Northeast Utilities and NSTAR have wrapped up their merger and created one of the nation’s largest utilities, with six regulated electric and natural gas companies serving 3.5 million customers in three states.

As of Tuesday, NU operates in Connecticut, Massachuetts and New Hampshire. NSTAR will become the subsidiary in Massachusetts. The new company will use NU's name.

NU says the merger will help it "be more diverse and better positioned to support economic growth and renewable energy opportunities in New England."

Thomas J. May is president and CEO of Northeast Utilities, and Charles W. Shivery is non-executive chairman of NU’s board of trustees.

The company will have dual headquarters in Boston and Hartford, Conn.