A federal appeals court today ordered a major New York bank to hold up refunding to Iran $30.2 million that represents a down payment the previous government made to an American Telephone & Telegraph subsidiary for work on a nation-wide communications system.

Monday, a district court judge said he could not stop Manufacturers Hanover Trust Co. from making the payment.

The bank issued a letter of credit last year to Iran to guarantee that the down payment could be refunded. The AT&T subsidiary, American Bell International, would then be liable to Manufacturers Hanover for the $30.2 million.

The appeals court enjoined the bank from making payments on the letter of credit for a week, until a full hearing could be held.

AT&T said that Iran owes it $32 million for work it did late last year and early this year on the communications project. AT&T pulled out of Iran during the revolution that ousted the shah early this year.

In a typically complicated international business transaction, AT&T and Iran renewed a contract for the communications project in July 1978. The contract called for a down payment from Iran of $38 million.

The $280 million contract was to be paid off in monthly installments on the basis of vouchers American Bell presented to the Iranian telecommunications ministry.

As the work was performed, the amount of the down payment that could be demanded was to be reduced. Manufacturers Hanover issued a letter of credit to Iran to guarantee that the down payment would be refunded.

By the time American Bell pulled out of Iran, the amount of the down payment that could be demanded was $30.2 million.

Last week, Manufacturers Hanover received a request from Iran's Bank Iranshahr for the $30.2 million. AT&T immediately went to court to block payment of the funds. But Monday, District Court Judge Lloyd F. Macmahon rulem that while AT&T would face hardship if Manufacturers Hanover honored the letter of credit, the bank would suffer even more.

Not only would Iran attach $30.2 million of Manufacturers Hanover's assets, the judge said, the bank could face a "loss of credibility in the international banking community" if it failed to make good on one of its letters of credit."

An AT&T spokesman said that although the company has made claims to Iran for $32 million to compensate the company for work it has performed on the communications system, Iran has not paid off. That is why, the spokesman said, the company wants to hold on to the down payment.

The AT&T contract was one of the first to become embroiled in the courts here after the Iranian revolution. AT&T filed suit in New York courts as a preventative measure, to block payment of the letter of credit if Iran should demand it.

But the courts denied the request. AT&T went to court again last week after Iran made the formal request of Manufacturers Hanover. Manufacturers Hanover wants to make good on the letter of credit, because the Iranian request conforms with the specifications set down in the letter.