Southeastern Idaho – Firth, Idaho Falls, Rigby, 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 is currently being 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 is scheduled for completion in 2019 with the final segment scheduled for completion in 2022.

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 October 12, 2018

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