The Senate yesterday approved stricter limitations on the use of taxpayer funds for "franked," or free postage, mail and told a House-Senate conference committee to reduce by 5 percent the $2.2 billion it authorized for the operations of the House, Senate and congressional agencies.

The Senate voted to exempt the Library of Congress from the reduction.

After more than eight hours of debate that began Wednesday, the Senate voted 72 to 24 to approve its version of a legislative appropriations bill for fiscal 1991.

The House version of the appropriations bill calls on the conference to reduce the overall total by 2 percent, setting the stage for a potentially difficult session to iron out the differences in the House and Senate versions.

The appropriations bill has become politically sensitive because it pays for the operation of Congress at a time when many voters seem discontent with incumbents.

Rep. Vic Fazio (D-Calif.), chairman of the House Appropriations subcommittee on legislative branch appropriations who will lead that body's conferees, reacted to the Senate version by saying that "the two percent cut will apply to the House accounts and the five percent will apply to the Senate," an aide said yesterday.

On the increasingly controversial issue of free mail, the Senate approved an amendment by Sen. Don Nickles (R-Okla.), the ranking minority member on the Senate Appropriations legislative subcommittee, to boost the overall franking allowance for the 100 senators to $30 million from last year's $24 million, but the amount is less than the $35.5 million recommended by the Senate Appropriations Committee.

The Senate also voted 50 to 44 to end the practice by which one senator transfers unused mail funds to another senator, usually one up for reelection.

The amendment, also from Nickles, prevailed in part with the votes of five Democrats up for reelection: Sens. Tom Harkin (D-Iowa), Carl Levin (D-Mich.), J. James Exon (D-Neb.), Joseph R. Biden (D-Del.) and John F. Kerry (D-Mass.). Nearly all Republicans backed Nickles.

Another conflict between the two bodies is expected over a House provision that would prohibit members of either body from using personal or political campaign funds to supplement official expense allowances.

Senators now are permitted to use their own or campaign funds for such things as additional staff, mailings and, most frequently, the cost of television tapes and transmission from the Senate television studio.

The Senate bill calls for a year's delay on the prohibition, which is aleady a House rule, so that its impact could be assessed.