When Comcast decided to suspend its 250GB per month caps on Internet service, the reprieve for customers was only temporary. New caps of 300GB to 600GB would hit two test markets by the end of the year, Comcast said, with the caps presumably affecting customers nationwide at some later, not-yet-announced date.

Comcast has now revealed some details on how those caps are being implemented as they hit test cities. The caps began in Nashville, Tennessee, on August 1, and go into place in Tucson, Arizona, on October 1. In Tucson, customers are getting at least 300GB, as promised, and can get up to 600GB if they pay for the Extreme 105 service, which offers 105Mbps download speed:

Those who go over their caps will automatically be billed $10 for another block of 50GB. Customers can view their usage online, and when they exceed their caps they will get an e-mail and an in-browser notification like this one:

In Nashville, the cap is 300GB no matter what tier of service you subscribe to. The difference between Nashville and Tucson makes sense given that Comcast said in May that it will pilot “at least two approaches in different markets.”

The automatic $10 charge for each additional block of 50GB applies to both Nashville and Tucson. Comcast customers in both test markets also get three “courtesy passes,” meaning that customers won’t be billed the first three times they exceed their monthly allotment in a 12-month period.

Comcast is not saying whether these details will remain the same when caps are implemented beyond Nashville and Tucson. “We need to run these trials for a few months so we can assess the customer experience and our network,” a Comcast spokesperson told Ars, adding that “we don’t have any additional plans to announce at this time.” An article in Broadband Reports citing a "reliable source" claims that Comcast's national rollout will adopt tiered caps, but this has not been confirmed.

How high can the caps go? Comcast is planning 305Mbps service in some markets, but not in Tucson or Nashville. If Comcast continues with the Tucson approach of giving progressively bigger caps to customers who pay for higher speeds, those who opt for 305Mbps could end up getting more than 600GB per month.

Even the previous 250GB cap affected less than 1 percent of customers, and median monthly usage is only 10 to 12GB per month, according to Comcast. But, people and families with multiple devices who download and stream lots of video could find themselves hitting Comcast’s limits.

Of course, Comcast makes exceptions. If you’re using Comcast’s Xfinity TV On Demand on an Xbox 360, for example, that traffic does not count against your usage cap. Criticism that Comcast is prioritizing its own services over those of video streaming companies like Netflix and Hulu has led to a Department of Justice investigation into possible antitrust violations.

Listing image by Comcast