quarter on the wages paid with respect to the employment of all individuals in the employ of the employer. (4) Carryover of unused credit If the amount of the credit allowed under paragraph (1) exceeds the limitation of paragraph (3) for any calendar quarter, such excess shall be carried to the succeeding calendar quarter and allowed as a credit under paragraph (1) for such quarter. (5) Eligible employer For the purposes of this section, the term eligible employer means a community center, library or museum that is described in section 501(c)(3) and exempt from taxation under section 501(a). (6) Aggregation rule All persons treated as a single employer under subsection (a) or (b) of section 52, or subsection (m) or (o) of section 414, shall be treated as one employer for purposes of this section. (7) Election not to have section apply This section shall not apply with respect to any employer for any calendar quarter if such employer elects (at such time and in such manner as the Secretary may prescribe) not to have this section apply. (8) Regulations The Secretary shall issue such regulations or other guidance as may be necessary or appropriate to carry out the purposes of this section. . (b) Effective date The amendment made by this section shall apply to amounts paid or incurred after the date of the enactment of this Act. III Housing 301. Grants for providing cooling systems in public housing units (a) Authority The Secretary of Housing and Urban Development (in this section referred to as the Secretary ) shall carry out a program under this section to make grants to public housing agencies for the installation of air conditioners or heat pumps for public housing dwelling units. (b) Eligible activities Amounts from a grant under the program under this section may be used only for costs of acquiring and installing air conditioners or heat pumps that meet such standards as the Secretary shall establish regarding cooling capacity, safety, reliability, and energy efficiency and that serves public housing dwelling