11 U.S.C. § 511: Rate of interest on tax claims

(a) If any provision of this title requires the payment of interest on a tax claim or on an administrative expense tax, or the payment of interest to enable a creditor to receive the present value of the allowed amount of a tax claim, the rate of interest shall be the rate determined under applicable nonbankruptcy law. (b) In the case of taxes paid under a confirmed plan under this title, the rate of interest shall be determined as of the calendar month in which the plan is confirmed.