Underground conduit on CNG site.

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sentrybryan

Member
Location
Nebraska
Hello all, first post.

Having an issue with cover requirements in an area with CNG compressors. The area has been determined to be Class1Div2 on the print. The issue I have is that i really cant get a full 24" of cover on all my pvc's running thru the area. Having discussed this with AHJ on the phone he is wanting the full 24" or RMC. Problem is, is that this trench has (1) 4", (3) 2-1/2", (25) 1" and (5) 3/4" conduits. In discussng this with a good inspector friend, he was seeing this area under the concrete as unclassified (CNG only is from grade up), and the sealoff first fitting out of the ground essentially blocks the conduit from any fumes. He was reading this as only needing 18" of cover according to Art 300.5.

My backup plan was to see if we could simply encase the conduits in concrete anywhere where I couldn't get the full 24". AHJ kind of scoffed at this. I had done this in the past with inspectors permission on regular gasoline sites. This being subgrade under CNG I would think this would be a no-brainer to pass.

Just wondering what your thoughts/opinions on this matter were. Thank you.
 

sameguy

Senior Member
Location
New York
Occupation
Master Elec./JW retired
rbalex, on this site is really good on this stuff, sure he will drop in soon.
I too think under slab is under slab and 18" seal with no coupling below until under slab should be fine; however no code book to look it up going by fading memory.
 

rbalex

Moderator
Staff member
Location
Mission Viejo, CA
Occupation
Professional Electrical Engineer
Your "good inspector friend" is correct. Essentially, it goes like this:

  • If electrical area classification was not a consideration, how should it be installed?
  • If some of the locations in the installation are classified, what modifications are necessary?
  • Below grade is not (or should not properly be) classified in a CNG facility.
Therefore, you are in a Table 300.5, Column 3 situation with regard to the raceway. Sections 501.10(B) and 501.15 should be observed above grade.

It should be noted, compressed or not, CNG is basically "natural gas" (methane) and lighter than air. Migrating gases and vapors below grade is not a consideration; nevertheless, boundary seals are.

Section 501.10(A)(1)(a) and its Exception can be confusing. The intent of part of the Exception is to recognize certain installation practices that are accepted in specific applications [Sections 514.8, Exception No.2. and 515.8(A)] where below grade is commonly classified. A CNG facility is not such an application, nor is underground classification necessary.
 

sentrybryan

Member
Location
Nebraska
Awesome, thank you. To clarify, this means even the pipe stubbing up in the cl1div2 area only needs to be 18" deep when transitioning because of the sealoff?
 

rbalex

Moderator
Staff member
Location
Mission Viejo, CA
Occupation
Professional Electrical Engineer
Awesome, thank you. To clarify, this means even the pipe stubbing up in the cl1div2 area only needs to be 18" deep when transitioning because of the sealoff?
Actually, the seal has nothing to do with the burial depth. However, "Threaded rigid metal conduit or threaded steel intermediate metal conduit shall be used for the last 600 mm (24 in.) of the underground run to emergence or to the point of connection to the aboveground raceway." [Section 501.10(A)(1)(a) Exception] The seal should be located consistent with Section 501.15 (B)(2) using grade (the "point of emergence") as the "boundary".
 

nhee2

Senior Member
Location
NH
Actually, the seal has nothing to do with the burial depth. However, "Threaded rigid metal conduit or threaded steel intermediate metal conduit shall be used for the last 600 mm (24 in.) of the underground run to emergence or to the point of connection to the aboveground raceway." [Section 501.10(A)(1)(a) Exception] The seal should be located consistent with Section 501.15 (B)(2) using grade (the "point of emergence") as the "boundary".

If we are saying the below grade is unclassified, then why does 501.10(A)(1)(a) Exception apply? And is the requirement for "Threaded rigid metal conduit or threaded steel intermediate metal conduit shall be used for the last 600 mm (24 in.) of the underground run to emergence or to the point of connection to the aboveground raceway." even applicable?

I'd think this section does not apply. Boundary seal required at the Class I Div 2 boundary, and all below grade installed per Article 300, not 501?
 

rbalex

Moderator
Staff member
Location
Mission Viejo, CA
Occupation
Professional Electrical Engineer
If we are saying the below grade is unclassified, then why does 501.10(A)(1)(a) Exception apply? And is the requirement for "Threaded rigid metal conduit or threaded steel intermediate metal conduit shall be used for the last 600 mm (24 in.) of the underground run to emergence or to the point of connection to the aboveground raceway." even applicable?

I'd think this section does not apply. Boundary seal required at the Class I Div 2 boundary, and all below grade installed per Article 300, not 501?
The Section applies via Section 501.10(B)(1)(1).

My first response in Post #3 answers the rest of your question. There is no basis for classifying the underground except for a few cases in Articles 514 and 515. (Note: below grade and underground are not necessarily the same thing) Section 501.10(A)(1)(a) Exception was developed to recognize those cases and not put them in direct conflict with Article 501. I was on CMP14 at the time and was basically opposed to the change, primarily because I believe Article 510, especially Section 510.2 covers the issue. Also, see the FPN/INs following the Titles of Articles 511 to 517. Articles 511 to 517 are actually controlled by NFPA Technical Committees that are not coordinated through the NEC Technical Coordinating Committee nor fully subject to CMP14. In general, most conflicts require CMP14 to comply. Any "hard" conflicts have to be reconciled by the NFPA Standards Council which generally takes about 2-3 Code cycles and details of the original issue are often forgotten because the Standard Council will be almost totally reconfigured in that time.
 

nhee2

Senior Member
Location
NH
The Section applies via Section 501.10(B)(1)(1).

My first response in Post #3 answers the rest of your question. There is no basis for classifying the underground except for a few cases in Articles 514 and 515. (Note: below grade and underground are not necessarily the same thing) Section 501.10(A)(1)(a) Exception was developed to recognize those cases and not put them in direct conflict with Article 501. I was on CMP14 at the time and was basically opposed to the change, primarily because I believe Article 510, especially Section 510.2 covers the issue. Also, see the FPN/INs following the Titles of Articles 511 to 517. Articles 511 to 517 are actually controlled by NFPA Technical Committees that are not coordinated through the NEC Technical Coordinating Committee nor fully subject to CMP14. In general, most conflicts require CMP14 to comply. Any "hard" conflicts have to be reconciled by the NFPA Standards Council which generally takes about 2-3 Code cycles and details of the original issue are often forgotten because the Standard Council will be almost totally reconfigured in that time.

I agree there is no basis for classifying the underground. But if the underground is unclassified (which I think was the basis for suggesting it could be buried at 18" vs. 24") then I would not think 501.10(B)(1)(1) or the exception in 501.10(A)(1)(a) would apply to the conduit once it leaves the boundary. A chapter 3 wiring method would be acceptable. Otherwise, if 501.10(B)(1)(1) does apply to this conduit, its not clear how an 18" burial depth would be acceptable - either its classified and Chapter 5 method, or not classified and its a Chapter 3 method (and doesn't require the rigid or imc for the last 24" of the run).
 
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