(c) Excluding contributions of significant withdrawn employers. Contributions of certain withdrawn employers are excluded from the denominator in each of the fractions used to determine a withdrawing employer's share of unfunded vested benefits under the presumptive, modified presumptive and rolling-5 methods. Except as provided in
paragraph (c)(1) of this section, contributions of all employers that permanently cease to have an obligation to contribute to the plan or permanently cease covered operations before the end of the period of plan years used to determine the fractions for allocating unfunded vested benefits under each of those methods (and contributions of all employers that withdrew before September 26, 1980) are excluded from the denominators of the fractions.
(1) The plan sponsor of a plan using the presumptive, modified presumptive or rolling-5 method may amend the plan to provide that only the contributions of significant withdrawn employers are excluded from the denominators of the fractions used in those methods.
(2) For purposes of this paragraph (c), “significant withdrawn employer” means—
(i) An employer to which the plan has sent a notice of withdrawal liability under
section 4219 of ERISA; or
(ii) A withdrawn employer that in any plan year used to determine the denominator of a fraction contributed at least $250,000 or, if less, 1 percent of all contributions made by employers for that year.
(3) If a group of employers withdraw in a concerted withdrawal, the plan sponsor must treat the group as a single employer in determining whether the members are significant withdrawn employers under
paragraph (c)(2) of this section. A “concerted withdrawal” means a cessation of contributions to the plan during a single plan year—
(i) By an employer association;
(ii) By all or substantially all of the employers covered by a single collective bargaining agreement; or
(iii) By all or substantially all of the employers covered by agreements with a single labor organization.