Omaha investor Warren E. Buffett, who sold a $50 million interest in the National Corporation for Housing Partnerships earlier this year, announced yesterday his Berkshire Hathaway Inc. would invest $25 million in the National Equity Fund (NEF), a private corporation that uses federal tax breaks to spur the construction of low-income housing.

The infusion of private capital would create 750 housing units at a cost of about $30,000 per unit during the next seven years, and it represents one of the largest single corporate investments ever made in nonprofit development of affordable housing.

Berkshire Hathaway shareholders will recoup their investment through federal low-income tax credits recently extended by Congress.

Buffett said there was no connection between yesterday's announcement and his decision earlier this year to sell his 50 percent stake in the partnerships corporation, which is the nation's largest owner and operator of low- and moderate-income housing. The stake was purchased by Harvard University for roughly $75 million.

The tax credit incentive, Buffett said, made the National Equity Fund "a perfectly intelligent investment." The fund, which was created by the Local Initiatives Support Corp. (LISC) to take advantage of the tax credit program in 1987, raised $141 million in corporate money by 1989 and expects to have amassed $250 million by the end of this year.

The projects funded with aid of the tax credits must remain low-income housing for 15 years. In addition, the NEF program provides for the properties to be sold after that time only to nonprofit groups that promise to retain them as low-income housing stock.

Buffett said he hoped news of his investment would inspire other businesses to follow suit "because they know I don't throw around $25 million that easily." Buffett is well-known in financial circles for his well-timed investments in corporations such as USAir Group Inc., Salomon Brothers Inc., Gillette Co. and Geico Corp.

He stressed that his decision to join forces with the National Equity Fund was not a philanthropic one but a moneymaking one with a "pro-social outcome." David Stanley, chairman and chief executive officer of Kansas City's Payless Cashways Inc. and chairman of the NEF board, estimated that Buffett's investment could earn his shareholders a 20 percent annual return on their investment.

LISC officials touted Buffett's decision as a development that could help them as they prepare for what has become an annual lobbying effort on behalf of legislation extending the tax credits. A General Accounting Office report criticized the program earlier this year, saying there is no system in place to assure compliance with the requirement that rents remain "affordable" for 15 years.

Paul Grogran, president of LISC, said Buffett's investment and smaller cash infusions from more than 70 other Fortune 500 corporations could form the backbone for an effective public-private partnership that would supplement the federal government's reduced role in subsidized housing.