Higher Education Act of 1965 ( 20 U.S.C. 1094 ) and is not enrolled in an elementary or secondary school. (2) Eligible employer (A) In general The term eligible employer means an employer who employed on average not more than 500 full-time employees on business days during the preceding calendar year. For purposes of the preceding sentence, any individual with respect to whom a credit is allowed under this section shall not be taken into account. (B) Rules for determining employer size For purposes of subparagraph (A)— (i) Application of aggregation rule for employers All persons treated as a single employer under subsection (b), (c), (m), or (o) of section 414 shall be treated as 1 employer. (ii) Employers not in existence in preceding year In the case of an employer which was not in existence throughout the preceding calendar year, the determination of whether such employer is an eligible employer shall be based on the average number of employees that it is reasonably expected such employer will employ on business days in the current calendar year. (iii) Predecessors Any reference in this paragraph to an employer shall include a reference to any predecessor of such employer. . (b) Credit made part of general business credit Subsection (b) of section 38 of such Code is amended by striking plus at the end of paragraph (36), by striking the period at the end of paragraph (37) and inserting , plus , and by adding at the end the following new paragraph: (38) the STEM internship credit determined under section 45T(a). . (c) Denial of double benefit Subsection (a) of section 280C of such Code is amended by inserting 45T(a), after 45S(a), . (d) Clerical amendment The table of sections for subpart D of part IV of subchapter A of chapter 1 of the Internal Revenue Code of 1986 is amended by adding at the end the following new item: Sec. 45T. STEM internship credit. . (e) Effective date The amendments made by this section shall apply to taxable years beginning after December 31, 2017. 5. STEM apprenticeship tax credit (a)