While it is technically feasible, there are significant challenges to building Energize Eastside underground. Overhead transmission lines are PSE's first option for their combination of reliability and affordability – both of which are important to our customers. While undergrounding is an available option, the biggest challenge to underground transmission lines is cost. For more in-depth information about underground transmission lines, read our fact sheet.
The construction costs for an overhead transmission line are about $3 million to $4 million per mile, versus $20 million to $28 million per mile to construct the line underground. When a new line is constructed overhead, project costs are distributed evenly between PSE’s 1.1 million customers and paid for over time. If a transmission line were to be constructed underground, PSE can’t justify asking customers across its entire service territory to pay the significant cost increases.
That’s why, per state-approved tariff rules, the requesting party, often the local jurisdiction, must ultimately decide whether to make this investment. The requesting party would then be responsible for paying the difference between overhead and underground costs.
In addition to cost, there are other factors to consider such as environmental and neighborhood impacts.
- Underground transmission lines require an easement 30 feet to 50 feet wide, which, unlike with overhead lines, must be completely free of trees.
- Underground transmission lines require large (20 feet x 30 feet) access vaults every quarter mile which can be very disruptive to adjacent neighborhoods and the environment.
- Repairs take much longer and can be more difficult with underground lines. While overhead lines can be repaired within hours or days, underground transmission line repairs can take days or even weeks.