The question whether a debt is a nonbusiness debt is a question of fact in each particular case. The determination of whether the loss on a debt's becoming worthless has been incurred in a trade or business of the taxpayer shall, for this purpose, be made in substantially the same manner for determining whether a loss has been incurred in a trade or business for purposes of section 165(c)(1). For purposes of subparagraph (2) of this paragraph, the character of the debt is to be determined by the relation which the loss resulting from the debt's becoming worthless bears to the trade or business of the taxpayer. If that relation is a proximate one in the conduct of the trade or business in which the taxpayer is engaged at the time the debt becomes worthless, the debt comes within the exception provided by that subparagraph. The use to which the borrowed funds are put by the debtor is of no consequence in making a determination under this paragraph. For purposes of section 166 and this section, a nonbusiness debt does not include a debt described in section 165(g)(2)(C). See § 1.165-5 , relating to losses on worthless securities.
26 C.F.R. §1.166-5