By Casey Sullivan

Jan 7 (Reuters) - The Chicago-based law firm Schiff Hardin has acquired the 10-lawyer Washington energy law boutique Bruder Gentile & Marcoux, the firms announced on Monday.

The combination, which became effective Jan. 1, nearly doubles the size of Schiff Hardin's 11-lawyer energy practice. Five Bruder Gentile members joined as partners.

The deal adds a mergers and acquisitions component to Schiff Hardin's energy practice, which until now has specialized in advising investor-owned utilities and electric nonprofit cooperatives in a variety of federal regulatory enforcement matters, according to leaders of the firms.

Among recent deals, Bruder Gentile advised Boston-based utility NStar, as part of its $5 billion April merger with Connecticut's Northeast Utilities. The combination formed one of the largest electric and natural gas utilities in the United States.

Leading up to the merger , Bruder Gentile handled NStar's application to the Federal Energy Regulatory Commission, which authorized the transaction, according to firm partner Carmen Gentile.

Schiff Hardin's acquisition underscores the growing interest among law firms' in recruiting lawyers with expertise in the energy field, the most active industry in United States for mergers and acquisitions in 2012, according to Thomson Reuters data.

Law firm managing partners believe the energy industry is one of the few sectors fueling growth in the U.S. economy, said Sabina Lippman, a legal recruiter with Lippman Jungers.

Roughly half of the exclusive searches Lippman Jungers handles are focused on energy lawyers, she said. For the company's international law firm clients, energy is the first or second most desired practice area to expand.

One driver of new energy work could be the tax law passed by Congress to avoid the fiscal cliff, lawyers said. The deal extended by one year a tax credit worth 2.2 cents per kilowatt-hour of energy produced by new wind installations for the first 10 years of operation.

Sherry Quirk, leader of Schiff Hardin's energy and public utilities group, said she expects to receive more business based on the construction of new wind operations because owners will need legal advice on federal compliance issues.

Schiff Hardin employs 385 lawyers in nine offices throughout the United States. Its lawyers specialize in a range of practices, including construction, real estate, corporate law and energy.

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