Washington Gas Light Co. received conditional approval yesterday to join Pepco in allowing D.C. customers on Social Security or welfare to pay their bills up to 10 days later each month.

Washington Gas customers whose major source of income is Social Security, Supplemental Security Income, Aid to Families with Dependent Children, or other low-income assistance will be eligible for payment extensions until the 5th of the month if they receive their bills between the 5th and 15th of the previous month.

Since Washington Gas bills normally are due within 20 days of receipt, eligible customers would receive extensions of between one and 10 days.

Washington Gas officials told the D.C. Public Service Commission yesterday that the purpose of the plan is to make it easier for low-income customers to pay their gas bills without incurring late payment charges. The extension is designed to coincide with the arrival of Social Security and public assistance checks.

Washington Gas officers told the PSC yesterday that they expect between 10,000 and 20,000 customers, of a total of 180,000 in the District, to participate eventually in the program.

Washington Gas already has received approval from Maryland and Virginia authorities to proceed with an identical program in Northern Virginia and suburban Maryland. Customers in those areas are receiving details this month along with their bills.

Company officials said they would start to mail eligible customers in the District of Columbia information about the program with their September bills, if the PSC grants final approval.

Commissioners asked Washington Gas to spell out exactly who will be eligible for the program, how customers will be informed and how much it will cost to administer before they allow the company to go ahead.

Company representatives said they would provide the information by the end of this week.

Potomac Electric Power Co. started an extended payment plan for its residential users in April. Pepco mailed notices to about 200,000 potentially eligible customers, according to Nancy Moses of Pepco.

So far the utility has received 287 requests for applications; 100 have been approved, 21 rejected, and 166 applications have not been returned to Pepco. Moses estimated that 20,000 Pepco users could qualify for the program.

A spokesman for the Virginia Electric and Power Co. said Vepco had no such plan in the works but will assist customers individually if they have trouble paying their bills on time.

Washington Gas consumer relations director Arthur Howard said the company will use "random validation" to make sure that customers signing up for the extensions do in fact qualify.