"In my view, this final rule may be the most significant step we have taken to reduce the systemic risk posed by these large, complex banking organizations," said Martin Gruenberg, chairman of the Federal Deposit Insurance Corp (FDIC).
The rule would apply to JPMorgan Chase, Citigroup, Bank of America, Wells Fargo, Goldman Sachs, Morgan Stanley, Bank of New York Mellon and State Street.
The Financial Services Roundtable, a trade group for large banks, issued a statement blasting the limits, which are more stringent than the international Basel III agreement.
"This rule puts American financial institutions at a clear disadvantage against overseas competitors," said Tim Pawlenty, the group's chief executive.
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The FDIC, Federal Reserve and Office of the Comptroller of the Currency approved the rules, implementing a portion of the Basel III agreement known as the leverage ratio, which is calculated as a percentage of a bank's total assets.
The rules require the eight biggest bank holding companies to maintain top-tier capital equal to 5 percent of total assets. Insured bank subsidiaries must meet a 6 percent ratio. That's higher than the 3 percent ratio included in the Basel agreement.
Smaller U.S. banks would be held to the 3 percent ratio, regulators said.
Scott Alvarez, the Fed's general counsel, told a congressional panel on Tuesday that the 18 biggest banks have already boosted top-tier capital levels by more than $500 billion since 2008. He did not break down the total to show how much the top eight banks increased their capital.
The Basel III accord included both a leverage ratio and risk-based capital requirements, which take into account the riskiness of banks' assets.
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Banks and other critics of leverage rules say they are draconian and that risk-based capital requirements are more tailored to banks' businesses.
While the rule may strengthen banks, "the potential for adverse effects on market liquidity and the strength of the system going forward could be real," Oliver Ireland, an attorney at Morrison & Foerster in Washington, said in an email.
U.S. regulators said risk-based capital requirements are easier to game than leverage rules. They said the 3 percent Basel leverage ratio would not have been high enough to sustain many banks through the financial crisis.
"The leverage ratio serves as a critical backstop to the risk-based capital requirements," said Fed Governor Daniel Tarullo, adding that regulators could write extra capital requirements for banks that rely on risky, short-term funding.
The agencies also voted to issue a separate proposal to adjust the way banks tally up their assets under the leverage rules. The proposal, which would have to be finalized later, changed those calculations to bring them more in line with the Basel rules.
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Regulatory officials said the proposed changes would apply to banks meeting the 3 percent Basel ratio as well as the eight biggest firms.
Regulators said the changes would result in a "modest" increase in the amount of capital banks would need to hold.
They said banks needed $22 billion in additional capital under the old calculation model, compared to $68 billion with the Basel method. The proposed changes count credit derivatives more and traditional loans less than the original model did.
Chip MacDonald, a banking lawyer at Jones Day, said the difference of $46 billion in capital between the two calculation systems was "more than a tweak."
"Unless something particularly compelling can be demonstrated through the comment process, we're going to end up with a much more stringent leverage requirement on the largest banks," said Kevin Petrasic, a partner in the global banking practice of law firm Paul Hastings.
Banks have until June to comment on the proposed changes.