A North Carolina appeals court on Tuesday upheld the 2012 regulatory decisions that approved Duke Energy's purchase of Progress Energy, making Charlotte-based Duke the country's largest electric company and a major utility in west-central Florida.The advocacy group NC WARN had challenged the North Carolina Utilities Commission's decision, arguing the panel did not fully consider the acquisition's impact on consumers. But a three-judge panel ruled unanimously the commission was not wrong in determining the deal was justified. NC WARN argued the merger's consumer impact should be analyzed and not just rely on Duke's claims of customer benefits. The group said it may challenge the ruling to the North Carolina Supreme Court.Duke Energy serves 7 million customers in the Carolinas and four other states, including 1.7 million Florida retail customers. It's the largest U.S. utility based on customers and market value.