Ambient Temperature and Conductor Bundling

[maallen]

My question is from the 2011 NEC. It involves 310.15(A)(2) exception with 310.15(B)(2)(3), adjustment factors.

Under the ?General? provision for Selection of Ampacity exception, am I allowed to use this exception for adjustment factoring in 310.15(B)(2)(3), bundling of cables, or do each stand alone? That is, if I have an elevated ambient temperature for a circuit I would apply the ambient temperature factor from Table 310.15(B)(2)(a) and could use the 10ft/10% exception as it may apply. If bundling of conductors would require Adjustment Factoring from Table 310.15(B)(3)(a), do I get to use the 10 ft/10% exception for the bundling since this exception since it appears under "General"? As an example if I had 100' run of 86 degree ambience I would not have to adjust for ambient temperature. If the run also introduced more circuits in the last 9' that would total more than 3 current carrying conductors could I use the General exception to avoid having to use bundling adjustment factors since the lesser ampacity of the run is less than 10% and less than 10 feet? If so, I am not sure how the fixed 24? rule would apply in every case

Also if a given length of a circuit passed through a high temperature zone and joined other circuits to for a greater than 24? bundling condition quite remote from the high temperature zone, would the bundling factor be multiplied with the temperature adjustment for the circuit?s overall ampacity calculation, or would I treat them individually?

I hope my question is clear enough. This is a complex matter will a myriad of possible scenarios.

 

[GoldDigger]

The code is not really clear on what you have to do when different adjustments and corrections apply to different parts of the same circuit, but common sense would say that if the area of the fill-based adjustment and the temperature based correction do not overlap you would just have to meet both individually and not multiply the two factors.
But the inspector is going to be the one making the interpretation.
If the two areas overlapped at all, then yes, you would have to multiply the two factors.
And I think that trying to apply the 10% exception several times over the run of the same circuit will raise some eyebrows, but as long as there is a sufficiently long normal ampacity stretch in between it should be within Code.

 

[maallen]

The code is not really clear on what you have to do when different adjustments and corrections apply to different parts of the same circuit, but common sense would say that if the area of the fill-based adjustment and the temperature based correction do not overlap you would just have to meet both individually and not multiply the two factors.
But the inspector is going to be the one making the interpretation.
If the two areas overlapped at all, then yes, you would have to multiply the two factors.
And I think that trying to apply the 10% exception several times over the run of the same circuit will raise some eyebrows, but as long as there is a sufficiently long normal ampacity stretch in between it should be within Code.

Thanks GoldDigger. You are thinking along the same lines as I. Let me ask a more simplified question. Can I simply use the exception for bundling, or am I taking an exception from one code section and shoe-horning it into another code section?

 

[Smart $]

Thanks GoldDigger. You are thinking along the same lines as I. Let me ask a more simplified question. Can I simply use the exception for bundling, or am I taking an exception from one code section and shoe-horning it into another code section?

If you have a portion of a run that is 'bundled' less than 24", then it is not considered a 'section' under (A)(2) Exception.

Also keep in mind the (A)(2) Exception is regarding the length of the 'target' section and its adjacent section(s), not the length of the entire run. For example, if in 100' example you had a 9' section at a lower ampacity at each end, the higher-ampacity middle section would only be 82'. You would not be able to apply the exception to either end because they both exceed 10% of the middle section length.

 

[GoldDigger]

Thanks GoldDigger. You are thinking along the same lines as I. Let me ask a more simplified question. Can I simply use the exception for bundling, or am I taking an exception from one code section and shoe-horning it into another code section?

IMHO the 10'/10% exception does not say anything about the reason that one section has a lower calculated ampacity than the other. So you can use it for fill, for bundling, for roof temperature, etc.
But there are those who argue that it does not apply when the reason for the ampacity reduction is something like cables running together through a fire caulked penetration of framing members. I think their reasoning is that if it applied to that case the whole ampacity reduction rule would be invalidated since the penetration distance is short enough that it is less than 10% of almost anything.