Frederick Wiseman has permitted his movies a running time inexcess in two hours on only two occassions. "Welfare," presented over CBS affiliates in 1975, ran close to three hours and so does "Canal Zone," his 11th documentary feature about American institutions and social problems, scheduled or showing tonight at 9 o'clock on WETA, Channel 26.

The expansion that enhanced the cumulative impart of "Welfare" seems to make "Canal Zone" dreadfully heavy going. It's not as if Wiseman had failed to return with some revealing observations from his trip to the Canal Zone in May and June of 1976.The problem is that the observations evoke such unrelieved and reiterated tedium and depression that one feels mightily tempted to abandon ship. "Canal Zone" is the first Wiseman movie whose cumulative effect proves more debilitating than illuminating.

Viewers anticipating a documentary about the history of the canal or about the current treaty controversy are almost certain to be disappointed. The information they're seeking can be found in the entertaining, stirring pages of David McCullough's new book "The Path Between the Seas" or in the daily press.

"Canal Zone" opens with a long sequence of a ship being piloted through the locks, and Wiseman eavesdrops on a guide in order to incorporate some basis facts and figures about the canal, but his subject is the apparently forlorn, homesick lifestly - you should excuse the expression - of the Zonians, the American dependents residing in the Canal Zone.

In a kind of theme-stating sequence we hear a middle aged employee of the Canal Company express the fundamental problem to a fellow HAM radio operator: "Many, many thanks for a nice contact. How are things up in the good country?" The movie is full of more or less sad-looking ordinary citizens searching for a nice contact of some sort and clinging to customs and remembrances of the good country while stationed in what must strike them as a tropical purgatory.

Wiseman's impressions never connect directly with the current political debate about Panama, although one get an awful sinking feeling imagining how the Zonians might be exploited, by either Panamanian insurgents or an American government determined to maintain the statues quo at any cost, in the event of a diplomatic impasse.

Up to a point it's interesting and probably valuable to observe the garrison mentality that affects the members of an anachtronistic colonial outpost. Traits that would look harmlessly parochial here may acquire a slightly neurotic or sinister aspect when indulsged in exile.

The frustrations and defense mechanisms Wiseman documents seem to meet the eye quicker than he thinks they do. Many scenes have a lingering quality that wears one down emotionally instead of adding or complicating impressions. For example, we watch a young wife respond to a drawing given to her by a psychologists and her description of what she sees reveals some of the fears and aspirations about her own marriage that must have led her to seek professional help.

The question is do we also need to see her respond to a second drawing, and a third, and fourth? Wiseman may not have lucked into some pertinent confrontation on this occasion, but that doesn't account for his almost superhuman tolerance of sermons, lectures and public addresses in the course of "Canal Zone."

There's a heavy concentration of oratory, none of it particularly inspiring, and the weight of it all becomes destructive. You cease caring about this environment and the people who feel trapped in it. You just want to get out.