In June, Time Warner Cable began a metering trial in one Texas city by offering various monthly plans and charging extra when consumers exceeded their bandwidth limit. AT&T has said that it is considering a similar pricing plan. The concept is not a foreign one; consumers already pay by usage for water and electricity. But broadband access has seemed unlimited, and any stifling of that is sure to concern some customers.

Until now, Comcast had not defined excessive use, but it had contacted customers who were using the heaviest amount of broadband and asked them to curb usage. Most do so willingly, the company said. The ones who do not curb their usage receive a second notice and risk having their accounts terminated.

Although the 250 gigabyte cap is now specified, users who exceed that amount will not have their access switched off immediately, nor will they be charged for excessive use. Instead, the customers may be contacted by Comcast and notified of the cap. The company did not say how 250 gigabytes was selected.

According to Comcast, a customer would have to download 62,500 songs or 125 standard-definition movies a month to exceed the caps. But high-definition video and video gaming require a higher amount of bandwidth. S. Derek Turner, the research director for the nonpartisan media policy group Free Press, said broadband caps could create a disincentive to view online video.

“As media companies put content online, consumers can bypass the cable companies and get their content directly from the Internet,” Mr. Turner said. “A 250 gigabyte cap may seem very high  and it is for today’s Internet use. But it’s essentially the equivalent of four hours of HD television a day.”

Critics have charged that Internet providers are trying to protect their cable TV and telephone businesses by stifling Internet access. Comcast says Fancast, its online video Web site, will count against the 250 gigabyte limit, but its digital voice service will not.

Comcast said there was no link between the caps, announced Thursday, and the Federal Communications Commission’s finding on Aug. 1 that the company was improperly inhibiting customers who used BitTorrent, a popular file-sharing program.

But Andrew Jay Schwartzman, the president of the Media Access Project, said the caps appeared to be a direct result of that finding. Mr. Schwartzman’s group represented Free Press in its complaint against Comcast about the file-sharing controls.