Entergy Corp. will spin-off its electric transmission business and merge it with Michigan-based ITC Holdings Corp. under a $2.5 billion deal announced Monday.

Entergy has 15,700 miles of transmission lines. After the deal, ITC will have more than 30,000 miles of transmission lines covering an area from the Great Lakes to the Gulf Coast, making ITC one of the largest electric transmission companies in the country.

Under the agreement, Entergy’s electric transmission business will be placed into a new company, Mid South TransCo LLC. Mid South will merge with a newly created subsidiary of ITC in an all-stock deal. Energy shareholders will get 50.1 percent of the new ITC subsidiary’s shares, while ITC shareholders will get 49.9 percent of them.

ITC will take on $2.5 billion in debt to finance the deal. Entergy expects to get $1.8 billion in cash from that and will use most of the money to retire debt.

The deal is expected to be completed in 2013. However, Louisiana’s Public Service Commission, other state regulators, the Federal Energy Regulatory Commission and ITC shareholders must approve the deal.

About 750 Entergy workers will be moved to ITC. ITC will assume the operations of Entergy’s transmission and transmission-related facilities and maintain a regional headquarters in Jackson, Miss. ITC’s corporate headquarters are in Novi, Mich.