Five of the region's biggest industrial power users say the proposed Black Hills Power rate increase would harm them by increasing their cost of business at a time they are already under economic stress.

The group filed a petition Thursday with the state Public Utilities Commission, asking to intervene in the issue. Intervenors become part of the case and are allowed to call witnesses at hearings on the proposed 26.6 percent rate increase, commission Vice Chairman Steve Kolbeck said.

Dakota Panel particleboard plant in Rapid City, the GCC Dacotah cement plant in Rapid City, the Sanford Underground Laboratory at Homestake in Lead, and Neiman Enterprises' two South Dakota sawmills -- Rushmore Forest Products in Hill City and Spearfish Forest Products -- have asked to intervene.

GCC Dacotah is the power company's single biggest user of electricity, plant manager Stephen Post told the Journal in an e-mail, and the increase would cost it nearly $1 million more per year.

The Sanford lab would pay about $500,000 more per year, spokesman Bill Harlan said.

"It's a huge amount of money, and in an economy that's stressed -- and we have a very tight budget -- this is potentially devastating," he said.

The lab's biggest uses of electricity are running hoists up and down two 5,000-foot mine shafts and pumping water and running exhaust fans 24 hours a day, he said.

Beyond the five intervenors, 23 residential customers and two other businesses, Coca-Cola Bottling Company High Country and Fischer Furniture, have filed comments with the Public Utility Commission opposing the rate increase.

"I believe the rate increase would place a significant hardship, not only on my business and other businesses in my area, but on my employees and their households," Fischer Furniture vice president Sam Fischer wrote in his letter to the commission. "In the current economy, when businesses are fighting to spend every penny wisely and employees are living on flat or reduced incomes, this proposed rate increase is out of line."