Which category does Vishwakarma caste belong to?

You can't predict the status of Hindu Caste under government reservation, Since the base of Hindus are Vedas.Vishwakarma or Dhiman (also known as Vishwabrahmin or Dhiman Brahmins) Are born Brahmins.

Vishwakarma acharya are divided into five gotras (or clans); each Rishi's name is mentioned in the Yajurveda (4.3.3)

1.Sanaga (Son of Manu (Shiva))

2.Sanatana (Son of Maya (Vishnu))

3.Abhuvana (Son of Tvashtha (Brahma of four faces))

4.Pratanan (Son of Silpi (Indra))

5.Suparna (Son of Vishvajna (Surya))

These Rishi's were called as Pancha Rishi Brahmins or Pancha Brahmam From this 5 Face vishwakarmas generations are started.100% Pure Brahmins as per vedas."catur-varnyam maya srstam".1.Brahmins, 2.Kshatriya,3.Vaishya 4.Sudra.Some places of India they are economically Backward. But According To Vedas Upper caste Hindu.