About the Project

Idaho Power’s Mayfield Substation and Transmission Line Projects are a coordinated group of energy infrastructure improvements designed to strengthen the regional power grid and support reliable, affordable energy for customers across southern Idaho.

Several related projects are planned or underway in the Mayfield area, including:

• Final design and future construction of the Peregrine Energy Center on Simco Road.
• Final design and construction of the new Mayfield Substation on Baseline Road.
• A new transmission line connecting Mayfield and Pleasant Valley Substations.
• A new transmission line, with a short segment rebuilding an aging transmission line, connecting between the Mayfield Substation and Peregrine Energy Center.
• A new transmission line, parallel to an existing transmission line, connecting the Peregrine Energy Center and Pleasant Valley Solar Substation.
• Rebuilding an aging transmission line that runs from north of the Mayfield Substation into the Boise area.
• Final design and future construction of Gateway West 500-kV transmission line, which will cross through the same area.

Idaho Power has coordinated the planning and permitting of these projects to help reduce impacts on nearby landowners and communities. By taking this approach, we can complete construction more efficiently, minimize disruptions where possible, and continue providing reliable energy service.

As a key hub in the system, the Mayfield Substation will help move electricity more efficiently across the region, especially during periods of high demand or changing operating conditions. Together, these projects will improve transmission reliability, support future growth, connect new energy resources, and enhance wildfire resilience in the area.

Purpose and Need

Southern Idaho continues to grow, increasing demand for electricity across the region. To maintain reliable service and prepare for future energy needs, Idaho Power must continue to expand, modernize, and strengthen its transmission system.

The Mayfield area has long been identified as a strategic location for these improvements because it is situated near major transmission infrastructure. Its location also allows multiple energy resources and transmission facilities to connect at a central point.

Together, the Mayfield area projects will:

• Strengthen the regional transmission system
• Improve how electricity moves throughout southern Idaho
• Connect new generation resources to the grid
• Reduce strain and transmission bottlenecks
• Provide additional system redundancy and reliability
• Support wildfire mitigation efforts through transmission line upgrades

These improvements will help ensure reliable electric service for customers throughout the Treasure Valley, Magic Valley, and surrounding communities in Idaho.

Benefits

Current Status and Timeline

Several energy infrastructure projects are planned or underway in the Mayfield area. Through a coordinated approach to planning, permitting, and construction, Idaho Power can complete this work as efficiently as possible while minimizing impacts to the community.

Mayfield Substation

• Final design underway.
• Public outreach completed in spring 2025, including two open houses.
• Construction anticipated to begin fall 2026.

Mayfield–Pleasant Valley Substation Transmission Line

• Easement acquisition complete.
• Public outreach completed in spring 2025, including two open houses.
• Final construction plans and access road locations are being developed.

Peregrine and Pleasant Valley Solar Interconnection Lines

• Planning and permitting underway.
• Public outreach and open house planned for September 2026.
• These transmission line connections will link the Peregrine Energy Center and Pleasant Valley Solar Substation with the Mayfield Substation.
• Final construction plans and access road locations are being developed.

Northern Transmission Line Rebuild

• Planning and permitting underway.
• Public outreach and open house planned for September 2026.
• Construction anticipated to begin in spring 2027.
• Project will replace aging wooden H-frame structures with single steel poles to improve reliability and support wildfire mitigation efforts.

Gateway West Segment E-8

Focused on Segment E-8, from Hemingway Substation (near Melba) to Midpoint Substation (near Jerome).

• Easement acquisition underway.
• Public outreach underway, first round of open houses in spring 2025 and another round of open houses planned for early 2027.
• Final construction plans and access road locations are being developed.

Additional updates, including detailed construction timing, will be shared as the project progresses.

Frequently Asked Questions

What will the substation and transmission lines look and sound like?

The substation will look similar to other Idaho Power substations with fencing around the entire site. Transformers can produce a slight hum when standing within the site. The new 230-kV transmission lines are single-poles with three lines of wire running between them. Depending on the weather, a faint sound may be heard from the transmission lines connecting the station to the grid.

How will the substation be kept safe and secure?

Safety is a top priority for Idaho Power. The planned Mayfield Substation will be designed to meet strict industry standards for both public and worker safety, as well as wildfire prevention. The location gives us enough space to follow best practices for substation layout and fire prevention.

The site will be fully enclosed with a high-security fence, including a stamped concrete wall along the frontage road and mesh steel fencing around the rest of the perimeter. It will also feature deterrent and detection devices to prevent unauthorized access. Only trained Idaho Power personnel will be allowed on site, and we work closely with local emergency responders to ensure they’re familiar with the facility.

Substations like this operate safely in communities across Idaho every day, helping ensure reliable energy while maintaining a strong focus on safety and security.

Why is this location for the Mayfield Substation and transmission lines?

This location offers a unique opportunity to strengthen the power grid efficiently and cost-effectively. Idaho Power purchased the land in the early 1980s when the first 500-kilovolt (kV) transmission line was built across southern Idaho, specifically with future infrastructure in mind.

The site is ideally situated near both the existing 500-kV line and several 230-kV transmission lines already in operation. It’s also close to the new Pleasant Valley Solar Substation. Tying this infrastructure together at a single point will improve the flow of electricity across the region, reduce strain on the system, and support the reliable, affordable service our customers count on.

Are the Mayfield Substation and transmission lines just being built to serve large businesses or new developments in the Treasure Valley?

No. The Mayfield Substation is needed for Idaho Power to meet growing energy demand in the Magic and Treasure valleys. The Mayfield Substation property was purchased over 40 years ago in anticipation of this substation hub being needed to serve long-term growth. Idaho Power is now at the point where the project is necessary to serve our growing customer base.  

Questions?

We understand there is a lot of planning and work occurring in this area, and we want to ensure neighbors and property owners are well informed of each project and phase of the process. If you have any questions or concerns, please contact Idaho Power project manager, Jeff Maffuccio at jmaffuccio@idahopower.com.