ISO-NE has published a summary of proposals submitted for its first longer-term transmission planning (LTTP) procurement, which is aimed at reducing transmission constraints between Maine and southern New England and supporting 1,200 MW of new onshore wind in northern Maine.

The solicitation is the first run of ISO-NE’s new LTTP process, which the RTO and the New England states established to select solutions to needs identified in long-term transmission studies. (See FERC Approves New Pathway for New England Transmission Projects.)

Four project sponsors responded to the first LTTP procurement, submitting six proposals in total. The proposals represent “a good diversity of solution designs,” ISO-NE said.

The cost projections range from $962 million to $4.04 billion, though these projections may change as the bidders and ISO-NE work to standardize the cost calculations. The expected in-service dates range from the fourth quarter of 2032 to the third quarter of 2035.

Four of the six projects are joint proposals submitted in collaboration with incumbent transmission owners. ISO-NE has not disclosed the identities of the companies that participated in the solicitation but noted that three of the lead project sponsors are incumbents and one is a non-incumbent.

Three of the submissions propose new HVDC lines running from Maine to Massachusetts, along with new and upgraded AC infrastructure. These proposals are:

• • A 151-mile 400-kV line between Wiscasset, Maine, and Everett, Mass., with a total cost of $2.55 billion.
  • A 144-mile 400-kV line between Wiscasset and Wakefield, Mass., projected to cost $2.6 billion.
  • A 164-mile 320-kV line between the retired Maine Yankee Nuclear Plant (in Wiscasset) and the retired Mystic Generating Station (in Everett), with an expected cost of $4.04 billion.

The three other proposals rely on new AC lines and line upgrades. They are:

• • A $2.2 billion proposal to build two new 345-kV lines totaling 70 miles, upgrade 16 miles of 115-kV line in Maine to 345 kV and upgrade existing 345- and 115-kV lines throughout Maine and New Hampshire.
  • A $2.14 billion proposal that is nearly identical to the prior proposal, but with a reduction in total mileage of 345-kV upgrades.
  • A $962 million proposal that includes a new 43-mile 345-kV line and three new substations.

ISO-NE said all the proposals claim to meet the minimum requirements of the RFP, which are to increase the Maine-New Hampshire interface limit to 3,000 MW and the Surowiec-South limit to 3,200 MW and support the interconnection of 1,200 MW of onshore wind in northern Maine.

For context, when the New England Clean Energy Connect transmission line is online — it is expected to achieve commercial operations this winter — the Surowiec-South limit will be 2,800 MW and the Maine-New Hampshire limit will be 2,200 MW.

ISO-NE said some proposals claimed to increase the limits beyond the minimum requirements. The RTO noted that it received proposals to increase the Surowiec-South limit to 3,800 MW and the Maine-New Hampshire limit to 3,600 MW.

All proposals would build a new substation near Pittsfield, Maine, to enable a 1,200-MW injection of onshore wind. No submissions proposed infrastructure that would accommodate more than the required 1,200 MW of offshore wind.

Separate from the LTTP process, Maine is seeking to procure 1,200 MW of wind in northern part of the state, along with transmission to connect the power to the proposed Pittsfield interconnection point in central Maine. Maine officials have expressed hope that other New England states will join in the solicitation.

Maine issued a draft RFP for this procurement in October 2025 (PUC Docket No. 2024-00099), noting that the procurement “is designed to leverage the LTTP solicitation and is contingent on ISO-NE selecting a longer-term transmission upgrade project.”

To select a preferred solution in the LTTP process, ISO-NE will review the projects to ensure they meet the minimum requirements, evaluate effects on other interfaces and screen for adverse system impacts.

ISO-NE also will rely on a consultant to evaluate the financial health of the project sponsors, the feasibility of the construction proposals and the cost estimates. The RTO will rely on the participating transmission owners to estimate the costs of corollary upgrades.

For projects that meet all the requirements, the RTO will quantify costs and benefits. (See ISO-NE Releases Longer-term Transmission Planning RFP.) Projects must have a positive benefit-to-cost ratio to be eligible to be selected by ISO-NE as the preferred solution.

ISO-NE said it expects to select a preferred solution by September 2026, noting that it is “cautious about committing to an earlier date” because the RFP “involves utilizing numerous new processes.”

By default, the costs of a solution would be allocated by load, though the states could submit an alternative cost allocation methodology or opt to terminate the process following ISO-NE’s selection.

If no proposals pass the benefit-cost threshold, the LTTP process allows one or more states to cover a project’s costs that exceed the threshold, enabling it to proceed.