Qualified residence interest is interest paid or accrued during taxable year on either (1) acquisition indebtedness, which is the amount incurred in acquiring, constructing, or substantially improving a qualified residence, I.R.C. §§ 163(h)(3)(A)(i), (B), or (2) home equity indebtedness, which is the amount of indebtedness equivalent to the fair market value of the qualified residence minus the amount of acquisition indebtedness for the residence, I.R.C. §§ 163(h)(3)(A)(ii), (C). A qualified residence is either (1) the taxpayer's principal residence, I.R.C. §§ 163(h)(4)(A)(i)(I), 121, or (2) a second residence selected by the taxpayer that is either used by the taxpayer or certain categories of persons for the greater of 14 days or ten percent of the number of days during the taxable year for which the unit is rented at fair rental value, I.R.C. §§ 163(h)(4)(A)(i)(II), 280A(d)(1), or not rented by the taxpayer at any time during the taxable year, I.R.C. § 163(h)(4)(A)(iii). A taxpayer may generally deduct interest on aggregate acquisition indebtedness that does not exceed $1,000,000; however, a married taxpayer filing a separate return may only deduct interest on acquisition indebtedness that does not exceed $500,000, I.R.C. § 163(h)(3)(B)(ii). A taxpayer may generally deduct interest on aggregate home equity indebtedness that does not exceed $100,000; however, a married taxpayer filing a separate return may only deduct interest on aggregate home equity indebtedness that does not exceed $50,000, I.R.C. § 163(h)(3)(C)(ii).