S.C. Code § 12-6-520
Beginning on December 15, 2018, and each December fifteenth thereafter, the department shall cumulatively adjust the brackets in Section 12-6-510 using the Chained Consumer Price Index for All Consumers, as published by the Bureau of Labor and Statistics of the Department of Labor, pursuant to Internal Revenue Code Section (1)(f). However, the adjustment may not exceed four percent a year, and notwithstanding the rounding amount provided in (1)(f)(7), the rounding amount is ten dollars. The brackets, as adjusted, apply instead of those provided in Section 12-6-510 for taxable years beginning in the succeeding calendar year. Inflation adjustments must be made cumulatively to the income tax brackets.
S.C. Code § 12-6-520
2018 Act No. 266, Sections 1, 4.B, 7, 8, and 9, provide as follows:
"SECTION 1. This act may be referred to as the 'South Carolina Taxpayer Protection and Relief Act'."
"[4.]B. This SECTION takes effect upon approval by the Governor and first applies to the adjustment made for tax year 2019, with the 2018 income tax brackets being used first as the base year upon which adjustments made pursuant to Section 12-6-520, as amended by this act, are made."
"SECTION 7. The Department of Revenue shall take actions necessary to implement the provisions of this act and take the appropriate actions to educate taxpayers of the amendments contained within this act, especially those, if any, that impact filing requirements or determination of taxable income.
"SECTION 8. By January 15, 2025, the Department of Revenue, in coordination with the Revenue and Fiscal Affairs Office, shall deliver a report to the General Assembly that specifies the many provisions of the Tax Cuts and Jobs Act of 2017 that expire after tax year 2025.
"SECTION 9. Except where specified otherwise, this act takes effect upon approval by the Governor and first applies to tax years beginning after 2017."