26 CFR §1.166-5

1. (a)Allowance of deduction as capital loss.
   1. (1)The loss resulting from any nonbusiness debt's becoming partially or wholly worthless within the taxable year shall not be allowed as a deduction under either section 166(a) or section 166(c) in determining the taxable income of a taxpayer other than a corporation. See section 166(d)(1)(A).
   2. (2)If, in the case of a taxpayer other than a corporation, a nonbusiness debt becomes wholly worthless within the taxable year, the loss resulting therefrom shall be treated as a loss from the sale or exchange, during the taxable year, of a capital asset held for not more than 1 year (6 months for taxable years beginning before 1977; 9 months for taxable years beginning in 1977). Such a loss is subject to the limitations provided in section 1211, relating to the limitation on capital losses, and section 1212, relating to the capital loss carryover, and in the regulations under those sections. A loss on a nonbusiness debt shall be treated as sustained only if and when the debt has become totally worthless, and no deduction shall be allowed for a nonbusiness debt which is recoverable in part during the taxable year.
2. (b)Nonbusiness debt defined. For purposes of section 166 and this section, a nonbusiness debt is any debt other than—
   1. (1)A debt which is created, or acquired, in the course of a trade or business of the taxpayer, determined without regard to the relationship of the debt to a trade or business of the taxpayer at the time when the debt becomes worthless; or
   2. (2)A debt the loss from the worthlessness of which is incurred in the taxpayer's trade or business.
3. (c)Guaranty of obligations. For provisions treating a loss sustained by a guarantor of obligations as a loss resulting from the worthlessness of a debt, see §§ 1.166-8 and 1.166-9.
4. (d)Examples. The application of this section may be illustrated by the following examples involving a case where A, an individual who is engaged in the grocery business and who makes his return on the basis of the calendar year, extends credit to B in 1955 on an open account: