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Seattle City Light DEBRA SMITH, General Manager and CEO
Denny Substation Project
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Future Transmission

As part of the Environmental Impact Statement (EIS), City Light evaluated three alternative routes for a new transmission line to connect the proposed Denny Substation to the existing Massachusetts Substation located in the SODO area.

Once constructed, the transmission line will operate at 115 kilovolts (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 considered through the SEPA EIS process were:
  • 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)

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

After additional engineering work City Light determined that the route through the Downtown Transit Tunnel was no longer feasible. A hybrid alternative – one that travels north on 6th Avenue, crosses under I-5 at James Street and connects to the substation primarily via 7th and Boren avenues – replaces the Transit Tunnel alternative. The Denny Substation Program map shows the three alternatives currently being considered. City Light will identify a preferred alternative in early 2017 after additional engineering work is conducted.

Design of the transmission line will begin once the preferred alternative is selected. The design process is anticipated to take place from mid-2017 through the end of 2018. Construction of the transmission line is anticipated to start in 2019 and to be complete in late 2020. An existing transmission line (the Pine-Broad Substation transmission line) will be used to power the new Denny Substation prior to completion of the new transmission line in 2020.

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