Most readers carry an instinct that the deductibility of loan interest depends on what secures the loan — a mortgage must be deductible because it is a mortgage; a margin loan must be investment interest because it is a margin loan. That instinct is wrong. The IRS cares about where the money goes, not where it came from. Treas. Reg. §1.163-8T (“temporary” since 1987 and still controlling) sets the rules: every dollar of borrowed funds is traced to a specific expenditure, and the interest on that dollar is categorized by what the expenditure was. A single physical loan can have its interest split across multiple deductibility categories if the proceeds were spent across multiple uses.
The categories matter because limits and forms differ across them. Table 16.5 “Tax treatment of loan interest by use of proceeds, per Treas. Reg. §1.163-8T” summarizes the framework.
The practical consequence: the same HELOC has three different tax treatments depending on where the cash went. Drawn to remodel the kitchen, it is qualified residence interest, deductible inside the $750K cap. Drawn to buy a taxable equity portfolio, it is investment interest, deductible against net investment income. Drawn to fund a vacation, it is personal interest — not deductible at all, even though the loan is secured by your home. A margin loan against a brokerage account follows the same logic: used to buy more securities it is investment interest; used to buy a car it is personal interest. The collateral is irrelevant.