Class II Div I and II Classification Question

[Rolson]

Is there any articles that shed more light on when a location is Class II DivI or DivII. I have read that in the NEC particle sizes must be under 420um to be considered a classified location (500.1 NEC) but under what concentration or amount of particles? The location is in a feed mill with the customer milling a particle size of 500um corn. There is some smaller particles but are usually under 10%. Does anyone know or have any information that would be helpful in determining classification NEC article 500 does not explain what the levels need to be to be classified.

 

[rbalex]

The proper reference is NFPA 499. It is one of the documents listed in Section 500.4(B) Informational Notes.