Nonresident allocation and apportionment formula.

Nonresident individuals and nonresident estates and trusts engaged in business within and without the state shall be taxed only on such income as is derived from business transacted and property located within the state. The amount of such income attributable to Wisconsin may be determined by an allocation and separate accounting thereof, when the business of such nonresident individual or nonresident estate or trust within the state is not an integral part of a unitary business, but the department of revenue may permit an allocation and separate accounting in any case in which it is satisfied that the use of such method will properly reflect the income taxable by this state. In all cases in which allocation and separate accounting is not permissible, the determination shall be made in the following manner: for all businesses except air carriers, financial organizations, telecommunications companies, pipeline companies, public utilities, railroads, and car line companies there shall first be deducted from the total net income of the taxpayer the part thereof (less related expenses, if any) that follows the situs of the property or the residence of the recipient. The remaining net income shall be apportioned to this state by use of an apportionment fraction composed of the sales factor under sub. (7).