20 CFR §404.1054

1. (a)We exclude as wages any payment by an employer (described in paragraph (b) of this section) that is not deducted from the employee's salary (or for which reimbursement is not made by the employee) of either—
   1. (1)The tax imposed by section 3101 of the Code (employee's share of Social Security tax); or
   2. (2)Any payment required from an employee under a State unemployment compensation law.
2. (b)The payments described in paragraph (a) of this section are not included as wages only if they are made by an employer on behalf of an employee employed in—
   1. (1)Domestic service in the private home of the employer; or
   2. (2)Agricultural labor.