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
- 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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