About Growth and Congestion

Learn how the Southline Transmission Project can help support and sustain growth and relieve system congestion.


The Southline Transmission Project will connect the Las Cruces/El Paso area in the east with the Tucson/Phoenix area in the west. The Project will expand opportunities for these and other regional load centers to import power to ensure economical electricity rates for the region as it grows. The Project could also facilitate the development of some of the region’s vast renewable resources to meet local power needs, or for export to other areas. The Project is intended to create a transmission link that provides benefits to population centers by interconnect ing with up to fourteen existing substation locations used by multiple transmission providers.

Population growth is expected to continue in the Southwest. As of the 2010 Census, Phoenix is the sixth largest city in the U.S. by population and Arizona boasts second in population growth at 24.6%, with New Mexico experiencing 13.2% population growth over a 9.7% national average.

The Western Electricity Coordinating Council (WECC), which coordinates and promotes electric system reliability in the western United States, has indicated in its 2011 10-Year Regional Transmission Plan Executive Summary that continued population growth in the West will drive demand, with 28% growth in Arizona and 10% growth in New Mexico anticipated for 2010–2020. These estimates are based on state and provincial population statistics and projections received by WECC. Local utilities have identified a need for additional transmission access to support the expected increased demand for energy. Requests for transmission interconnection from renewable energy generators in Arizona and New Mexico indicate a need for increased transmission capacity.


Due to a lack of sufficient transmission capacity in the Desert Southwest, local electric utilities are constrained in their ability to consistently access the most cost-efficient energy resources. Since 2009, the Southline Transmission Project team has worked to incorporate the needs of regional utilities, other transmission providers, and their customers into the design and routing of the Project in order to improve the region’s electric system.

The Southline Transmission Project team is working through the guidelines established by the Western Electric Coordinating Council (WECC) to secure a rating to use the line in both directions. This bi-directional rating is unique to interstate transmission projects that are planned in the region. It will enable the Project to significantly improve the existing transmission transfer capacity in the region by working with existing electrical facilities. This will help relieve existing congestion in the electrical system in southern New Mexico and southern Arizona.