Boundary Seal Exception for Pressurized Rooms

[J_W_]

Long time lurker, but first post...

Per 501.15(B)(2) Exception 3 (2011 NEC): Boundary seals aren't required passing from a pressurized room into a C1,D2 area.

Is that exception only valid where the transition is immediate, or could the boundary seal be omitted in a conduit that leaves the room, travels some distance (lets say 10' - 20'), and then enters a C1D2 area?

The latter situation occurs regularly in the types of facilities I work in because a room may get pressurized due to impingement along 1 wall, but along the other walls conduits cross an unclassified area before going into the classified process area.

 

[rbalex]

Ya gotta love English ambiguity. This is what Exception 3 says:

Exception No. 3: Conduit systems passing from an enclosure or room that is unclassified as a result of pressurization into a Class I, Division 2 location shall not require a seal at the boundary.

Informational Note: For further information, refer to NFPA 496-2008, Standard for Purged and Pressurized Enclosures for Electrical Equipment.​

The simplest reading, "... passing from an enclosure or room ... into a Class I, Division 2 location ... " would strongly imply the boundary is at the wall; but it doesn't exactly say that. If I were the AHJ, since Division 2/unclassified boundary seals aren't required to be explosionproof in the first place, I might be convinced a remote boundary was covered if it could be shown the pressure at the boundary was equivalent to that required throughout a Type Z system by NFPA 496. It isn't very much [25 Pa (0.1 in. of water)] but you might be surprised how difficult it would be to maintain with threaded raceways.