The information you need is too broad and generalized for this type of forum. An example of your installtion or more defined details will assist in providing assistance.
Article 500 covers Hazardous Locations. Under the definition of "Explosionproof Apparatus", a FPN note directs you to see the publication: ANSI/UL 1203-1994 Explosion-Proof and Dust-Ignition-Proof Electrical Equipment for Use in Hazardous (Classified) Locations. Start there.
What is the classification of your area? Are there boundry areas where you will enter other classified zones? Will you require seals? See section 501.5(C)(4).
There are no special requirements for splicing in classified areas. There are requirements for raceways, enclosures and seals, but as others have said, more information is needed for a specific answer.