Wis. Stat. § 701.1132: Transfers from income to principal for depreciation.

(1) In this section, “depreciation” means a reduction in value due to wear, tear, decay, corrosion, or gradual obsolescence of a fixed asset having a useful life of more than one year.

(2) A trustee may transfer to principal a reasonable amount of the net cash receipts from a principal asset that is subject to depreciation, but may not transfer any amount for depreciation:

(a) Of that portion of real property used or available for use by a beneficiary as a residence or of tangible personal property held or made available for the personal use or enjoyment of a beneficiary.

(b) During the administration of a decedent’s estate.

(c) Under this section if the trustee is accounting under s. 701.1117 for the business or activity in which the asset is used.

(3) An amount transferred to principal need not be held as a separate fund.