Before coming here on a visit in February, President Barack Obama assured anxious Canadians in a television interview that the United States' trading partners had nothing to fear from the 'Buy America' provisions in federal stimulus bills. But now that the stimulus money is flowing, many Canadian companies are finding that, despite the president's assurances, the North American Free Trade Agreement does not in fact offer them much protection. Charles A. Cartmill buys about half of the components he uses to make street lamps based on energy-efficient light-emitting diodes from American companies. But the Buy America provisions have still made the lamps from his company, LED Roadway Lighting, increasingly unwelcome in the United States. 'We've had contractors call and say that they want certificates that everything is from the U.S.,' Cartmill said from Halifax, Nova Scotia. 'It will definitely cost us business.' Cartmill's case is not isolated, particularly in the case of companies that make the parts used to build the infrastructure of American cities. Canadian Manufacturers and Exporters, a trade group, estimates that about 250 companies have been affected to date. While the dispute has attracted relatively little notice in the United States, it has reopened the debate in Canada about the value of NAFTA and the trustworthiness of the United States as a trade partner. On Friday, the premiers of Canada's 10 provinces concluded a meeting by urging the federal government to confront the United States over the issue. They will not have to wait long for satisfaction. A spokesman for Prime Minister Stephen Harper said on Friday that Buy America would be on the agenda when two days of meeting between him, Obama and President Felipe Calderon of Mexico begin in Guadalajara, Mexico, on Sunday. 'No country should add any protectionist measures to what they are doing,' Stockwell Day, Canada's international trade minister, said in a recent interview. 'Congress should realize that they are risking retaliatory actions from Canada. Retaliatory trade measures always end up with both sides getting hurt.' 