At least 100 more savings and loan associations have been added to the troubled list since the end of May, Federal Home Loan Bank Board Chairman Richard Pratt told a congressional subcommittee yesterday.

That brings to around 360, out of a total of 4,700, the number of S&Ls, whose net worth has fallen to 2 percent of assets. Last week Pratt said privately that one-third of all S&Ls will no longer be viable if interest rates do not subside or relief is not forthcoming.

This week the S&L regulatory agency asked the Federal Reserve Board to allow ailing savings institutions access to the central bank's discount window without first requiring them to exhaust all other sources of credit. This would lower their cost of funds considerably. Pratt also proposed legislation this week to help S&Ls.

In the short term, he calls for permitting mergers of S&Ls across state lines and with banks if necessary. Over the long term, he aims to give S&Ls the same powers and functions as commerical banks.

Despite the pessimistic outlook, Pratt repeated his contention before the House subcommittee on commerce, consumer and monetary affairs that the Federal Savings and Loan Insurance Corp.'s $6 million fund is sufficient to meet any emergency.

Irvin Sprague, chairman of the Federal Deposit Insurance Corp. which insures banks, said the situation is not as acute for them. But he added he expects more bank failures in the next year than the customary average of 10.

Subcommittee Chairman Benjamin Rosenthal (D-N.Y.) questioned the adequacy of the fund as well as the procedures for estimating the costs involved in failures. He noted that federal regulators underestimated by 70 percent the cost of closing out falling banks between 1973 and 1978. The final cost to the FDIC of arranging the acquisition of all failed banks with assets over $90 million in those six years amounted to $150.1 million, up from the original estimate of $88.4 million.

Sprague said some of the difference could be explained by court decisions increasing the failed banks' liabilities.

Rosenthal then questioned the wisdom of the Bank Board's recent decision to let S&Ls hedge interest rates in the futures markets, given the number of S&Ls that got burned speculating in Government National Mortgage Association securities known as Ginnie Maes