IRS Pub 946 https://www.irs.gov/publications/p946#en_US_2019_publink1000107523

What Is the Basis for Depreciation?

The basis for depreciation of MACRS property is the property's cost or other basis multiplied by the percentage of business/investment use. For a discussion of business/investment use, see Partial business or investment use under Property Used in Your Business or Income-Producing Activity in chapter 1. Reduce that amount by any credits and deductions allocable to the property.