S.C. Code Ann. § 12-6-2210 — Taxation of business; determination whether entirely or partly transacted or conducted within State

(A) If the entire business of a taxpayer is transacted or conducted within this State, the income tax as provided in this chapter is measured by the entire net income of the taxpayer for the taxable year. The entire business of the taxpayer is transacted or conducted within the State if the taxpayer is not subject to a net income tax or a franchise tax measured by net income in another state, the District of Columbia, a territory or possession of the United States, or a foreign country, and would not be subject to a net income tax in another taxing jurisdiction if the other taxing jurisdiction adopted the net income tax laws of this State.
(B) If a taxpayer is transacting or conducting business partly within and partly without this State, the South Carolina income tax is imposed upon a base which reasonably represents the proportion of the trade or business carried on within this State. A taxpayer subject to taxation under this section is considered to have been transacting or conducting business partly within and partly without the State if the taxpayer is subject to a net income tax or a franchise tax measured by net income in another state, the District of Columbia, a territory or possession of the United States, or a foreign country, or would be subject to the net income tax in any other taxing jurisdiction if the other taxing jurisdiction adopted the net income tax laws of this State.