If the income of the corporation is not effectively connected with, or if the corporation did not receive income that is treated as being effectively connected with, the conduct of a United States trade or business, the tax will be assessed under § 1.882-1(b)(1) on a gross basis, without allowance for any deduction (other than that allowed by section 170) or credit (other than the credits allowed by sections 33, 34 and 852(b)(3)(D)(ii)). If the income is effectively connected, or treated as effectively connected, with the conduct of a United States trade on business, tax will be assessed in accordance with either section 11, 55 or 1201(a) without allowance for any deduction (other than that allowed by section 170) or credit (other than the credits allowed by sections 33, 34 and 852(b)(3)(D)(ii)).
26 C.F.R. §1.882-4