Transmission System Projects

Hydro One’s proactive planning process helps us prepare for the future, provide a reliable power supply, and improve the efficiency of the electricity system. We integrate local area plans with larger system plans over a 10-year period.

Load Forecasting

Hydro One works with the Independent Electricity System Operator (IESO) and local distribution companies to determine the need for new facilities to keep up with growth. It’s our responsibility to make sure we can deliver power to the people of Ontario when they need it. Hydro One also uses a number of forecasting tools including econometric and end-use models to prepare load forecasts. Provincial GDP, population growth, industrial production, residential housing starts, commercial construction activities, and conservation efforts are some of the factors we consider.

Technical Assessments

Technical studies are carried out in accordance with accepted industry criteria and practices set by the Northeast Power Coordinating Council (NPCC), the North American Electric Reliability Council (NERC), the Independent Electricity System Operator (IESO), and the Ontario Energy Board’s (OEB) Transmission System Code (TSC).

In consultation with customers, possible transmission solutions are developed and assessed on various factors including cost, value, lead-time, and impacts on the environment and local community.


Once we identify possible transmission solutions, we work with various rights-holders and stakeholders to get their input. Our staff go out into the field and hold community open house events to share our ideas and obtain input. These groups include Indigenous communities, customers, provincial, regional and municipal officials, local community members, and other key stakeholders who have an interest in the proposed transmission solution. We explain the need for the work, provide a description of the possible solutions, discuss any potential effects on the environment or local community, estimated cost, and when the facility is needed. Based on an assessment of all possible options and including feedback from the community, the best option is selected.


A variety of external approvals may be required and rights-holders and stakeholders are able to give input along the way:

  • Technical: The IESO, through their Connection Assessment and Approval Process, confirms the impact of proposed new transmission projects on the reliability of the provincial transmission system.

  • Environmental: Where applicable, we submit Environmental Assessment reports regarding proposed transmission system development projects to the Ontario Ministry of Environment, Conservation and Parks. Most projects follow the Class Environmental Assessment for Minor Transmission Facilities*, an approved process for assessing environmental effects, undertaking engagement activities, and evaluating alternatives.

  • Ontario Energy Board (OEB) approval is required before we can make most additions to our transmission system. This legislated requirement gives the OEB the opportunity to assess the proposed transmission solution from the perspective of price, availability, reliability, and quality of electricity.

  • Canada Energy Regulator (CER) approval for construction and operation of an international power line is also required for interconnection projects.

* Ontario Regulation (O. Reg.) 852/21 came into effect on January 1, 2022. This regulation outlines amendments to the Class EA Document that align specific provincial and federal transmission line EA thresholds. These amendments comprise changes to clause a. of Section 1.1, clause c. and the final paragraph of Section 3.3.3, and Section 5.3. In addition, administrative amendments to update terminology that came into effect in July 2020 have also been incorporated.

Engineering and Construction

Once all approvals are in place, detailed project engineering, material procurement, construction, and commissioning take place. The construction of new transmission facilities can take anywhere from months to years depending on the scope and complexity.