Fed Issues Rule to Prevent Oversized U.S. Financial Firms The rule is designed to prevent large financial firms from becoming so big that their failure could shake the core of the U.S. financial market.

The U.S. Federal Reserve unveiled a final rule on Wednesday designed to prevent large financial firms from becoming so big that their failure could shake the core of the U.S. financial market.

The final rule, required by the 2010 Dodd-Frank Wall Street reform law, prohibits banks and certain large financial firms from acquiring another company if that merger would cause their liabilities to exceed 10 percent of the total consolidated liabilities for all financial firms.

The Fed said on Wednesday that its final rule is "substantially similar" to the one it proposed in May, but contains a few changes.

For instance, the final rule has an exemption that would permit firms to continue securitization activities even if they have reached the limits set forth in the rule.

The final rule also prohibits a company from acquiring another company under "merchant banking authority" if it has reached the 10 percent limit.

In addition, it spells out more details for how to properly calculate financial sector liabilities, among other things.

The rule is slated to take effect on January 1, 2015.

Wednesday's rule applies to banks and to large financial firms who are designated as "systemic" by the Financial Stability Oversight Council, a federal government panel of regulators that polices for emerging market threats.

The FSOC has already designated General Electric Co's GE Capital, American International Group Inc and Prudential Financial Inc as systemic. It has also proposed designating Metlife Inc, although the company has hired a lawyer to fight the proposal.

(Reporting by Sarah N. Lynch; Editing by Susan Heavey)