SALT LAKE CITY (AP) _ Utah Gov. Norm Bangerter announced today he won't seek a third term in 1992, saying he always had seen the post as ''an eight-year commitment.''

The surprise announcement came after ''careful consideration'' over the Thanksgiving weekend, he said, adding that he and his family decided eight years as chief executive was enough.

''Someone once told me that politics is dependent on a never-ending supply of people who don't know what they're in for. After six years, I know too well,'' Bangerter said at a hastily arranged news conference at the Capitol.

The governor's board room was overflowing with reporters, cameras, state department heads and members of Bangerter's family. His wife, Colleen, was at his side.

Bangerter said that when he was elected to the Utah House in 1974 he ''came not as a career politician. Serving in the Legislature is all I intended to do.''

Instead, his initial election in 1984 over Wayne Owens ended 20 years of Democratic control of the governorship.

His voice breaking, Bangerter, 57, said the rigors of public office have interfered somewhat with his role as a father and grandfather. He has seven children.

''I want to go down in history as the governor who didn't spend eight years worrying about how he'd go down in history,'' Bangerter said.

He said that once his term was over in January 1993, he planned to return to the jobs he left for the governor's chair, those of builder and contractor.

''But we need to finish this administration with the same energy we started it with. The people of Utah should know they have my full energy and attention in the rest of this term,'' he said.

The former speaker of the state House of Representatives was re-elected in 1988 in a come-from-behind victory over Democratic former Salt Lake City mayor Ted Wilson. Bangerter had trailed Wilson by as much as 30 percentage points early in the campaign.

Today he called that victory the high point of his administration.

''It was kind of fun to get all those phone calls from people who forgot to send a check,'' he said.

Polls showed his job approval rating drop severely after a record $168 million tax increase he pushed through the Legislature in 1987. But it climbed after Bangerter's re-election, until this year.

In August, a Deseret News-KSL-TV poll showed Bangerter's popularity had fallen from 61 percent in December 1989 to 56 percent. A disapproval rating of 41 percent in the latter survey was by far the highest of any of the state's elected officials, the poll found.

Bangerter had been sharply criticized earlier this year for vowing to spend state funds to defend a lawsuit by the American Civil Liberties Union seeking to ban prayers at high school graduations.

In October he also raised the possibility of tax hikes in areas other than the property tax to deal with a Utah Supreme Court decision that Amax, a minerals company, had been denied equal taxation. Analysts said the ruling could cost state and local governments $56 million a year.