The Potomac Electric Power Co. and the Federal National Mortgage Association have helped finance the purchase of 32 homes for rental to low- and moderate-income families in Montgomery County, officials with the Montgomery County Housing Opportunities Commission said yesterday.

The commission said the purchases constitute its first investment partnership with private companies as part of its effort to provide more affordable rental housing in the county. Such homes are at a premium in Montgomery, one of the nation's wealthiest jurisdictions.

"We do not have adequate low- and moderate-income housing in this county," said County Executive Sidney Kramer. "We're trying to increase the supply of affordable housing without using state, local or federal money."

The program is expected to help 32 families rent homes scattered throughout the county and give federal housing tax credits to Pepco and Fannie Mae, as the secondary mortgage market investor is commonly known. Under the tax credit program, companies are allowed to write off against other tax liabilities 9 percent of a project's acquisition or construction costs over 10 years.

Most of the 32 homes are town houses. They are located in 11 subdivisions in Silver Spring, Potomac, Rockville, Gaithersburg and Germantown. The commission paid an average of $72,000 for each house excluding land costs, which were donated by the developers as required by county law.

To qualify for the program, families must earn between $19,936 and $28,035; the county'smedian income for a family of four is $46,900. There are between 30,000 and 40,000 households in the county that would be eligible, said James Brodsky, chairman of the housing commission.

Many workers in the county, such as teachers, firefighters, policemen, electricians and secretaries, have a difficult time finding an affordable home that is large enough for their families, Brodsky said. Under the commission's new program, a four-member family with an annual household income of $26,000 who wants to rent a three-bedroom house would pay $533 a month. Such a house would have a fair market rent of $743 a month, he said.

"This arrangement means that this year 32 more Montgomery County families will have affordable housing," Brodsky said. "Next year, the number could be as high as 100."

The housing commission formed the Montgomery Homes Limited Partnership I to purchase the homes for $2.3 million on Dec. 31. The commission financed part of the deal with a total of $700,000 in cash from Fannie Mae and Potomac Capital Investment Corp. (PCI), an investment arm of Pepco, and financed the balance with a mortgage from Perpetual Mortgage Co.

The commission, formed in 1966, manages 5,293 units of subsidized housing in the county and has worked to increase the county's supply of affordable rental housing. The commission has been aided by the county's moderately priced dwelling unit law, which requires developers that build projects with 50 homes or more to devote at least 12 1/2 percent of the houses or apartments to assisted housing. One-third of these units must be offered for sale to the housing commission at a price that excludes land costs.

Currently, the commission has more than 5,000 families on a housing waiting list. Some of these families may qualify for one of the 32 homes, but applications from other families will also be considered.