S.C. Code Ann. § 12-6-2230

Any income, less all related expenses, which is not allocated under Section 12-6-2220 and not properly includable in the net apportionable income of taxpayers engaged in interstate commerce under the Constitution of the United States because it is unrelated to the business activity of the taxpayer conducted partly within and partly without this State, is allocated to the state in which the business situs of the investment is located. If the business situs of the investment is partly within and partly without South Carolina, the investment is apportioned using the same formula used for apportioning the net income of the corporation.