What is the minimum depth at which it will be considered as excavation work and permit will be required ?

Aman,

Any excavation require work permit

If it is more than 4 ft, you may need confined space entry work permit

1.2 m required PTW or 5Feet

Based on Osha 29 CFR Part 1926 under subpart P (Excavation) 1926.651 that a specific excavation requirements of an excavation is 4 feet or more in depth will be considered as excavation work and permit will be required.

working at depths in excess of 5 ft (1.5 m) for safety reason