The Washington Suburban Sanitary Commission is asking its 1.2 million customers in Prince George's and Montgomery Counties to voluntarily curtail their use of water by at least 20 per cent for a three-day period beginning today.

During this period, the WSSC's Potomac River Water Filtration Plant, one of two serving the area, will be out of operation while the Potomac Electric and Power Co. prepares the system for the peak summer demands.

"While this work is being done there will be times when the plant's pumping capacity will be curtailed," explained WSSC maintenance director James H. Lee. "We'll have only one electrical feeder instead of the normal two to power the water distribution."

The WSSC is asking customers to refrain from watering lawns and washing motor vehicles. Customers are also being asked not to fill swimming pools and to curtail the use of water-using appliances."We'd like people to eliminate all unnecessary usage, period," said Lee.

WSSC officials said its customers use between 148 million gallons of water on an average weekdays. "If we can cut that down to about 140 million," said Lee, "we'll be comfortable for these three days."