(a) “Contributions made” and “total amount contributed”. Each of the allocation fractions used in the presumptive, modified presumptive and rolling-5 methods is based on contributions that certain employers have made to the plan for a five-year period. For purposes of these methods, and except as provided in paragraph (b) of this section, “the sum of all contributions made” or “total amount contributed” by employers for a plan year means the amounts (other than withdrawal liability payments) considered contributed to the plan for the plan year for purposes of section 412(b)(3)(A) of the Code. For plan years before section 412 applies to the plan, “the sum of all contributions made” or “total amount contributed” means the amount reported to the IRS or the Department of Labor as total contributions for the plan year; for example, for plan years in which the plan filed the Form 5500, the amount reported as total contributions on that form. Employee contributions, if any, shall be excluded from the totals.

(b) Changing the period for counting contributions. A plan sponsor may amend a plan to modify the denominators in the presumptive, modified presumptive and rolling-5 methods in accordance with one of the alternatives described in this paragraph. Except as provided in paragraph (b)(4) of this section, any amendment adopted under this paragraph shall be applied consistently to all plan years. Contributions counted for one plan year may be not counted for any other plan year. If a contribution is counted as part of the “total amount contributed” for any plan year used to determine a denominator, that contribution may not also be counted as a contribution owed with respect to an earlier year used to determine the same denominator, regardless of when the plan collected that contribution.

(c) Excluding contributions of significant withdrawn employers. Contributions of certain withdrawn employers are excluded from the denominator in each of the fractions used to determine a withdrawing employer's share of unfunded vested benefits under the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.