The MSTI 500kV alternating current (AC) transmission line would interconnect the new Townsend Substation with IPCO’s existing Midpoint Substation. The MSTI 500kV transmission line would be 390 to 400 miles long.
Various alternative routes have been identified as part of the siting study for the transmission line. A preferred route was selected based on environmental and other considerations. Alternative route links, shown in Figure 1, cross Silver Bow, Jefferson, Broadwater, Deer Lodge, Beaverhead, and Madison counties in southwestern Montana and Clark, Jefferson, Blaine, Butte, Bingham, Booneville Power, Minidoka, Lincoln, and Jerome counties in southeastern Idaho. The links cross private, state (Idaho and Montana) and federal (primarily Bureau of Land Management [BLM] and U.S Forest Service [USFS]) land. There are a total of 1,150 miles of alternative route links being evaluated, 582 miles in Montana and 568 miles in Idaho.
The MSTI 500kV transmission would be constructed mainly on guyed-V steel lattice structures approximately 125 feet high. Less frequently, self-supporting steel lattice structures or self-supporting tubular steel structures approximately 125 feet high would be used. The guyed V structure would be used for most tangent segments of the line. Self-supporting steel lattice structures would be used in mountainous areas and at points where a line changes direction or terminates. Tubular steel monopoles may be used in areas of narrow right-of-way or where permanent land disturbance or the amount of land required for the structure must be minimized (e.g., agricultural land and developed areas). The land permanently required for the structures would vary depending on structure type and terrain, ranging from 100 square feet for steel monopoles to 22,500 square feet for the guyed V structures. An area of approximately 200 by 200 feet (0.9 acre) per structure may be temporarily disturbed during construction.
The required right-of-way (ROW) width is 220 feet and the average span length between the transmission structures would be approximately 1,400 feet (4 per mile) for the guyed V structures, 1,200 feet (4 per mile) for the self-supporting steel lattice structures, and 900 feet (6 per mile) for the self-supporting tubular steel monopole structures.
Access along the transmission line ROW would include using existing improved roads, using existing roads that require improvement, and building new roads. Permanent new roads would be graded to a travel service width of 14 feet.
In addition, during construction of the transmission line there would be temporary pulling and tensioning sites, material staging sites, and concrete batch plants.