Section 179 expensing has both a dollar limitation and an income limitation. The income limitation is as follows:
A taxpayer’s § 179 deduction for any taxable year...is limited to the taxpayer’s taxable income for that taxable year that is derived from the taxpayer’s active conduct of any trade or business during that taxable year.
See https://www.irs.gov/pub/irs-drop/rp-19-08.pdf
See also https://www.irs.gov/publications/p946#en_US_2018_publink1000299340
Further, the Section 179 deduction is not allowed for property used in connection with residential rental property.
See https://www.irs.gov/publications/p527#en_US_2018_publink1000219023
