We are wiring a explosion proof exhaust fan in a
car auction barn mounted below 18".I need some help on placement of the seal offs,disconnects,
how and where to go back to EMT
What's the classification of the building according to the Building Code? It sounds like you are assuming this comes under Article 511 for commercial garages. You might look at NEC Handbook 511.1 for a description of what is include or not included in this article.