The National Association of Manufacturers Wednesday roundly endorsed the European Community's moves toward a free internal market.

NAM executives, in perhaps the most upbeat assessment yet by a major U.S. business group of the EC initiative, said it has helped spur a "big demand" for U.S. capital goods.In turn, they said, that demand has helped convert a more than $20 billion U.S. trade deficit with the EC to a trade surplus.

U.S. exports to the EC are projected to exceed $90 billion this year, a new high, and to top $100 billion within the next two to three years, said Stephen Cooney, director of the NAM's office of international investment and finance.

Since 1985, expanding U.S. exports to the EC have created more than a million new American jobs, added Jerry Jasinowski, NAM president.

Both NAM officials lauded the EC for trying to minimize any adverse impact of its market unification program on U.S. industry.

"It's been a very positive process over the last year," Mr. Jasinowski said. "We don't think there are any unresolvable issues."

Under the program, due to be largely in place by 1992, the EC aims to establish the free flow of people, capital and goods among its 12 member nations.

The NAM's comments came as the business group released a report, "Update on EC-92," describing the EC program's main trade and investment issues of interest to U.S. manufacturers.

"Some tough issues," Mr. Jasinowski said, "need to be resolved," but, ''fears that EC-92 means an emerging 'Fortress Europe' have been overblown."

Compared with other foreign markets, the NAM said, "the EC is both open and booming."

Though the EC so far has adopted only about half of the nearly 300 market reforms it plans, most of the reforms that might have major impact on U.S. industry have been or are being "cast," Mr. Cooney said.

Problems that remain to be resolved for U.S. industry, the report said, include the EC's product testing and certification procedures, potentially restrictive government procurement practices and rules relating to the treatment of products on the basis of their foreign content or origin.

The NAM, however, reports encouraging signs that the United States and the EC can reconcile not only these but other matters of concern.

It particularly praised EC and U.S. officials for recently resolving a potential dispute over the EC's setting of product standards. As a result, Mr. Cooney said, any U.S. company can bring a standards problem directly to the EC's standards-setting agencies.