Duke Energy and Piedmont Natural Gas are seeking proposals to build and operate a second major natural gas pipeline into North Carolina.

Duke has increasingly relied on gas as prices fell and coal-fired power plants shut down in the face of looming environmental standards. Piedmont’s customer growth last year was the highest since 2008 and continues to climb.

North Carolina is now served by Transco, a 10,200-mile pipeline owned by Williams Partners L.P. that sends Gulf of Mexico gas from south Texas to New York City. The line runs toward the Northeast in a diagonal route through Western North Carolina, including Mecklenburg County.

Massive new gas supplies are being developed in Pennsylvania and other northeastern states as the drilling technique called fracking taps shale-gas deposits.

Duke and Piedmont offer few details but say they have a “strong preference” for an interstate pipeline with…