D.C. Mayor Marion Barry will have to wait at least 30 days to shorten prison sentences under an emergency law enacted Tuesday, preventing him from using the law to meet a court-ordered July 1 deadline to relieve overcrowding at the city's three Occoquan prisons.

D.C. corrections officials said yesterday that a provision of the law requires that capacity in the prisons be above the official limit for 30 consecutive days before Barry could declare a state of emergency and trigger the early parole mechanism.

Edward D. Sargent, public information officer for the Corrections Department, said that, although the system has been overcrowded for more than 30 days, the law does not grant the mayor the power to declare the emergency retroactively.

The officials said interpretation of the 30-day provision of the bill, which was hurriedly drafted by council member Wilhelmina Rolark (D-Ward 8) at the request of Barry, means the new law may have a more symbolic than substantive value in the city's effort to comply with U.S. District Judge June Green's order to reduce the population of the Occoquan facilities at the Lorton Reformatory in Fairfax County.

According to D.C. government statistics, 1,953 prisoners are in the three Occoquan facilities -- 672 more than the number permitted under Green's order.

The sources said the mayor's sponsorship of the measure was intended to satisfy Green showing that officials are making a good-faith effort to solve the crowding crisis that resulted in court-ordered limits at most of the city's nine prison facilities and the D.C. Jail.

Green "wants the city to be able to do this so the prisoners won't have to live like cans of tomatoes stacked on top of each other," said one high-ranking corrections official.

Under the new measure, the Corrections Department would be authorized to reduce some inmates' sentences by 90 days, making them eligible for early parole. Inmates serving life sentences or terms for convictions of homicide, rape, extortion, assault with a deadly weapon or other serious crimes would not be eligible for sentence reduction under the measure.

The 30-day countdown, according to Corrections Department officials, can not begin until Barry signs the bill. John C. White, the mayor's spokesman, said "the bill has not yet arrived at the mayor's desk."

After the Corrections Department determines which sentences to reduce, the prisoners must go before the D.C Parole Board for a hearing. Bernice Just, a member of the Parole Board, said the board has the capacity "to gear up" for an increased number of hearings.

But she said parole would not be automatic and the board would take into consideration whether the inmate is a repeat offender. "Each decision would be based on a review of the individual case," Just said.

Corrections Director Hallem H. Williams Jr. said yesterday that he hopes other means of reducing the prison population will be found, adding that the department "will be working to prevent" the necessity of implementing the legislation.

"Everyone is eager not to use it," Williams said. "It is not the answer."

Legal experts involved in the overcrowding controversy said the bill's exact provisions are unclear.