If the governing instrument of the trust so provides and if the trust income for any taxable year exceeds the amount determined under (ii) above, the payment to beneficiaries also must include the excess income to the extent that the aggregate of the amounts paid to beneficiaries in prior years is less than the percentage of the aggregate net fair market value of the trust assets, which percentage is defined in the governing instrument of the trust, for these years. Payments to beneficiaries must be made annually or in more frequent equal or unequal installments if the governing instrument so provides. Any income of the trust in excess of these payments must be added to principal.
Notwithstanding the foregoing sentence, in computing any underpayment or overpayment of the unitrust amounts, if the governing instrument was executed or last amended before August 9, 1984, and if the governing instrument does not specify that a ten percent (10%) rate of interest shall be used, the underpayment or overpayment of the unitrust amounts shall be computed using an interest rate of six percent (6%) a year, compounded annually.
N.C. Gen. Stat. § 36C-4B-6