The merger between PacifiCorp and Utah Power & Light Co., the largest in the electric utility industry in 50 years, is officially under way.

Papers filed Monday with the state's Department of Business Regulation officially make UP&L a division of PacifiCorp, a $5.7 billion diversified utility company based in Portland, Ore. In addition to owning Pacific Power & Light and now UP&L, PacifiCorp also owns a phone utility, mining company and financial services firm.Based on projected savings of about $150 million annually resulting from merging the two systems, PacifiCorp has promised that rates for UP&L customers will fall at least 5 percent within five years after the merger, with 2 percent of that decrease coming in March.

Announcements on how the merger will change operations at UP&L and Pacific Power are expected later this week. UP&L will retain its name and headquarters in Salt Lake and management of the power supply for both utilities will be based in Utah.

Also pending are instructions for UP&L shareholders on how to convert their stock into shares of PacifiCorp.