See https://www.irs.gov/publications/p527#en_US_2020_publink1000219087
5-, 7-, or 15-year property.
For property in the 5- or 7-year class, use the 200% declining balance (DB) method and a half-year convention. However, in limited cases you must use the mid-quarter convention, if it applies. For property in the 15-year class, use the 150% declining balance method and a half-year convention.
You can also choose to use the 150% declining balance method for property in the 5- or 7-year class. The choice to use the 150% method for one item in a class of property applies to all property in that class that is placed in service during the tax year of the election. You make this election on Form 4562. In Part III, column (f), enter "150 DB." Once you make this election, you can’t change to another method.
If you use either the 200% or 150% declining balance method, figure your deduction using the straight line method in the first tax year that the straight line method gives you an equal or larger deduction.
