Create a dual wall to circumvent Classification?

[Mr. Pickle]

NEC states spaces adjacent to class 1, div 1, group D locations is a Class 1, Div 2, Group D location.

On a sanitary lift station project, we are looking to declassify an electrical space without providing ventilation per NFPA 820. See left side diagram of attached sketch for existing conditions.

We thought of sealing the door in the wall between the wetwell and electrical room (refer to space designations on attached sketch), but per NEC the electrical room would still be adjacent to a class 1, div 1 location, thus making it class 1, div 2.

What if we build a dual wall system? See right side of attached sketched. Red lines are proposed.

If we construct a new wall in the wetwell side, say 6-12" from existing wall shared with the electrical room and by doing so creating a new void or space. Then the void would be Class 1, Div 2, and now since the electrical room wall is shared with the new void space, the electrical room is now adjacent to a class 1, div 2 location, and such the electrical room wouldn't be classified.

This seems like a fairly simple solution but I have never seen this done before. I am wondering what I might be missing?

 

[petersonra]

What makes you think you have to have a wall to separate a division one area from a division two area?

In any case I don't believe there's any requirement that an impermeable wall that is division 1 on one side has to be division 2 on the other side. How would fumes get through the wall?

 

[rbalex]

NEC states spaces adjacent to class 1, div 1, group D locations is a Class 1, Div 2, Group D location.

On a sanitary lift station project, we are looking to declassify an electrical space without providing ventilation per NFPA 820. See left side diagram of attached sketch for existing conditions.

We thought of sealing the door in the wall between the wetwell and electrical room (refer to space designations on attached sketch), but per NEC the electrical room would still be adjacent to a class 1, div 1 location, thus making it class 1, div 2.

What if we build a dual wall system? See right side of attached sketched. Red lines are proposed.

If we construct a new wall in the wetwell side, say 6-12" from existing wall shared with the electrical room and by doing so creating a new void or space. Then the void would be Class 1, Div 2, and now since the electrical room wall is shared with the new void space, the electrical room is now adjacent to a class 1, div 2 location, and such the electrical room wouldn't be classified.

This seems like a fairly simple solution but I have never seen this done before. I am wondering what I might be missing?

I’m going to suggest you become familiar with NFPA 497, particularly Part 5.3.2.4:

In cases in which an unpierced barrier, such as a blank wall, completely prevents the spread of the combustible material, the area classification does not extend beyond the barrier.

I will note you will have a Class I, Division 2 envelope extending beyond the entrance door (the red one), consistent with the meaning of NEC [2023] Section 500.5.(B)(2)(3).

 

[Mr. Pickle]

What about NEC 500.5 (B)(2)(3)? This is a sanitary lift station of which NFPA 820 and the NEC applies

 

[rbalex]

What about NEC 500.5 (B)(2)(3)? This is a sanitary lift station of which NFPA 820 and the NEC applies

What about it? It’s the NEC Section that declares a Class I, Division 2 location is a location…, “That is adjacent to a Class I, Division 1 location, and to which ignitible concentrations of flammable gases, flammable liquid–produced vapors, or combustible liquid–produced vapors above their flash points might occasionally be communicated…”. It also mentions ventilation as a possible mediation.

The problem with so many Standards that peripherally address a subject is almost none of them completely cover the all possible aspects of the subject.

Forgetting NEC Zones, IEC Zones don’t necessarily require an adjacent Zone 2 to a Zone 1. The ‘classic” NEC philosophy, as stated in 500.5(B)(2)(3), is an open space next to a Division 1 location must at least occasionally have flammable material from the the Division 1 occur. But, where an impervious barrier exists, there is no need. [NFPA 5.3.32.4]

English ambiguity can also add to the confusion. In this case, NEC 500.5 (B)(2)(3) uses the 1b definition from Merriam-Websters 11 Collegiate Dictionary. (The “official” NFPA dictionary for common terms.

Definition of adjacent​

1a: not distant : NEARBY the city and adjacent suburbs
b: having a common endpoint or border
c: immediately preceding or following

In the case of your rooms, the Division 1 and 2 locations don’t actually have a common border if they are separated by a wall. Using your original analysis, the location just outside the exterior walls would also need to to be Division 2; however, only the location just outside the entrance needs attention.

 

[Mr. Pickle]

I don't understand. how is a shared wall not a common border?

 

[rbalex]

I don't understand. how is a shared wall not a common border?

Ok, all of your exterior walls around the Division 1 location are “common” in the same sense too, so why don’t you have a Division 2 classification around the exterior of the entire room?

• Seal the original common door as you proposed and forget a new wall.
• The “new” Division 2 is limited to an envelope around the exterior door.

OR

properly pressurize the adjoining room. [NFPA 496]​

BTW, your solution will work - you will just be wasting time and money.

OH - and it’s a common wall - not border; the “border” (or boundary in this case ) is the interior surface of the the impervious wall. If it isn’t impervious, a good coat of paint will usually do.

 

[rbalex]

I should have noted, that while your solution will work, there will still be a Division 2 envelope around the exterior door.