Electricity demand in Big Sky grew by more than 60 percent in the last decade, outstripping what NorthWestern Energy is able to deliver with its power line that stretches to the mountain town from a substation in Four Corners.

A second line running from Ennis over the southern Madison Range to Big Sky allows the energy company to meet peak monthly winter demands of more than 29 megawatts — about enough electricity to power a home for three years. But high winds and falling limbs that are not uncommon in the rugged terrain can cause that 69-kilowatt line to fail. When it does, Big Sky can be flushed into darkness.

“They’re using a lot more power up there these days. The current line (from Four Corners) couldn’t handle that capacity on a peak day,” said Emmett Riordan, NorthWestern’s director of electric transmission.

NorthWestern Energy is going through the final stages of approval for a roughly $34 million, two year project to upgrade the transmission line running from Four Corners to Big Sky. The upgrade will more than double the line’s capacity, Riordan said. The utility company began the permitting process six years ago.

Already the energy company has installed 12 miles of new line from Four Corners to the mouth of Gallatin Canyon. The project, which will employ 10 NorthWestern workers and about 30 outside contractors, is expected to start work in the Gallatin National Forest by May 20 at the earliest, said Amy Waring, National Environmental Policy Act planner with the U.S. Forest Service.

Those that live and work in Big Sky are looking forward to having the town’s infrastructure shored up. Development appears ready to ramp back up. Contractors are seeing more housing work, notably for full-time residents rather than predominantly second homes or vacation homes. Developers are beginning to discuss building plans.

Bob Foster, general manager of Lone Mountain Ranch, said there has been talk of potentially building a branch hospital and a larger grocery store, such as an Albertson’s or a Safeway. Having more reliable electricity — and an expanded sewer and water system, he noted — would help clear the path for those businesses to come into the town straddling Gallatin and Madison counties.

“As a businessman, I’m always eager to have infrastructure that can support more growth and be ahead of the curve rather than behind the curve,” said Foster, who also sits on the board of directors for Visit Big Sky, a destination-marketing group.

One of the more recent power outages occurred on March 25, said Kitty Clemons, executive director of the Big Sky Chamber of Commerce. The outage occurred when a surge in electricity blew through “two of the really high-tech” surge protectors in the chamber office. Still, she said sewer and water availability is brought up by businesses much more frequently than electricity issues.

The outage came during a heavy snowstorm in which a tree fell across the power line coming from Ennis, said Riordan. Winter storms are dangerous for the town’s power supply, because Big Sky draws the most electricity when its ski areas are running lifts full-time and vacation homes are occupied during cold weather.

“Right now, today, we can only supply about half of the peak (demand) from the Jackrabbit side, that’s why we’re rebuilding the line,” Riordan said.

The line is about 35 miles long, most of which runs through the Gallatin National Forest, he said. It will roughly follow the same path as the existing power line. Where the new line’s path will differ is by the Cave and Cascade creek cabins. The line will run west of the cabins on Cave Creek and along the eastern side of U.S. Highway 191, cutting out two river crossings, by Cascade Creek. Building the new line won’t affect the Gallatin River’s eligibility for Wild and Scenic designation, Waring said.

If NorthWestern receives the necessary permits from the Forest Service, Riordan said the utility company would work this summer to clear timber from the power line’s right of way. Work connecting the new higher-capacity line from the mouth of the canyon to Big Sky’s Meadow Village would begin in 2014, Riordan said.

The utility company told the Forest Service that it expects work to conclude in 2015, allowing for wiggle room because of the complexity of the terrain, said Butch Larcombe, a NorthWestern spokesman.

Jason Bacaj may be reached at jasonb@dailychronicle.com or 582-2635.

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