Southeastern Idaho

Firth, Idaho Falls, Rigby and Rexburg

Project overview

Electrical power consumption in southeastern Idaho has grown steadily over the last several years and this increased demand for electrical power is forecasted to continue into the future. By around 2021, demand for electrical power is projected to surpass the capacity of the electrical system. To continue providing safe, reliable, efficient and cost effective power to customers, Rocky Mountain Power must upgrade and expand existing facilities.

Rocky Mountain Power has identified the lowest cost, highest value solution, which requires various substation upgrades and a 161 kilovolt (kV) transmission line to provide improved capacity and reliability to the region.

About the project

The project requires a multi-stage upgrade to the electrical system from Goshen Substation, near Firth, Idaho, to Rexburg Substation, in Rexburg, Idaho.

From Goshen Substation through Ammon Substation and to Sugarmill Substation, an existing 69 kV transmission line will be rebuilt for 161 kV operation. Goshen Substation will also be upgraded with an additional 345 kV/161 kV transformer and an expanded 161 kV breaker yard. Ammon Substation will be converted to receive 161 kV transmission, and Sugarmill Substation will be upgraded with a 161 kV breaker yard expansion.

From Sugarmill Substation to Rigby Substation a new 161 kV transmission line will be constructed. After generally exiting north out of Idaho Falls, the new transmission line will follow Ririe Outlet Canal and then go north along Lewisville Highway to 400N in Rigby where the existing 69 kV line from Roberts will be rebuilt to 161 kV. Rigby Substation will be upgraded with a 161 kV breaker yard.

From Rigby Substation to Rexburg Substation, an existing 69 kV line has been rebuilt to 161 kV. Also, Rexburg Substation will be upgraded to receive 161 kV transmission.

Also, for a certain portion of this project, Rocky Mountain Power is working with Idaho Falls Power to jointly construct and operate a portion of the transmission line. This coordination eliminates the need for two separate lines (one for each company) and reduces costs.

These improvements will provide increased capacity and reliability to the area.

To maintain reliability and serve peak load, the construction of the facilities will be completed in segments. The first segment was completed in 2019 with the final segment scheduled for completion in 2022.

Project timeline

  • Permitting & Rights of Way – 2017 - 2020
  • Engineering & Equipment Procurement – 2017 - 2021
  • Construction
    • Rexburg – Rigby 161 kV Line – 2018 - 2019 (complete)
    • Goshen (Firth) – Sugarmill (Idaho Falls) 161 kV Line – 2019 - 2021 (complete)
    • Goshen Substation – 2020 (complete)
    • Ammon Substation – 2020 (complete)
    • Sugarmill Substation – 2020 - 2021 (complete)
    • Rigby Substation – 2020 - 2021 (complete)
    • Rexburg Substation – 2020 - 2021 (complete)
    • Sugarmill (Idaho Falls) – Rigby 161 kV Line – 2021 - 2022 (complete as of April 21, 2022)

Additional information about the project


If you have additional questions, please call 801-220-4221 or email us at Please be sure to include the project name – "Southeastern Idaho – Firth, Idaho Falls, Rigby, Rexburg" – in your inquiry.

Updated April 8, 2021