A federal judge yesterday set Wednesday as the effective date for her order sending as many as 300 D.C. inmates a month to federal prisons until crowding subsides at Lorton Reformatory.

U.S. District Judge June L. Green, in a written order issued one day after her oral opinion, said the federal government must begin sharing the burden of incarcerating District inmates sentenced in D.C. Superior Court because Lorton crowding has reached a "crisis situation" that shows no sign of easing.

"Indeed, with continued innovative law enforcement efforts such as the laudable 'Operation Clean Sweep,' and the open court orders imposing population limits on {Lorton's} Maximum and Occoquan facilities," Green said, violations of the court-ordered inmate population ceiling at Lorton's Central facility will likely "become more severe."

Central is the District's main medium-security facility and houses 1,238 prisoners, 72 more than its limit. The prison is the focus of a suit brought in 1980 by Central inmates. Two weeks ago, Green made Attorney General Edwin Meese III a defendant in that suit, and on Thursday she held the city in contempt for exceeding the population limit there.

Green's order barring the referral of D.C. prisoners to Lorton and sending them to the federal prison system marks the first time that the federal government has been told to step in and help the city house its prisoners. Under a 1932 law, the attorney general has responsibility for designating where District inmates will be confined.

In her written ruling, the judge said the order did not apply to prisoners whose charges are still pending in D.C. Superior Court, those sentenced under the D.C. Work Release Act or those sentenced under the D.C. Youth Rehabilitation Amendment Act.

A spokesman for the U.S. attorney's office said the Justice Department would file a motion with the Court of Appeals Monday seeking an emergency stay of Green's order.

City officials, however, hailed the judge's order sending new prisoners to federal facilities.

"It pleased me very much in that the U.S. attorney and the attorney general are supposed to take every person sentenced," Mayor Marion Barry said in an interview on WDCU-FM. "I can't continue to build any more prisons."

But Barry said he disagreed with Green's imposition of a new inmate limit of 1,281 on Lorton's three Occoquan facilities, which now hold 2,000 prisoners. The new limit took effect yesterday.

"We think we can safely keep about 1,600 people at Occoquan," Barry said. "We are going to try to renegotiate that number and, in addition, appeal that order."

Green, citing plaintiffs' arguments and an affidavit of the District's corrections department chief, Hallem H. Williams Jr., said the District-operated Lorton complex in Fairfax County no longer qualifies as suitable for housing additional inmates sentenced in D.C. Superior Court.

Williams, in his affidavit, stated that after a personal inspection of Lorton facilities and after conferring with officials there, "It is my considered judgment . . . that those institutions are not suitable or appropriate for the housing of additional prisoners and that they should not be available for that purpose."

The inmates' attorney, Peter J. Nickles, had asked Green to require that D.C. inmates be sent to nearby federal prisons and that the transfers continue until 1989. But Green's written order yesterday contained neither stipulation.

Assistant U.S. Attorney Royce C. Lamberth had argued at the hearing that federal prisons are 56 percent above capacity and that D.C. prisoners would have to be sent to the least crowded institutions.

According to court documents, the only federal prison operating under capacity is the penitentiary at Marion, Ill., reserved for the nation's most violent federal prisoners.

In her written order yesterday, Green described the court as being in a no-win situation because both the city's correctional facilities and the federal prison system are "suffering from overcrowding." But she said the federal system "is clearly not faced with the crisis state in which the {District} finds itself."