Clarification on Article 334.80

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Henley

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Chesapeake Bay
I have a question on Article 334.80 where the Ampacity reduction of multiple NM thru sealed holes are covered. I think I am ok in the main Article, the question comes in where is states the Exception in 310.15 a(2) shall not be allowed.

I recently had an inspector suggest a 14/2 and 12/2 thru same insulated hole would result with being required to use Ampacity of 14/2 due to this wording and therefore OCPD for 12/2 lowered to 15 A. I can't see this logically, but reading the code doesn't prove him wrong. What does NOT Allowing the Exception mean as it applies to 334.80?

Thanks

Sorry, I couldn't figure out how to paste Text of Articles
 

augie47

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The exception in 310.15(A)(2) allows that when the conductor is subject to two different ratings, you to use the higher of ampacities under certain conditions. Since the distance in the framing members is such a minor portion of the entire run if you applied the exception the derating rule would never apply.
However, as don states with just the two cables the rule does not even apply.
 

Dennis Alwon

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Chapel Hill, NC
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Retired Electrical Contractor
I have a question on Article 334.80 where the Ampacity reduction of multiple NM thru sealed holes are covered. I think I am ok in the main Article, the question comes in where is states the Exception in 310.15 a(2) shall not be allowed.

I recently had an inspector suggest a 14/2 and 12/2 thru same insulated hole would result with being required to use Ampacity of 14/2 due to this wording and therefore OCPD for 12/2 lowered to 15 A. I can't see this logically, but reading the code doesn't prove him wrong. What does NOT Allowing the Exception mean as it applies to 334.80?

Thanks

Sorry, I couldn't figure out how to paste Text of Articles

The inspector is incorrect. The ampacity of nm at 90C can be used for de-rating purposes and Table 310.15(B)(3)(a) gives the percentage of reduction. Basically the de-rating is affected after 9 current carrying conductor.
 

Dennis Alwon

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Chapel Hill, NC
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Retired Electrical Contractor
The exception in 310.15(A)(2) allows that when the conductor is subject to two different ratings, you to use the higher of ampacities under certain conditions. Since the distance in the framing members is such a minor portion of the entire run if you applied the exception the derating rule would never apply.
However, as don states with just the two cables the rule does not even apply.


334.80 states that 310.15(A)(2) does not apply
 

Dennis Alwon

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Chapel Hill, NC
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Here it is

Where more than two NM cables containing two or
more current-carrying conductors are installed, without
maintaining spacing between the cables, through the same
opening in wood framing that is to be sealed with thermal
insulation, caulk, or sealing foam, the allowable ampacity
of each conductor shall be adjusted in accordance with
Table 310.15(B)(3)(a) and the provisions of 310.15(A)(2),
Exception, shall not apply.
 

infinity

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New Jersey
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Journeyman Electrician
This was added to the 2011 NEC because when this rule first appeared in the 2008 they had not prohibited the use of the exception in 310.15(B)(2) so for the 2008 code cycle this was/is basically unenforceable.
 

Henley

Member
Location
Chesapeake Bay
Since I have some confirmation the Inspector is off on this, my question then is; What does the exception to 310.15 A(2) have to do with Cable assemblies ? What is the "exclusion" of the "exception" even mean? I guess you have to be a Lawyer/Politician/Electrician to understand some Articles.


Thanks in Advance
 

infinity

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Location
New Jersey
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Journeyman Electrician
Since I have some confirmation the Inspector is off on this, my question then is; What does the exception to 310.15 A(2) have to do with Cable assemblies ? What is the "exclusion" of the "exception" even mean? I guess you have to be a Lawyer/Politician/Electrician to understand some Articles.


Thanks in Advance


If you had cable assemblies bundled together for more than 24" you might be able to apply the exception and not need to apply any derating.

Mike Holt spells out the exception application here: http://ecmweb.com/code-basics/conductor-size-matters

808ecmCBfig3.jpg
 

JDB3

Senior Member
As said before, you got to be a lawyer, politician or---------------- to understand some (actually most) of the language used in the NEC.
So if I have 3 NM (romex) 12/2 or worse yet 12/3 going through a top plate 9one hole) framing member above my panel, and the opening is going to be sealed with foam, they need their ampacity needs to be adjusted ?
But this does not apply to SE cable? Any thoughts why?
 

infinity

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Location
New Jersey
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Journeyman Electrician
So if I have 3 NM (romex) 12/2 or worse yet 12/3 going through a top plate 9one hole) framing member above my panel, and the opening is going to be sealed with foam, they need their ampacity needs to be adjusted ?

Yes and no. Yes an adjustment calculation is required and no because it doesn't change the final ampacity. Using your 3-12/3 cables as an example, since you have more than two cables in that sealed hole you would need to derate, if these are homeruns out of the panel then you would have 2 CCC's in each cable so you end up with:

3 cables * 2 CCC = 6 CCC's
6 CCC's = 80% ampacity
30 amps * 80% = 24 amps so your 12/3 cables are still good for the standard 20 amp circuit.
 
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