Comcast will start enforcing a 300GB data cap in Atlanta next month, with overages resulting in extra charges of $10 for each additional 50GB.

The move expands the list of cities subject to Comcast's data overage charges. In May 2012 the company raised its data cap from 250GB to 300GB while getting more serious about penalizing customers who go over the new limit.

Caps were in place in Nashville, Tennessee, and Tucson, Arizona, a year ago.

"The 300GB has been in trials in Nashville for some time, but back in August Comcast expanded these trials into the Central Kentucky; Savannah, Georgia; and Jackson, Mississippi markets," DSLReports wrote today.

The report pointed to a notice to Atlanta customers posted by a reddit user, saying the data overage charges will take effect December 1. A Comcast pricing FAQ updated yesterday said, "The data amount included with all XFINITY Internet tiers will be 300GB per month. We will offer additional gigabytes in increments/blocks of 50GB for $10 each." That pricing applies to Huntsville and Mobile, Alabama; Atlanta, Augusta, and Savannah, Georgia; Central Kentucky; Maine; Jackson, Mississippi; Knoxville and Memphis, Tennessee; and Charleston, South Carolina.

Those same markets are also being offered a "Flexible-Data Option" that assumes customers would use only 5GB per month. "The Flexible-Data Option provides a $5 credit if a customer’s total monthly data usage is less than or equal to 5GB per month," Comcast says. "However, if these customers use more than 5GB of data in any given month, they would not receive the $5 credit and would be charged $1 per gigabyte for each gigabyte of data used over the 5GB included in the Flexible-Data Option."

The 5GB plan is also available in Fresno, California.

We've asked Comcast when the overage charges will come to other cities or be spread nationwide, but we haven't heard back yet.

If you were hoping the cable industry would move away from data caps instead of toward them, you might be disappointed. Michael Powell, a former Federal Communications Commission chairman who is now the cable industry's top lobbyist, recently said that cable companies "should be moving with some urgency and purpose" toward implementing data caps.

UPDATE: Comcast has gotten back to us, noting that it has "No further announcement than just doing a pilot in Atlanta, which is our largest market trial to date." Comcast further said: