As the nation woke up yesterday to its first day of federally mandated energy conservation, phone operators in the Forrestal Building were ready.

At 8:30 a.m., the Department of Energy's Emergency Building Temperature Hotline opened to the expected onslaught of callers ready to turn in violators of the new law which makes it illegal (up to a maximum $15,000 in fines) to air condition most commercial establishments below 78 degrees.

They got an onslaught - of excuses. In keeping with the American spirit of enterprise, innovation (and a bit of genuine confusion), most callers had thought up reasons why they might be exceptions to the rule:

- A clothing store operator in Nashville, Tenn., said the lapels on his jackets were wrinkling and so was his rug. (Response: If humidity goes over a certain point, the thermostat can be set below 78.) The store also has an employe with emphysema who cannot work unless the room is quite cool. (Get a doctor's certification, and the store will be exempt.)

- A caller from a bank in Minnesota said its envelopes stick together when it's too warm, and some machines won't work. (If machines are essential to operation, that is a basis for an exemption.)

- A health food store operator in Berkeley, Mich., said there are no preservatives in the store's food, and a higher room temperature will attract bugs. (Keep room temperature as high as you can without attracting bugs.)

- A caller from a cookie factory in Orlando, Fla., said the thermostat is never set below 70 degrees and the room temperature never drops below 80 at that seting. (The law says room temperature must not be below 78. Turn the thermostat to whatever reading is necessary to keep within the requirement.)

- A man from Frazier, Penn., who owns a chain of small drugstores, said his vitamins would melt if he kept room temperature at 78 degrees. (check with the Food and Drug Administration.)

- A man called up after an argument with the owner of a gift and card shop. The man claimed the owner was lowering the store's temperature to keep the candy bars from melting. Can that be done? (Perhaps.)

- One grinning phone operator asked another during a break between phone calls: "Do Eskimos in Alaska have to comply?"

- A funeral home director in Indianapolis called to ask if he could lower the funeral home's temperature. Are people in grief considered to have a health-related problem, he wanted to know. (Henry Bartholomew, director of the Hotline, shook his head.)

- "What am I supposed to do with my dead animals?" asked a clothing store owner from the Midwest. He meant stuffed animals. (No exemption for stuffed animals in clothing stores.) Operator Christopher Freitas, who took this call, is well-versed in hotline operation. He usually works for the Energy Information Center where someone once called him for the fare to Mars and back.

Some were inventive:

- One woman who owns a bowling alley called DOE last week to complain that the higher temperature would make bowlers score worse. Because they were hot and disgusted by their bad games, they would not bowl as much. (She got some sympathetic words - but no exemption.)

- One woman called and asked if she could cut down on her hours of work from 66 to 44 and lower the temperature, because she was open fewer hours . . .(No.)

Some violators were reported - a shopping center in Cincinnati, a post office in Washington, an office in Dallas. "He said the people in his office have had pneumonia it's so cold there," explained phone operator John Muller.

"Just write the complaints down," said Bartholomew. "I don't know what we want to do with them."

Chances are there will not be much anyone at the Department of Energy can do with the complaints until they have some staff or commitments from state governments to do inspections.

Bartholomew, a calm man with longish gray hair who vaguely resembles Lee Marvin, walked around the Hotline phone room, his collar open, a gold chain around his neck.

A little after noon, he looked around the room, pleased with the operation. "We're gotten about 350 to 400 calls," he estimated. "I don't think we got any calls from Puerto Rico or Hawaii, because we didn't publicize the number."

He was interrupted by another phone call.

"Hi," he said, "I hear you have a veterinary hospital. Well, there is an exemption . . ."