Of the 397,000 Americans who declared bankruptcy last year, 38 percent did so because of the more liberal provisions of the new bankruptcy law, a Senate subcommittee was told yesterday.

Andrew F. Brimmer, a former governor of the Federal Reserve Board and now a consulting economist, presented the results of his survey of 1,000 debtors to the Senate Judiciary subcommittee which is investigating why individual bankruptcies soared by 75 percent during the first year after the code was revised. The new code went into effect Oct. 1, 1979.

Personal defaults cost industry between $2 billion and $3 billion last year, and business failures amounted to $4.5 billion during the same period, according to committee estimates.

The study, which was commissioned by a consumer finance company, concluded that between 136,000 and 151,000 of the total bankruptcies could not be explained by demographic trends or the state of the economy and therefore were due to "broadened opportunities" for debtors to wipe out their debts. These include increased federal exemptions which remove large amounts of property for use in debt repayment and the ease and social acceptability of filing, as exemplified in the increasing number of advertisements by attorneys emphasizing how little debtors actually lose.

The major reason for defaulting, listed by 57 1/2 percent of the debtors, was "using too much credit." Overspending was cited by 43 percent; loss of employment, 39 percent. Marital difficulties, medical expenses and "not taking financial matters serious enough" were mentioned by about one-quarter of the sample. Nearly two-thirds of the respondents (64.7 percent) said bankruptcy had had no social impact on their lives, and 47.8 percent said the only real effect of declaring bankruptcy was their loss of credit.

The tally of what subcommittee Chairman Bob Dole (R-Kan.) called "epidemic bankruptcy" was given by Richard F. Kerr, speaking for the National Retail Merchants Association. Last year his Federated Department Stores reported that losses attributable to customer default amounted to $2.8 million, up 100 percent over the previous year; J.C. Penney, $20.7 million, up 102 percent; Montgomery Ward, $50.6 million, up 117 percent; and Sears Roebuck, $46.2 million, up 120 percent. "In recent months many creditors have experienced a doubling of bankruptcy cases over last year, with potential dollar losses as much as triple those of last year," Kerr said. Losses, of course, are passed on to cash-paying customers as well.

The current bankruptcy code -- the first revision in nearly 40 years -- was eight years in the making. During that time consumer bankruptcy was not thought to be a big problem; the emphasis was on business bankruptcy and administrative reorganization. So the finance industry did not object unduly to giving debtors "a fresh start." After barely a year -- and well publicized abuses of bankrupts keeping up to $100,000 of their property -- the industry is having second thoughts.

Finance companies, banks, retail merchants, credit unions, oil companies: and other credit grantors have zeroed in on what they consider the biggest abuse: straight bankruptcy filing or court cancellation of all debts for people whose income would allow them to repay at least a portion of them. Unlike the credit process, where money is lent not on the basis of a person's current assets but on the future ability of the borrower to pay, the bankruptcy process considers only a debtor's assets and liabilities, not his or her income. This means that a person with a steady job can still erase all debts by delcaring bankruptcy.

Many creditors would like to revise the code so that a bankruptcy judge could order a debtor to pay a reasonable percentage of his or her existing obligations over time. Jonathan Landers, a University of Illinois Law School professor, believes one-third of all bankruptcy cases might be affected by such a change. At present the debtor can decide whether to repay, and how much. Credit unions would go one step further and allow a judge to refuse to discharge the debts of a person believed to be able to repay out of future income.

In related news, the Mortgage Bankers Association reported this week that short-term mortgage delinquencies (payments past due 30 or more days) declined for the first time in more than a year during the last quarter of 1980. The rate dropped to 4.87 percent of all loans outstanding from 5.03 percent during the third quarter. Long-term delinquency (90 days or more) rose, however, to 0.62 percent, reflecting continued high unemployment in many parts of the country. The National Association of Mutual Savings Banks, which is most active in the hard-hit Northeast, reported a modest increase in long-term delinquencies during the final quarter of last year to 0.73 percent.