Why is Accumulated Depreciation an asset account?

Acc Dep is the depreciation till date on an asset. Regardless of the method used to calculate it, the depreciation of an asset during a single period is added to the previous period's accumulated depreciation to get the current accumulated depreciation.

A company buys an asset for $5,000 that has a five-year lifespan and zero salvage value. The company uses straight-line depreciation, and the asset depreciates at a rate of $1,000 per year. In year one, depreciation will be $1,000, as will accumulated depreciation, and carrying value of the asset will be $4,000. In year two, depreciation will be $1,000, accumulated depreciation will be $2,000 ($1,000 from the current year + $1,000 accumulated from previous years) and carrying value will be $3,000. Each subsequent year will follow the same process.