N.C. Gen. Stat. § 96-11.6
The Division must determine the ratio of the credit balance in each employer's account to the total of the credit balances in all employers' accounts as of the computation date. The Division must allocate an amount equal to the interest credited to this State's account in the Unemployment Trust Fund for the four completed calendar quarters preceding the computation date on a pro rata basis to these accounts. The amount must be prorated to an employer's account in the same ratio that the credit balance in the employer's account bears to the total of the credit balances in all the accounts. Voluntary contributions made by an employer after July 31 of a year are not considered a part of the employer's account balance used in determining the allocation under this section until the computation date in the following year.
N.C. Gen. Stat. § 96-11.6