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Transmission and Distribution


City Light is evaluating three alternative routes for a new transmission line to connect the proposed Denny Substation to the existing Massachusetts Substation located in the SODO area. Denny Substation map shows all of these project components and locations.

When installed, the transmission line will be operated at 115 kV with the capability to operate at 230 kV in the future as electrical demand increases and more areas of North Downtown Seattle are switched to a network distribution system.

The three transmission alternatives currently under consideration through the SEPA process are:
  • Alternative TL1 - A route primarily along the eastern edge of the downtown commercial core
  • Alternative TL2 - A route through the Downtown Seattle Transit Tunnel (sometimes referred to as the bus tunnel)
  • Alternative TL3 - A route along the east side of Interstate-5 (I-5) - NOTE: this is a new route - added in June 2013. For more information on this route, visit the SEPA Review page.

All three alternatives - as currently conceived - transition from underground to overhead structures at the southern end before entering the Massachusetts Substation.

Construction of the transmission line is scheduled to start in 2018 and be complete in late 2020. An existing transmission line (the Pine-Broad Substation transmission line) will be used to power the proposed new Denny Substation prior to completion of the new transmission line in 2020.

'Transmission Line Alternatives'