Topic 608 – Excess Social Security and RRTA Tax Withheld
Most employers must withhold social security tax from your wages. Certain government employers (some federal, state and local governments) don’t have to withhold social security tax.

If you work for a railroad employer, your employer must withhold Tier 1 Railroad Retirement Tax Act (RRTA) tax and Tier 2 RRTA tax. Tier 1 RRTA provides social security and Medicare equivalent benefits, and Tier 2 RRTA provides a private pension benefit.

Employer’s error – If any one employer withheld too much social security, Tier 1 RRTA tax, or Tier 2 RRTA tax, you can’t claim the excess as a credit against your income tax. Your employer should adjust the excess for you. If the employer doesn’t adjust the overcollection, you can use Form 843 (PDF), Claim for Refund and Request for Abatement, to claim a refund.

Two or more employers – If you had more than one employer during the taxable year and your total wages and compensation were over the wage base limit for the year, the total social security tax or social security equivalent Tier 1 RRTA tax withheld may have exceeded the maximum amount due for the tax year. If you had more than one railroad employer, and your total compensation was over the maximum amount of wages subject to Tier 2 RRTA, the total Tier 2 RRTA tax withheld may have exceeded the maximum due for the tax year. You can find the wage base limits for the RRTA tier 1 and the RRTA tier 2 tax for the year in Chapter 38, Other Taxes, of Publication 17, Your Federal Income Tax For Individuals.

If you had more than one employer and too much social security tax or Tier 1 RRTA tax withheld, you may be able to claim the excess as a credit against your income tax on your income tax return. Use either Worksheet 3-1 or 3-2 in Publication 505, Tax Withholding and Estimated Tax, to help you figure the excess amount.
If you had more than one employer and too much Tier 2 RRTA tax withheld, you may request a refund of the excess Tier 2 RRTA tax using Form 843 (PDF). Attach copies of your Forms W-2, Wage and Tax Statement, for the year to Form 843. Use Worksheet 3-3 in Publication 505, to help you figure the excess amount.
Joint returns – If you’re filing a joint return, you and your spouse must figure any excess social security tax or Tier 1 RRTA tax separately.

Additional Information
For details, including how to compute the amount of excess credit, refer to Publication 505, Tax Withholding and Estimated Tax. Also, see the Form 843 Instructions