ISO-NE's Transitional Cluster Study (TC2) Results Are In: What Developers Need to Know

ISO-NE has released the interim results of its Transitional Cluster Study (TCS), and developers now have until July 7 to respond before results are finalized.

The TCS is ISO-NE's first cluster study under the Order 2023 framework. The interim report covers 23 qualified projects representing roughly 4.8 GW of capacity, with total interconnection costs estimated at approximately $3.2 billion. Of that, about $2.5 billion falls into System Network Upgrades, putting the average cost at roughly $664K per MW.

That average is the number most people will quote. But it could be misleading if you stop there.

The Average ISO-NE Interconnection Cost Hides the Real Story

The $3.2 billion headline is striking, but cost doesn't track evenly with capacity — and that gap is what actually matters for your project.

Southeast Massachusetts (SEMA) carried the second-highest volume in the study (approximately 1,460 MW across five projects) and came in at roughly $177K per MW — among the lowest in the study. West/Central Massachusetts (WCMA) brought a similar volume, approximately 2,000 MW across eight projects, and landed at over $1 million per MW, driven by approximately $1.5 billion in thermal upgrades. That's roughly six times higher —, same study, same process. The difference comes down to regional headroom — and understanding those nuances is critical before you make any decisions.

One Cost Category Dominates

Dig into the upgrade breakdown and a clear pattern emerges. Of the $2.5 billion in network upgrades, roughly $1.8 billion is attributable to deliverability thermal upgrades. Energy upgrades are a distant second at approximately $0.5 billion.

Deliverability costs are rarely cheap, but seeing this level of concentration in a handful of zones raises a strategic question: does the choice between Capacity Network Resource Interconnection Service (CNRIS) and Network Resource Interconnection Service (NRIS) deserve a closer look going forward? For many projects, the answer is yes.

A Compressed Transitional Cluster Study Timeline with Real Consequences

The TCS process is unlike anything that came before it. There's no formal results meeting and no facility study. Developers get 30 days to comment, ISO-NE finalizes the TCS in early August, and projects move directly into Large Generator Interconnection Agreement (LGIA) negotiations.

That's a tight window to validate hundreds of millions in allocated upgrades — with real money at risk on both sides of the decision. Withdrawal carries a steep penalty: nine times your actual study cost. Staying in without fully understanding your upgrade assignments carries its own risks.

Your 30-Day ISO-NE Roadmap Forward

The developers who come out of this in the best position will use this window to pressure-test their upgrade assignments and separate shared costs from project-specific ones. That work requires both deep familiarity with ISO-NE's cost-allocation methodology and the modeling expertise to validate what's been assigned to your project.

We're actively supporting clients through the TCS process and can help you move from raw results to an informed decision before the window closes. If the interim results caught you off guard, or if you want a partner to work through them with you, contact EPE's team of experts using the form below.