Article 505 / Article 501.5
Traditionally both the NEC and CEC codes have used a different way of categorising the Explosive Locations, which was by Class and Division, but since the NEC Code 505 change in 1996 which introduced the Zoning method for the first time a different approach is now possible.
This move brings the potential for alternative protection methods adopted in the IEC standards and European Normatives, to be accepted within the classified zones, and could be seen as the first major step in a process that could allow a degree of harmonisation from developing.
A similar move has occurred within the Canadian market with Code changes that embrace IEC concepts being adopted within the established CEC regulations.
In accordance with the NEC 2002 Code, Article 501.5(E) - Cable Seals, Class 1 Div 2, cables need only be sealed if the equipment is required to be explosion proof. i.e. contains an ignition source. This is highlighted in section (1) of 501.5(E). Additionally, section (2) of 501.5(E) - Cables That Do Not Transmit Gases or Vapors, states that if this type of cable is used, it does NOT require to be 'Sealed' at the entry unless the equipment is required to be 'explosion proof', as required in 501.5(E)(1)
For your information, here are the extracts from from the NEC Code book.
501.5(E) - Cable Seals, Class 1 Div 2, - In Class 1 Div 2 locations, cable seals shall be located in accordance with 501.5(E)(1) through (E)(4).
(1) Terminations. - Cables entering enclosures that required to be explosionproof shall be sealed at the point of entrance. The sealing fitting shall comply with 501.5(B)(1). Multi-conductor cables with a gas/vaportight continuous sheath capable of transmitting gases or vapors through the cable core shall be sealed with a listed fitting in the Division 2 location after removing the jacket and any other coverings so that the sealing compound surround each individual insulated conductor in such a manner as to minimize the passage of gases and vapors. Multi-conductor cables in conduit shall be sealed as described in 501.5(D).
Exception No 1: - Cables passing from an enclosure or room that is unclassified as a result of Type z pressurization into a Class 1, Div 2 location shall not require a seal at the boundary.
Exception No 2: - Shielded cables and twisted pair cables shall not require the removal of the shielding material or separation of the twisted pairs, provided the termination is by an approved means to minimize the entrance of gases or vapors and prevent propagation of flame into the cable core.
(2) Cables That Do Not transmit Gases or Vapors. - Cable that have a gas/vapor tight continuous sheath and do not transmit gases or vapors through the cable core in excess of the quantity permitted for seal fittings shall not be required to be sealed except as required in 501.5(E)(1). The minimum length of such cable run shall not be less than that length that limits gas or vapor flow through the cable core to the rate permitted for seal fittings.
Basically, what the above says is that cables only need to be 'sealed' in Class 1 Div 2 location only if the enclosure needs to be 'explosionproof'