29 C.F.R. § 4219.32 : Labor — REGULATIONS RELATING TO LABOR — PENSION BENEFIT GUARANTY CORPORATION — WITHDRAWAL LIABILITY FOR MULTIEMPLOYER PLANS — NOTICE, COLLECTION, AND REDETERMINATION OF WITHDRAWAL LIABILITY — OVERDUE, DEFAULTED, AND OVERPAID WITHDRAWAL LIABILITY — Interest on overdue, defaulted and overpaid withdrawal liability

(a) Interest assessed. The plan sponsor of a multiemployer plan-
(1) Shall assess interest on overdue withdrawal liability payments from the due date, as defined in paragraph (d) of this section, until the date paid, as defined in paragraph (e); and
(2) In the event of a default, may assess interest on any accelerated portion of the outstanding withdrawal liability from the due date, as defined in paragraph (d) of this section, until the date paid, as defined in paragraph (e).
(b) Interest rate. Except as otherwise provided in rules adopted by the plan pursuant to §4219.33, interest under this section shall be charged or credited for each calendar quarter at an annual rate equal to the average quoted prime rate on short-term commercial loans for the fifteenth day (or next business day if the fifteenth day is not a business day) of the month preceding the beginning of each calendar quarter, as reported by the Board of Governors of the Federal Reserve System in Statistical Release H.15 ("Selected Interest Rates").
(c) Calculation of interest. The interest rate under paragraph (b) of this section is the nominal rate for any calendar quarter or portion thereof. The amount of interest due the plan for overdue or defaulted withdrawal liability, or due the employer for overpayment, is equal to the overdue, defaulted, or overpaid amount multiplied by:
(1) For each full calendar quarter in the period from the due date (or date of overpayment) to the date paid (or date of refund), one-fourth of the annual rate in effect for that quarter;
(2) For each full calendar month in a partial quarter in that period, one-twelfth of the annual rate in effect for that quarter; and
(3) For each day in a partial month in that period, one-three-hundred-sixtieth of the annual rate in effect for that month.
(d) Due date. Except as otherwise provided in rules adopted by the plan, the due date from which interest accrues shall be, for an overdue withdrawal liability payment and for an amount of withdrawal liability in default, the date of the missed payment that gave rise to the delinquency or the default.
(e) Date paid. Any payment of withdrawal liability shall be deemed to have been paid on the date on which it is received.

29 C.F.R. §4219.32