The key data point in Moody’s view is the size of federal interest payments on the public debt as a percentage of tax revenue. For the U.S., debt service of 18%-20% of federal revenue is the outer limit of AAA-territory, Moody’s managing director Pierre Cailleteau confirmed in an e-mail. Under the Obama budget, interest would top 18% of revenue in 2018 and 20% in 2020, CBO projects. But under more adverse scenarios than the CBO considered, including higher interest rates, Moody’s projects that debt service could hit 22.4% of revenue by 2013.

”

— U.S. Debt Shock May Hit In 2018, Maybe As Soon As 2013: Moody’s - IBD - Investors.com