CHARLOTTE, N.C. — So you're at the gas station filling up your vehicle, and without warning the gas pump shuts off.

What? The tank isn't full, and you know your credit card isn't over its limit.

"Using my Visa card, I commonly hit a limit and I would be standing there scratching my head," Shawn Bloomfield, who pumps premium gas into his SUV, said from Allentown, Pa. "I would always assume it is the gas station setting a limit on how much gas I could purchase. It felt like a ration scenario."

As the price of gasoline continues to rise, rules to prevent credit card fraud at the nation's pumps are confusing consumers who just want a full tank of gas.

Caps on transaction amounts — or the total dollar amount of gas a customer can pump into their car — are limiting some drivers of gas-guzzling vehicles.

"When I go to the gas station, I now have to use two credit cards just for one tank of gas," said Paul Brisgone of Oxford, Pa. "Kind of defeats the convenience of pay-at-the-pump."

Credit-card companies say the policies, which aren't new, are to ensure that merchants and consumers are protected from fraudulent transactions that could occur at a gas pump.

When a customer uses their credit card at a cardholder-activated terminal, such as a gas pump, the transaction is authorized without knowing the final bill of sale.

Typically, consumers who use their credit card are not liable for any fraudulent purchases, and gas merchants are not liable either.

But credit-card companies have established a protective layer by setting caps on how much gas a consumer can pump at any one given time.

That means in the event of any fraud, "the merchant is protected from bearing the cost of the fraudulent transaction," said MasterCard spokeswoman Joanne Trout.

But only up to a certain amount. For MasterCard customers, it's $75. Visa and Discover users have a $50 pay-at-the-pump limit. Transaction limits vary for corporate card holders and American Express users.

Not all gas stations have to abide by the cap. And there are no limits if a customer goes inside and pays with their credit card at the counter.

The caps went unnoticed when gasoline prices were low.

The average price of regular unleaded gasoline increased from $1.50 a gallon at the start of the decade to $2.28 a gallon in 2005, according to the American Automobile Association.

Today, gasoline prices are topping $3 a gallon.

"Yes, it's an inconvenience," said Bloomfield, who often reaches his $50 limit when filling up his Nissan Pathfinder. "I guess you could say it's a necessary inconvenience for more secure transactions."