About the Project
Idaho Power is planning the Mayfield Substation and Transmission Line Project to strengthen the regional power grid and support reliable, affordable energy for customers across southern Idaho.
The project includes a new substation and new 230-kilovolt (kV) transmission lines that will connect north and south of Interstate 84. As a key hub in the system, Mayfield will help move electricity more efficiently across the region, especially during times of high demand or changing conditions.
The substation will be built on land Idaho Power has owned for decades, about 40 acres purchased in 1984, with an additional 25 acres added in 2025. The site is largely non-irrigated and not currently used for agriculture.
Background and Need
Southern Idaho is growing, and so is the demand for electricity. To keep service reliable, Idaho Power must continue to expand and strengthen the transmission system.
The Mayfield site was identified many years ago as a strategic location because it sits near major transmission lines, including an existing 500-kV line and several 230-kV lines. It is also near other energy infrastructure, allowing multiple power sources to connect at one location.
The new substation will connect these lines and improve how electricity moves across the grid by:
- Bringing power from a high-capacity 500-kV line (up to 1,000 megawatts) into the system
- Connecting multiple 230-kV lines (each capable of moving about 400 megawatts)
- Balancing power flows and reducing strain, or “bottlenecks,” on the system
- Providing additional backup (redundancy) for nearby substations like Hemingway and Midpoint
This helps ensure consistent, reliable service for customers across the Treasure Valley, Magic Valley, and surrounding areas.
Benefits
More Reliable Service – Reduces congestion on the grid and adds backup support to help prevent outages.
Ready for Growth – Supports increasing energy needs for homes, businesses, and agriculture.
Improved Energy Flow – Helps move electricity more efficiently across southern Idaho.
Cost-Effective Solution – Uses an existing, well-located site near major transmission lines to help keep costs down.
Stronger Regional Grid – Connects major transmission paths and energy sources in one central location.
Current Status and Timeline
The Mayfield Substation project is currently in the planning and design phase. All required easements for transmission lines have been acquired. Public outreach was conducted in spring 2025 and included two open houses, with more than 200 nearby landowners invited to each meeting within a three-mile area. Final construction plans and access road locations are still being developed.
Project Schedule
- 2025 – Begin permitting, easements, land acquisition
- 2026 – Continue permitting; complete design, and begin site work
- 2027 – Complete site work; begin fabrication and construction
- 2028 – Complete substation and line construction. In-service summer 2028
Additional updates, including 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 being considered 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?
Contact Idaho Power project manager, Lisa Loomis at lloomis@idahopower.com.