Cox has started charging overage fees to customers who exceed their data limits in California, bringing the nation's third largest cable company a bit closer to nationwide deployment of data caps.

Previously, Cox's California customers technically had monthly caps, but there was no enforcement of overage fees. Moving forward, Cox subscribers in the state will pay $10 for each additional block of 50GB after they pass the monthly cap.

Customers who signed up for the 1Gbps "Gigablast" service prior to September 26 of this year will get unlimited data plans at no extra charge, while all other California customers will have a 1TB (1,024GB) cap.

But while Cox customers in some other states can pay an extra $50 for unlimited data, that option doesn't seem to be available in any of Cox's four California markets (Orange County, Palos Verdes, San Diego, and Santa Barbara). Cox posts region-by-region details of its data caps and overage fees at this webpage. (UPDATE: Cox tells us that California customers can pay $50 extra for unlimited data or $30 for an extra 500GB, despite its website not including that detail.)

Before this week, Cox was already enforcing data caps and overage fees in Arizona, Arkansas, Connecticut, Florida, Georgia, Idaho, Iowa, Kansas, Louisiana, Massachusetts, Nebraska, Nevada, Ohio, and Oklahoma. Cox users in North Carolina, Rhode Island, and Virginia technically have monthly caps but no enforcement of overage fees. Cox operates in 18 states and has about 6 million residential and business customers.

The Comcast model

Cox has largely followed the Comcast data cap model, with the same overage charges and the gradual city-by-city rollout.

The new overage charges drew the attention of The San Diego Union-Tribune. "Just under 2 percent of Cox Communications’ subscribers in San Diego blow past their one-terabyte data cap each month, said [Cox] spokeswoman Ceanne Guerra," the paper reported yesterday.

But that percentage will almost certainly rise over time unless Cox adjusts the cap upwards. In late 2013, when Comcast enforced a 300GB monthly cap, the company said that only 2 percent of its customers used more than that. By late 2015, that was up to 8 percent, and Comcast boosted the cap to 1TB.

Many customers have disputed overage fees, saying the Comcast meter is wrong, and in some cases the customers have been proven correct.

Regardless of what caused an overage, Comcast customer service reps often tell customers who complain about high data readings that someone else might be using their network, and Cox appears to be heading down the same path.

"Some people are just heavy users for whatever reason," Guerra said, according to the Union-Tribune. “But there are some customers, we think, who have in-home Wi-Fi and maybe they have not secured it with a password. So it may be open and other people might be tapping into it.”

Correction: This story originally said that "Massachusetts and North Carolina seem to be exempt from the Cox data caps altogether." A closer examination of Cox's various data cap webpages showed that two of three Massachusetts towns that Cox operates in are subject to data cap overage fees. North Carolina technically has a cap, but no enforcement of data overage fees.