Ontario Approves New Toronto Transmission Line Following IESO Recommendation

The Ontario government has approved the Independent Electricity System Operator’s (IESO) recommendation to develop a new high-voltage underwater electricity transmission line from near the Darlington Nuclear Generating Station to downtown Toronto and the city’s eastern areas. The project is intended to address forecast growth in electricity demand, which is expected to double by 2050.

The proposed transmission line would deliver up to 900 megawatts (MW) of clean, reliable power to Toronto. The government states that the additional capacity is needed to support housing development, transit expansion, and economic activity. The project aligns with ongoing investments to upgrade and expand nuclear generating capacity, including the Darlington Nuclear Refurbishment Project and the Darlington New Nuclear Project, which includes the construction of small modular reactors.

“As we start 2026, we are focused on strengthening Ontario’s economy and creating jobs,” said Stephen Lecce, Minister of Energy and Mines. “Without a new transmission line, Toronto would have to turn down job-creating investments and reduce housing, which is simply unacceptable. We are thinking ahead and building for the future by approving a new transmission line that will enable 285,000 new homes, power new businesses, and create thousands of jobs. We are also initiating the first IESO-led competitive transmission procurement to reduce costs and maximize benefits for families.”

Toronto is currently supplied by two transmission lines: one from the west at Manby Transmission Station and one from the east at Leaside Transmission Station. According to the government, these lines are expected to approach their capacity limits in the 2030s. Electricity demand is highest in the downtown core, where growth has been particularly strong. The proposed third transmission line would provide an additional 900 MW of capacity, equivalent to the output of three small modular reactors or the electricity needed to power approximately 900,000 homes.

“Toronto is the fastest-growing city in North America, and that growth means we need more power to fuel our homes, transit, and businesses,” said Olivia Chow, Mayor of Toronto. “I want to applaud the Ontario government for moving forward with this first-of-its-kind underwater transmission line. It’s an exciting example of the province and city working together to build resilient, clean energy infrastructure that keeps costs down, supports good-paying jobs, and powers the projects that will shape Toronto’s future. We’re committed to creating an affordable and caring city that delivers for Torontonians.”

To manage costs, the government is proposing that the IESO launch a competitive procurement process to select a transmitter. The procurement will be informed by consultation on its design. The government states that this approach is intended to deliver value for ratepayers, accelerate construction timelines, and create opportunities for Indigenous participation and employment across the province.

The underwater transmission line is expected to improve system resilience by reducing vulnerability to extreme weather events such as flooding and ice storms, which have previously resulted in electricity supply disruptions and estimated costs and productivity losses of $100–$200 million. The government also notes that the underwater option would deliver greater capacity than other alternatives and meet system needs beyond 2044, potentially reducing the need for additional upgrades and expansions. The project could also reduce strain on the bulk power system by avoiding reinforcements elsewhere in the Greater Toronto Area, with estimated preliminary savings of $100–$300 million.

The development, construction, and commissioning of the third transmission line are expected to take between seven and 10 years. The IESO’s recommendation is based on technical analysis of multiple options and prioritizes routes that minimize land-use impacts by using existing corridors, underground infrastructure, and established pathways. Details of the proposal are included in the 2025 Toronto Integrated Regional Resource Plan.

Transmission expansion is identified as a component of Energy for Generations, Ontario’s integrated energy plan. The government states that the plan is intended to ensure the province has sufficient reliable and affordable electricity to meet future demand and support long-term economic activity.

Quick Facts

• High-voltage underwater transmission lines are in use in other Canadian jurisdictions, including Newfoundland and Labrador, Nova Scotia, and Prince Edward Island.

• 900 MW is equivalent to powering approximately 900,000 homes.

• Peak electricity demand in Toronto is approximately 4,700 MW, representing about 20 per cent of Ontario’s total peak demand.

• After approximately 20 years of flat demand, Toronto’s peak electricity demand is expected to roughly double by 2050, according to the IESO. Contributing factors include:

  • Housing development: Electricity to support at least 285,000 new homes as part of Toronto’s 10-year housing target, including developments at Downsview, Port Lands, East Harbour, and Golden Mile.

  • Economic growth: Toronto’s role as a provincially and nationally significant centre for investment and job creation.

  • Transit expansion: GO Transit electrification and the Ontario Line, which together are expected to add approximately 100 MW of electricity demand.

  • Electrification: Increased adoption of electric vehicles.

• The Integrated Regional Resource Plan for Toronto, developed by the IESO, is part of a province-wide effort to plan for localized electricity needs across Ontario.