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The Missouri Public Service Commission has approved an electric rate increase for Ameren Missouri that is estimated to raise the typical residential customer's bill by about $8 per month.

The increase is expected to take effect in August.

The commission on Wednesday voted 5-0 to grant a rate increase of about $172 million to Ameren Missouri, about $91 million less than what the company requested in its September 2010 filing. The increase reflects Ameren Missouri's increase in costs to provide service to its customers, according to a PSC news release.

The commission denied Ameren Missouri's request to include any of the cost to rebuild the upper reservoir of the Taum Sauk plant to the rate base. After the December 2005 Taum Sauk incident, Ameren Missouri promised to protect its ratepayers from the consequence of that collapse, and the commission said it intended to hold the company to that promise.

As part of its decision, the commission set Ameren Missouri's return on equity, or profit margin, at 10.2 percent. Ameren Missouri sought 10.7 percent.

When Ameren Missouri filed its rate request, it sought to increase annual electric operating revenues by about $263 million, according to the PSC's release. The company cited costs associated with energy infrastructure investments, higher fuel costs for its power plants and the installation of two scrubbers at its Sioux Power Plant in northern St. Charles County as major reasons for seeking a rate increase. The scrubbers are designed to remove in excess of 95 percent of the sulfur dioxide gas generated by the plant as well as removing oxidized mercury, sulfur trioxide, particulate, hydrogen chloride and hydrogen fluoride. Ameren Missouri installed the scrubbers to comply with various federal clean air rules.

Ameren Missouri provides electric service to about 1.2 million customers in Missouri.