TransCanada Corp., the nation’s largest natural-gas pipeline operator, said third-quarter profit fell 12 percent on lower power prices and volumes.

Net income fell to C$345 million ($324.8 million), or 50 cents a share, from C$390 million, or 67 cents, a year earlier, the Calgary-based company said today in a statement on Canada Newswire.

TransCanada has 59,000 kilometers (36,670 miles) of gas pipelines, stretching from Alberta to Vermont to Nevada. Its 19 power plants have the capacity to generate 10,900 megawatts, or enough for 8.7 million average homes, based on an estimate from the U.S. Energy Department.

The company said Sept. 30 it plans to build the Oakville Generating Station near Toronto for about C$1.2 billion. The plant, which would be TransCanada’s third-biggest with a capacity of 900 megawatts, would be fueled by gas and start up by the end of 2013. One megawatt is enough power for about 800 average homes.

TransCanada and Exxon Mobil Corp. are proposing a $27 billion pipeline to ship gas from Alaska’s North Slope through Canada to markets in the U.S. The project, which may start up in 2018, won the support of the Alaska government with a $500 million subsidy last year and competes with a similar plan by BP Plc and ConocoPhillips.

The earnings statement was released before the start of regular trading on U.S. stock markets. TransCanada rose 62 cents, or 1.9 percent, to C$33.50 yesterday on the Toronto Stock Exchange.