Electric utility regulators in Maryland, Virginia and West Virginia yesterday ordered a 10 percent reduction in electricity supplies to industries and businesses served by Potomac Edison Co., a small utility hit hard by the coal strike.

The cutback, which goes into effect at 12:01 a.m. Thursday, affects 240,000 customers in parts of eight Maryland panhandle counties, seven West Virginia counties and 10 northwest Virginia counties.

A 30 percent cut will take effect March 2 if the coal strike continues. Industries and businesses will be reduced to "plant protection levels" sufficient only to keep machinery from being damaged by the cold, if the situation continues to March 15.

"Beyond that, the next step to be considered would be rotating blackouts, but no potential date for these was agreed upon," the public service commissioners said in a joint statement. Residential customers, who have been urged in newspaper advertisements to conserve energy, would not be affected by the mandatory cuts until the blackout occur, according to Virginia State Corporation Commission spokesman John F. Daffron.

Enforcing the cutbacks, Daffron said, "is a very thorny problem." Businesses are under orders to cut back but penalties for failing to do so have not been set. Daffron said the formal order to be issued today might address the problem.

Utilities have essentially relied in the past on an honor system bolstered by informants and public pressure to obtain compliance.

John M. McCardell, executive vice president of Potomac Edison, had asked the commissioners for an immediate 30 percent cut in electricity consumption and expressed some doubt the lesser level could do the trick. The utility uses coal for 100 percent of its power generation and has only 24 days' supply left.

"With our plan it would have been done sooner. We'll have to see as we go along if this can do the job," he said.

The company has been buying 42 percent of its generating capacity from other utilities for several weeks in order to stretch out its coal supplies and can continue for 45 days under current demand and temperature conditions, McCardell said. However, any power outage in utilities supplying Potomac Edison could force them to reduce their power sales from the current high level. That makes the current situation unreliable and drastic measures necessary, McCardell explained.

In addition, Potomac Edison customers will face a "substantial" rate increase in order to pay for the borrowed power, which is produced by oil-fired electric plants that are more expensive to run than coal-fired ones.

The 10 percent reduction, McCardell said, "should be attainable without a great deal of hardship" on the part of stores and industries. In newspaper advertisements, the company will suggest ways to do it, he said: reduce lighting, cut back operating hours, close nonessential areas and turn down electric heat.

"We can't control delivery or use. We supply what the demand is . . . it has to be a government order. We'll do whatever we can to back it up, but only the authority of the government can enforce (large-scale) curtailment," McCardell said.

Commissioners from the three states met all day at the Virginia State Corporation Commission offices to arrive at the joint statement, and left immediately to inform their respective governors, John Wessels, a spokesman for Virginia Gov. John N. Dalton, said "It's up to the utilities to figure out" how to attain the reduced consumption levels. Penalties for noncompliance were not discussed during the meeting, he said.