco says will cost its customers $450 million.

EPA recently ordered Pepco to install a $50 million pollution-control device called a sulfur scribber on an oil-burning power plant now being built at Chalk Point on the Patuxent River in southern Prince George's County. a

Pepco Chairman W. Reid Thompson said yesterday the EPA decision is "based on a technicality" and imposes "enormous and unreasonable burdens on Pepco customers."

Not only will Pepco have to invest $50 million in scrubbers, it also will have to spend $100 million to buy low-sulfur oil for the plant until the scrubbers are installed and will have to spend $10 million a year maintaining the pollution-control gear, Thompson said.

D.C. People's Counsel Brian Lederer said Pepco customers wouldn't be facing the $450 million pollution bill if the power company had taken his advice and stopped construction of the Chalk Point plant or converted it to burn coal.

"It's another consequence of them building an oil plant," said Lederer. Lederer contends Pepco didn't need to build a fourth unit at the Chalk Point complex and that, if it had to build sometime, it should have built a coalburning power plant.

Lederer also accused Pepco of exaggerating the cost of the pollution-control gear to its customers. There's no need to buy low-sulfur oil until the scrubbers are installed because Pepco won't need the plant for several years anyway, he said.

The power company cited the high cost to customers in asking the U.S. Court of Appeals in Richmond to overturn the EPA decision. At issue in the dispute is whether the Chalk Point plant was already "under construction" on Aug. 17, 1971, when tough new EPA pollution standards went into effect. Plants already being built at that time were exempt from the new rules.