Where more than one ampacity applies

[ramsy]

According to Jim Dollard, NEC code-making panel member.

2020 NFPA-70, 310.14(A)(2) Where more than one ampacity applies..
Exception: ..the higher ampacity shall be permitted..

applies only to the impact of ambient temperature..
"..cannot be applied to correction factors on rooftops or for adjustment factors for more than three current-carrying conductors."

Source Q&A Article

This forum uses 210.14(A)(2) Exception on rooftops, regardless of CCC's that exceed table limits, contrary to Jim Dollard & inspector red tag.

If 310.14(A)(2) Exception excludes rooftops, and Adjustment factors that exceed table CCC's, where is that stated in the code language?

 

[ramsy]

2020 NEC, 310.15(C)(1) Adjustment Factors, for current-carrying-conductors, omits any reference to 310.14(A)(2) Exception, or similar language.

 

[augie47]

Be interesting to hear his reassoning as the wording itself seems to be all encompassing.

 

[infinity]

I agree that this would apply to a wireway but I don't see language that says this applies to raceways in general.

 

[don_resqcapt19]

It is my opinion that the exception applies in all cases where there are multiple ampacities for the circuit.

 

[Dennis Alwon]

It is my opinion that the exception applies in all cases where there are multiple ampacities for the circuit.

I agree. I don't see any wording that states what he stated.

There are multiple requirements in play here and we need to start in 310.14, Ampacities for Conductors Rated 0 Volts–2,000 Volts. Section 310.14(A)(2), Selection of Ampacity, requires that where more than one ampacity applies for a given circuit length, the lowest value must be used. The exception that follows this general rule is what you are referencing in your question. It applies only to the ampacity chosen (tables or through engineering supervision) and the impact of ambient temperature on a small portion of the circuit. This exception permits the higher ampacity to be used where the lower ampacity (as adjusted due to ambient temperature) does not exceed the lesser of 10 feet or 10% of the total circuit. This adjustment cannot be applied to correction factors on rooftops or for adjustment factors for more than three current-carrying conductors.

 

[infinity]

When I read his interpretation I see wording that just doesn't exist for all raceways.

 

[don_resqcapt19]

Public Input No. 3279-NFPA 70-2020 [ Section No. 376.22(B) ]

(B) Adjustment Factors.
The adjustment factors in 310.15(C)⁠(1) shall be applied only where the number of current-carrying conductors, including neutral conductors classified as current-carrying under 310.15(E), exceeds 30 at any cross section of the wireway. Conductors for signaling circuits or controller conductors between a motor and its starter and used only for starting duty shall not be considered as current-carrying conductors. Where adjustment factors are required to be applied, it shall be permitted to apply the provisions of the Exception to 310.14(A)(2).

Statement of Problem and Substantiation for Public Input
There has been much debate on the internet code forums about the use of the Exception to 310.14(A)(2) for ampacity adjustment in wireways and gutters. Respected code authorities are not in agreement on the use of the exception. Where code experts are not in agreement on what the code requires, that is a strong indication that the code is not clear on the issue. This proposal is intended to add language to the code to make the issue clear.

Committee Statement

Resolution:

The existing language is clear and the proposed text is not necessary. The requirements in 376.22(B) are based on the number of current carrying conductors in a wireway. These requirements modify the “adjustment factors for more than three current-carrying conductors” where there are 30 or less current carrying conductors in the wireway. The requirements in 376.22(B) do not modify the adjustment factors where there are more than 30 current carrying conductors. Further, 376.22(B) does not modify the ambient temperature correction factors. As there is no modification for ambient temperature correction, the ambient temperature correction factors in 310.15(B) and the requirements and exception in 310.14(A)(2) apply.

 

[dm9289]

It is my opinion that the exception applies in all cases where there are multiple ampacities for the circuit.

Yes it says adjustment or correction

 

[ramsy]

..the requirements and exception in 310.14(A)(2) apply.

Thanks for sharing this Public Input No. 3279-NFPA 70-2020, specific to 376.22(B) wireways only.

It appears each wiring method must clarify how it deals with 310.14(A)(2) Exception.

For example, 334.80 prohibits the Exception.

334.80
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.

The code is silent where conduit is concerned, except in Appendix D Example D3a, which only applies this exception to conduit / vault ambient conditions.

 

[don_resqcapt19]

Thanks for sharing this Public Input No. 3279-NFPA 70-2020, specific to 376.22(B) wireways only.

It appears each wiring method must clarify how it deals with 310.14(A)(2) Exception.

For example, 334.80 prohibits the Exception.


The code is silent where conduit is concerned, except in Appendix D Example D3a, which only applies this exception to conduit / vault ambient conditions.

That PI, as well as one for 366, was generated because there were specific comments by "experts" (statements like the one in the original post of this thread) saying that the exception does not apply to those applications. It was only a PI to clarify what the code means.

I see no need for any additional clarification as the exception, as written, applies for all applications where there is more than one ampacity for the circuit conductors as a result of any code required ampacity adjustment and/or ampacity correction.

If there is a wiring method where that exception does not apply, that needs to be stated in the wring method article, like you pointed out for Article 334.