Code - I.R.C. § 163(h)(3) -
"(C) Home equity indebtedness -
The aggregate amount treated as home equity indebtedness for any period shall not exceed $100,000 ($50,000 in the case of a separate return by a married individual)."
"(3) Qualified residence interestFor purposes of this subsection—
(A) In generalThe term “qualified residence interest” means any interest which is paid or accrued during the taxable year on—(i)
acquisition indebtedness with respect to any qualified residence of the taxpayer, or
home equity indebtedness with respect to any qualified residence of the taxpayer.
For purposes of the preceding sentence, the determination of whether any property is a qualified residence of the taxpayer shall be made as of the time the interest is accrued.
(B) Acquisition indebtedness(i) In generalThe term “acquisition indebtedness” means any indebtedness which—(I)
is incurred in acquiring, constructing, or substantially improving any qualified residence of the taxpayer, and
is secured by such residence.
Such term also includes any indebtedness secured by such residence resulting from the refinancing of indebtedness meeting the requirements of the preceding sentence (or this sentence); but only to the extent the amount of the indebtedness resulting from such refinancing does not exceed the amount of the refinanced indebtedness.
(ii) $1,000,000 limitation
The aggregate amount treated as acquisition indebtedness for any period shall not exceed $1,000,000 ($500,000 in the case of a married individual filing a separate return)."
One of the links to the full code: