26 CFR §1.411(a)-6

1. (a)Year of service. Under section 411 (a)(5)(A), for purposes of the regulations thereunder, the term “year of service” is defined in regulations prescribed by the Secretary of Labor under section 203(b)(2)(A) of the Employee Retirement Income Security Act of 1974. For special rules applicable to seasonal industries and maritime industries, see regulations prescribed by the Secretary of Labor under subparagraphs (C) and (D) of section 203(b)(2) of the Employee Retirement Income Security Act of 1974.
2. (b)Hours of service. Under section 411(a)(5)(B), for purposes of the regulations thereunder, the term “hours of service” has the meaning provided by section 410(a)(3)(C). See regulations prescribed by the Secretary of Labor under 29 CFR Part 2530, relating to minimum standards for employee pension benefit plans.
3. (c)Breaks in service. Under section 411(a)(6), for purposes of § 1.411(a)-5(b)(4) and of this paragraph—
   1. (1)In general
      1. (i)Year of service after 1-year break in service. In the case of any employee who has incurred a 1-year break in service, years of service completed before such break are not required to be taken into account until the employee has completed one year of service after his return to service.
      2. (ii)Defined contribution plan. In the case of a participant in a defined contribution plan or in an insured defined benefit plan (which plan satisfies the requirements of section 411 (b)(1)(F) and § 1.411(b)-1) who has incurred a 1-year break in service, years of service completed after such break are not required to be taken into account for purposes of determining the nonforfeitable percentage of the participant's right to employer-derived benefits which accrued before such break. This subdivision does not permit years of service completed before a 1-year break in service to be disregarded in determining the nonforfeitable percentage of a participant's right to employer-derived benefits which accrue after such break.
      3. (iii)Nonvested participants. In the case of an employee who is a nonvested participant in employer-derived benefits at the time he incurs a 1-year break in service, years of service completed by such participant before such break are not required to be taken into account for purposes of determining the nonforfeitable percentage of his right to employer-derived benefits if at such time the number of consecutive 1-year breaks in service included in his most recent break in service equals or exceeds the aggregate number of his years of service, whether or not consecutive, completed before such break. In the case of a plan utilizing the elapsed time method described in § 1.410(a)-7, the condition in the preceding sentence shall be satisfied if the period of severance is at least one year and the consecutive period of severance equals or exceeds his prior period of service, whether or not consecutive, completed before such period of severance. In computing the aggregate number of years of service prior to such break, years of service which could have been disregarded under this subdivision by reason of any prior break in service may be disregarded.
   2. (2)One-year break in service defined. The term “1-year break in service” means a calendar year, plan year, or other 12-consecutive month period designated by a plan (and not prohibited under regulations prescribed by the Secretary of Labor) during which the participant has not completed more than 500 hours of service. In the case of a plan utilizing the elapsed time method, the term “1-year break in service” means a 12-consecutive month period beginning on the severance from service date or any anniversary thereof and ending on the next succeeding anniversary of such date; provided, however, that the employee during such 12-consecutive-month period does not complete any hours of service within the meaning of 29 CFR Part 2530.200b-2(a) for the employer or employers maintaining the plan. See regulations prescribed by the Secretary of Labor under 29 CFR Part 2530, relating to minimum standards for employee pension benefit plans.
4. (d)Examples. The rules provided by this section are illustrated by the following examples: