Developers installed wind turbines with capacity of 6,810 megawatts in the U.S. last year, 31 percent more than in 2010, as they rushed to qualify for a federal-tax grant that expired last month, according to an industry group. Fourth-quarter installations reached 3,444 megawatts, topping the first three quarters combined, led by California, Illinois and Ohio, the fastest-growing state, the American Wind Energy Association said today in a report. New wind farms with capacity of more than 8,300 megawatts are under construction now. Developers will try to complete them before another federal incentive expires Dec. 31. The Washington-based trade group is pressing Congress to extend the Production Tax Credit of 2.2 cents a kilowatt-hour for wind power to prevent manufacturers from firing workers by 2013.