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Denny Substation Project
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Investing in the transmission and distribution of electricity to maintain reliability and meet future needs

Seattle City Light's (City Light's) Denny Substation Project consists of the following components:
  1. A new electrical substation on Denny Way in the Cascade area of Seattle's South Lake Union neighborhood;

  2. A new electrical network distribution system to serve the South Lake Union area; and

  3. A new high-voltage transmission line to connect the new substation to the existing Massachusetts Substation in the South of Downtown (SODO) neighborhood;
City Light also proposes to construct a new inductor at the Broad Street Substation to help balance the regional transmission system. For purposes of the project's environmental review, installation of this new inductor is incorporated as part of the Denny Substation Project. Outside of the project, City Light is also proposing to build a network distribution system to serve the Denny Triangle area.


The South Lake Union area of Seattle has been experiencing rapid redevelopment over the past 15 years and in 2004 was listed as one of five urban centers in the Seattle Comprehensive Plan. This designation is intended to lead to creation of a high-density urban area where low-density commercial and industrial development is replaced with multistory residential and commercial development, providing high-tech/professional jobs, increasing the tax base, and adding retail services and housing in the city center. Pursuant to these goals, the City has been upgrading transportation, road, and water management infrastructure in South Lake Union. As development occurs, City Light expects electrical load density in South Lake Union and north downtown to increase dramatically.

City Light considered how best to serve the emerging high load density and provide system flexibility to accommodate planned and unplanned outages, and determined the need to convert the existing overhead and underground radial distribution system to an underground network distribution system. City Light further determined that both the expanded and existing network distribution systems in the project vicinity would require a new substation designed to function as both a distribution station and a transmission station. A new network distribution system and substation system would provide the needed capacity and flexibility to manage load growth in other nearby urban centers.

The new substation would both distribute electrical power to the South Lake Union area and transmit power to other City Light substations to alleviate electrical system congestion and accommodate expected growth. The proposed Denny Substation would initially be served by the existing transmission line that connects the Broad Street and East Pine Substations, which runs through the proposed site.

The project would install a new underground network distribution system in the South Lake Union area. The network distribution system service area extending out from the new substation would be developed in phases, with a first set of new conduits and cables installed within street rights-of-way through the neighborhood adjacent to and surrounding the substation and then with connections to be constructed within street rights-of-way as needed in the future. Network improvements within the Denny Triangle area of City Light’s existing Downtown Network Service Area are planned in 2015 as a separate project. This project bolsters network distribution system service to this area whether or not the Denny Substation is completed as planned.

City Light's analysis also determined that a new (third) transmission line would be needed to provide reliable power to the new network area and First Hill network. A new transmission line would help meet the distribution load requirements and contribute to system reliability by providing more than one source of power to the network.


The capital improvements being constructed as part of the Denny Substation facility and its associated transmission circuits are system improvements paid for by all rate payers. This is because the Denny Substation facility and associated transmission circuits (which will provide energy service in the areas of the north downtown Urban Centers) also serve as backup in case of outages or loss of equipment at adjacent substations in the region; therefore, they are part of the system improvements necessary to maintain reliable operations and service to our entire service territory, as well as proper operation of the regional transmission grid. If Seattle City Light does not provide the capacity of the new Denny Substation, the reliability and service of the other substation service areas would be diminished, and they would not meet the service standards of Seattle City Light.

The new Denny Network System being constructed as part of this project will serve the new high-load customers in the north downtown Urban Centers of South Lake Union, Uptown, and Denny Triangle. As this is a premium level of reliability, these capital improvements will be paid for by the Network Service Customers in these areas as part of the Network Ratepayer class.
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    P.O. Box 34023
    Seattle, WA 98124-4023

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