MOUNT SI
SubstationWashington · PUGET SOUND ENERGY INC
At a glance
MOUNT SI is an electric transmission substation in Washington, on PUGET SOUND ENERGY INC's transmission network. It is classified as sub-transmission (138 kv) and connects 3 transmission lines. Substation proximity is one of the strongest speed-to-power signals for siting a datacenter — interconnection distance to an energized, high-voltage bus is often the binding constraint on time-to-energization.
Voltage & classification
Ownership
Connected transmission lines
Lines terminating at or passing through this substation, by HIFLD line ID, highest voltage first.
| Line ID | Voltage | Owner | Status | Length |
|---|---|---|---|---|
| #207625 | 138 kV | PUGET SOUND ENERGY INC | — | 0.4 mi |
| #207626 | 138 kV | PUGET SOUND ENERGY INC | — | 0.5 mi |
| #200966 | 115 kV | BONNEVILLE POWER ADMINISTRATION | — | 4.3 mi |
Nearby datacenter candidate sites
Scored candidate sites within roughly 25 miles of MOUNT SI, ranked by DC Readiness. Sites this close to an energized substation carry a strong speed-to-power advantage.
| Site | Type | State | County | Voltage | Capacity | Queue wait | Score |
|---|---|---|---|---|---|---|---|
| MAPLE VALLEY | Substation-Adjacent | WA | King County | 500 kV | 1,000 MW | 3.5 yrs | 68.4 |
| COVINGTON | Substation-Adjacent | WA | King County | 500 kV | 1,000 MW | 3.5 yrs | 68.2 |
| SNO-KING | Substation-Adjacent | WA | Kitsap County | 500 kV | 1,000 MW | 3.5 yrs | 68.2 |
| TAP200917 | Substation-Adjacent | WA | Snohomish County | 500 kV | 1,000 MW | 3.5 yrs | 68.1 |
| MONROE | Substation-Adjacent | WA | Snohomish County | 500 kV | 1,000 MW | 3.5 yrs | 67.8 |
| ECHO LAKE | Substation-Adjacent | WA | King County | 500 kV | 1,000 MW | 3.5 yrs | 67.4 |
| RAVER | Substation-Adjacent | WA | King County | 500 kV | 1,000 MW | 3.5 yrs | 66.6 |
| SNOHOMISH | Substation-Adjacent | WA | Snohomish County | 345 kV | 600 MW | 3.5 yrs | 66.5 |
How does MOUNT SI compare with other substations in Washington?
Washington has 2,106 substation records in HIFLD, of which 785 carry a station name; the rest are unnamed tap points, risers and line ends. 515 named substations in the state are in MOUNT SI's 115–229 kV band (66% of named stations). 137 sit in a higher band. 34 named substations in Washington operate at 345 kV or above. The highest voltage recorded in the state is 500 kV, and the state median is 115 kV.
Who owns the transmission lines at MOUNT SI?
The 3 connected lines listed above are owned by PUGET SOUND ENERGY INC, BONNEVILLE POWER ADMINISTRATION. Line voltages range from 115 kV to 138 kV. Their combined recorded length is about 5.2 miles.
How do candidate sites near MOUNT SI score?
The 8 highest-scoring candidate sites within about 25 miles range from 67 to 68 on the 0–100 DC Readiness scale. They fall in King County, Kitsap County, Snohomish County. The Washington average across 6,019 scored sites is 51.7. 1,343 of those Washington sites are classed as substation sites (candidate locations generated at an existing substation).
Frequently asked questions
What voltage is MOUNT SI substation?
MOUNT SI has a maximum voltage of 138 kV and a minimum of 115 kV. That places it in the 115–229 kV band. The median named US substation in this dataset is 115 kV, so MOUNT SI is above the national midpoint.
How many transmission lines connect to MOUNT SI substation?
3 transmission lines terminate at or pass through MOUNT SI in the HIFLD line data. The connected lines listed here run at 138 kV, 115 kV. Together the listed lines cover about 5.2 miles.
Who owns MOUNT SI substation?
HIFLD does not publish substation owners directly. Most of the transmission lines that end at MOUNT SI belong to PUGET SOUND ENERGY INC, and others to BONNEVILLE POWER ADMINISTRATION. GridCensus derives this from the line data, so confirm ownership with the utility for anything contractual.
Where is MOUNT SI substation located?
MOUNT SI is in Washington at 47.5247, -121.8624. The datacenter candidate sites closest to it are in King County, Kitsap County, Snohomish County.
Are there datacenter candidate sites near MOUNT SI substation?
GridCensus lists at least 8 scored candidate sites within roughly 25 miles of MOUNT SI. The highest-scoring is MAPLE VALLEY at 68/100. Across Washington, the average DC Readiness score is 51.7.
Other 138 kV substations in Washington
- MARCH POINT· 10 lines
- COSMOPOLIS· 6 lines
- SNOQUALMIE· 6 lines
- COWLITZ· 5 lines
- STRATFORD· 4 lines
- TARLETT· 4 lines
- CEDAR FALLS· 3 lines
- INDUSTRIAL WAY· 3 lines
- All named substations in Washington· 785 stations
- Datacenter candidate sites in Washington· 6,019 sites
- Brownfield and retired plant sites in Washington
- Substations by state
About this data
Source: HIFLD Electric Substations and Transmission Lines (U.S. Department of Homeland Security), as published June 29, 2026. See the methodology for how records are matched, cleaned and scored.
Cite as: GridCensus, “MOUNT SI substation, Washington”, https://gridcensus.com/substations/washington/mount-si-477de5a3 (data as of June 29, 2026).
Dataset updated . Screening estimates derived from public data sources.
Substation attributes are derived from public HIFLD / EIA infrastructure data and may lag real-world upgrades or retirements. Voltage and connected-line counts are screening figures — confirm available capacity and interconnection terms directly with the operating utility and ISO.
Go deeper than the public screen
Every site and score for MOUNT SI substation, Washington is free to browse. Pro adds Watchtower on 25 sites, alerts when new candidate sites, datacenters or interconnection-queue requests appear in your markets, score and capacity history, and CSV export. Enterprise adds team seats, weekly data files for all 147,272 sites, the REST API and an MCP server for AI tools.