Los Angeles-based mutual fund manager Capital Group Cos. is set to start building a 520,000-square-foot Irvine campus that has been years in the making.

Capital Group, which operates the American Funds Group of about 30 mutual funds, plans to start work on a five-building complex in the Irvine Spectrum in September, according to spokesman Chuck Freadhoff.

The campus is set for 35 acres of land bought from The Irvine Company. The project stands to be among the largest office developments and among the biggest of any kind in Orange County this year.

It could end up being second only to chipmaker Broadcom Corp.’s 700,000-square-foot campus set to break ground in Irvine’s University Research Park later this year.

The first phase of Capital Group’s Irvine campus is set to be done in 2007. Along with the five buildings, plans call for three parking garages. Capital Group has plans for an additional 80,000-square-foot building at the campus if needed, Freadhoff said.

San Francisco’s Hathaway Dinwiddie Construction Co. has been tapped to build the campus. Prominent New York architectural firm Pei Cobb Freed & Partners LLP is working on the project with Irvine-based LPA Inc.

The Irvine campus could employ as many as 3,000 people a few years after opening. Initially, Capital expects to employ 2,300 people at the site, according to Freadhoff.

The company has no plans to shift its Los Angeles headquarters to Irvine, according to Freadhoff.

*Kate Berry of the Los Angeles Business Journal contributed to this story.