Deluged with requests for credit from corporations wary that the Carter administration may impose credit controls, major banks have granted tens of billions of dollars of increased borrowing privileges to business customers in the last few weeks.

For the most part, the companies aren't actually taking out loans, but rather are seeking lines of credit and other loan commitments from banks so that the money will be there when -- and if -- they need it, bankers said. Banks charge a fee for such credit commitments.

Besides fearing that credit controls will make it difficult for them to obtain loans later, companies are worried that they will be unable to sell long-term debt securities when they want to because the bond markets are in such disarray, banking officials report.

As a result, companies are seeking bank loan commitments that could be activated to tide them over until the bond market is less chaotic, according to George Baker, executive vice president of Continental Illinois National Bank.

"These aren't your Joe Doe's hardware stores either. These are major name corporations," said the chief lending official at a big money center bank. n

Several top banks report privately that the level of their loan commitments has risen by 5 percent in the last few weeks.

"It's a big number. And it all happened in 10 days time," said a major banker.

Bankers are mum about their individual loan customers and won't divulge the amounts of credit they are committed to grant their customers.

But Edward Palmer, chairman of the executive committee at Citibank, New York's largest, said that the rise in loan commitments at major banks across the country probably is measured "in the tens of billions of dollars."

The unprecedented demand for loan commitments comes as interest rates already are at record levels and are expected to rise further. The prime lending rate at most major banks is 17 3/4 percent, and most analysts expect the rate will pass 18 percent soon.

If many businesses find it necessary to "draw down" the increased loan commitments they have negotiated in the last few weeks, the costs could be much higher than the current prime rate.

Last October, in an effort to hold down the amount of credit that banks grant their customers, the Federal Reserve set out special new regulations to make it more expensive for banks to borrow the funds they need to lend to their business customers.

The Fed put an extra 8 percent reserve requirement on all managed liabilities -- such as certificates of deposit of $100,000 or more -- whenever those liabilities exceed whatever a bank's total was between Sept. 13 and Sept. 26. There already was an 8 percent reserve requirement.

For example, if a bank had $1 billion in managed liabilities during the base period and today finds it necessary to sell an extra $50 million in CDs to meet loan demand, it can loan out only $42 million of the $50 million. The remaining $8 million must be left in a non-income-earning account with the Federal Reserve.

Because the banks also must pay about 15 percent to borrow the money on the open market, the cost of raising those additional funds is high, perhaps as much as 30 percent.

Most banks' managed liabilities have been below the base level since October. "But I've got the feeling that we'll be above it permanently soon, and most other big banks will be in the same situation," said a major banker.

Bankers say that they have taken a careful look at all the requests for loan commitments. "But with the kinds of companies we're talking about, our ability to tell them 'no' is pretty remote. What we're doing is charging them a lot of money for the comitments. We're doing it through price," a big banker said.

Even so, another said, competition for the loan commitments is strong, especially because foreign banks aggressively seeking to increase their business with U.S. corporations are offering loan commitments for small fees.

"We've seen some deals go through at a quarter-percent commission," said another lending official at another U.S. bank. "Those are the deals where foreign banks are taking charge."