Time Inc. announced yesterday that it plans to sell up to 20 percent of its cable television subsidiary to the public to raise money that can be used for acquisitions.

The company's American Television and Communications Corp. subsidiary (ATC) is the nation's second-largest cable system operator, behind Tele-Communications Inc. of Denver. ATC has 2.7 million subscribers, and the sale of a 20 percent interest would raise more than $500 million, based on an estimated value of $1,000 per subscriber.

Time, which publishes Sports Illustrated, People, Fortune and Money, has grown historically through internal development of magazines. But after difficulty developing new magazines, Time purchased Southern Progress Corp., publisher of Southern Living, for $480 million last year.

Time President J. Richard Munro indicated yesterday that the company would consider additional magazine acquisitions. He said Time will "consider acquisitions on an opportunistic basis throughout the communications industry."

Company spokesman Mike Luftman said Time's program to enhance shareholder value recently has included the purchase of its own stock. Luftman said Time has been buying its shares this spring, under authorization granted in 1984 to purchase 4 million shares. He said that management is considering asking the company's board of directors for additional authorization to buy shares.

Time, which has about 64 million shares outstanding, closed yesterday at 79, up 2. The announcements were made after the close of trading.

In the midst of a program to reduce operating expenses by $75 million, Time also announced yesterday that its board of directors has approved the "poison pill" antitakeover defense. The defensive maneuver is designed to make a takeover bid that does not have the support of Time's board of directors prohibitively expensive.

"The company is not aware of any present takeover attempt," Munro said. "The purpose . . . is to discourage coercive takeover tactics that may not be in the best interest of the company and its stockholders."

Under the terms of the poison pill, Time stockholders have been granted the right, under certain conditions, to buy stock for half price, in a company attempting a hostile takeover. This is designed to make a hostile takeover of Time too costly.

The poison-pill rights become effective 10 days after anyone acquires 20 percent of Time's common stock, or after disclosure of plans to acquire 30 percent or more of Time.

Commenting on Time's decision to sell 20 percent of its cable subsidiary to the public, Munro said, "As the cable industry has entered the mainstream of communications, investors have increasingly recognized the potential of well-managed cable companies. By establishing a market for ATC stock, this proposed offering will provide added financial flexibility to support Time Inc.'s strategy to extend its commitment to the cable television industry."