Married individuals. If a separate return is filed by a husband or wife, the tax liability limitation is computed by substituting a $12,500 amount for the $25,000 amount that applies under section 46(a)(3). However, this reduction of the $25,000 amount to $12,500 applies only if the taxpayer's spouse is entitled to a credit under section 38 for the taxable year of such spouse which ends with, or within, the taxpayer's taxable year. The taxpayer's spouse is entitled to a credit under section 38 either because of investment made in qualified property for such taxable year of the spouse (whether directly made by such spouse or whether apportioned to such spouse, for example, from an electing small business corporation, as defined in section 1371(b)), or because of an investment credit carryback or carryover to such taxable year. The determination of whether an individual is married shall be made under the principles of section 143 and the regulations thereunder.