Virginia Electric and Power Co. cut its power to thousands of Northern Virginians by 5 percent yesterday as sweltering temperatures on the hottest day of the year triggered demands for more electricity than the utility could produce.

Vepco appealed to its 1.2 million customers in three states to cut electric consumption for a 4-hour period yesterday so it could avoid buying more expensive power from other utilities.

Other Washington area electric utilities did not have to cut their power yesterday, as temperatures in the nation's capital climed to 97 degrees.

Vepco spokesmen in Richmond said that the voltage cut should not have been noticeable to most customers and that no industries were forced to close because of the action.

It was the first time since January 1977 that the Richmond-based utility has been forced to reduce its power.

Utility officials said the Virginia cutback which began at 3:34 p.m. was not a prelude to either brownouts or blackouts.

The voltage cut and the request for consumer cutback in energy use was "primarily directed toward keeping costs down," said Vepco spokesman August Wallmeyer. "We have an ample supply," Wallmeyer added, saying that the utility did not want to have to pass the higher costs of electricity purchased from other utilities on to its customers. The company has complained that its ability to produce power has been frustrated by federal actions that have shut two of its nuclear-powered generating units.

In Washington, the National Weather Service issued a special weather statement yesterday morning warning that high humidity and the afternoon heat would "tax all outdoor activities" and urging anyone venturing outside to exercise "caution."

Temperatures at Dulles International Airport, west of the District, reached 95 degrees yesterday afternoon, two degrees higher than the record for the day, set in 1977.