In service.
It's not an expense, unless the total cost is less than $2500. It's an asset, and assets are added to your depreciation schedule when they are placed in service.
You begin to claim depreciation when your property is placed in service for either use in a trade or business or the production of income. The placed in service date for your property is the date the property is ready and availa- ble for a specific use.
PAge 34 here https://www.irs.gov/pub/irs-pdf/p946.pdf
