Slow economic growth and low natural gas prices are putting the investment-grade ratings of some of North America's largest publicly traded power companies at risk, warns Moody's Investors Service.

"A triple whammy of low natural gas prices, tepid volume growth expectations, and rising operating costs continues to compress the margins and weaken the cash flows of most of the unregulated power companies," the New York-based rating agency says in a report dated November 9.

"As this tough operating