Carryforwards of unused credit

If the portion of the credit allowable under subsection (a) which is allowed under this subpart for any taxable year exceeds the limitation imposed by section 26(a) for such taxable year reduced by the sum of the credits allowable under this subpart (other than this section and section 25D), such excess shall be carried to the succeeding taxable year and added to the portion of the credit allowable under subsection (a) which is allowed under this subpart for such taxable year.

No credit may be carried forward under this subsection to any taxable year following the fifth taxable year after the taxable year in which the credit arose. For purposes of the preceding sentence, credits shall be treated as used on a first-in first-out basis.