§ 681. Limitation on charitable deduction
(a)
Trade or business income
In computing the deduction allowable under section
642
(c) to a trust, no amount otherwise allowable under section
642
(c) as a deduction shall be allowed as a deduction with respect to income of the taxable year which is allocable to its unrelated business income for such year. For purposes of the preceding sentence, the term “unrelated business income” means an amount equal to the amount which, if such trust were exempt from tax under section
501
(a) by reason of section
501
(c)(3), would be computed as its unrelated business taxable income under section
512 (relating to income derived from certain business activities and from certain property acquired with borrowed funds).