¿Ampacity Calculation for Conductors in Auxiliary Gutters ?

[Emiliano]

Hello everyone,

I have a question regarding the ampacity calculation for conductors installed in auxiliary gutters.

According to NEC Table 310.16, ampacity is provided for conductors installed in raceways or buried. However, auxiliary gutters are not specifically listed in this table. Despite this, auxiliary gutters function similarly to raceways since they enclose conductors, affecting the heat dissipation of the system.

Given that auxiliary gutters are not explicitly defined as raceways, I understand that we would still calculate the base ampacity of conductors using Table 310.16 (since it's the standard reference for conductors in enclosures), and then apply adjustment factors based on the number of conductors, fill, and other environmental conditions.

Can anyone confirm that this is the correct approach, or is there another table or method recommended for this specific scenario?

 

[don_resqcapt19]

First of all, auxiliary gutters, Article 366, are very rare. Most of what electricians call auxiliary gutters, are really wireways, Article 376.
However in both cases the fill is limited to 20% of the interior cross sectional area, and there is no required ampacity adjustment until you have more than 30 current carrying conductors at a cross section. See 376.22.

 

[Dsg319]

 

[Emiliano]

First of all, auxiliary gutters, Article 366, are very rare. Most of what electricians call auxiliary gutters, are really wireways, Article 376.
However in both cases the fill is limited to 20% of the interior cross sectional area, and there is no required ampacity adjustment until you have more than 30 current carrying conductors at a cross section. See 376.22.

Hi Don.
I appreciate your response and completely understand the distinction between auxiliary gutters (Article 366) and wireways (Article 376). I also acknowledge that, under normal conditions, ampacity adjustment isn’t required unless there are more than 30 current-carrying conductors in a cross-section.

That said, my question is more focused on why Table 310.16 is used for ampacity calculations in an auxiliary gutter when the NEC explicitly states that auxiliary gutters are not classified as raceways. The general consensus among electricians may be to treat them as metal wireways, but from a strict definition standpoint, they are not raceways, which raises the question of whether Table 310.16 is the appropriate reference.

In my specific case, I have a transformer where the secondary side connects to the main panel through an auxiliary gutter provided by the transformer manufacturer. The issue is that treating this auxiliary gutter as a raceway for ampacity calculations leads to a requirement for larger conductor sizes or more parallel conductors per phase than if I were to run the conductors directly from the transformer throat into a cable tray and then to the panel—even though the distance in both cases is extremely short.

Would you still consider Table 310.16 the correct reference for ampacity in this case, or is there an alternative approach that could be justified based on NEC definitions and installation conditions?

Thanks again for your insights!

 

[infinity]

In my specific case, I have a transformer where the secondary side connects to the main panel through an auxiliary gutter provided by the transformer manufacturer. The issue is that treating this auxiliary gutter as a raceway for ampacity calculations leads to a requirement for larger conductor sizes or more parallel conductors per phase than if I were to run the conductors directly from the transformer throat into a cable tray and then to the panel—even though the distance in both cases is extremely short.

So in your case the conductors never enter or leave a raceway or cable?

 

[wwhitney]

That said, my question is more focused on why Table 310.16 is used for ampacity calculations in an auxiliary gutter when the NEC explicitly states that auxiliary gutters are not classified as raceways.

What's the reference for the explicit such statement? I see the definition in the 2023 NEC does not mention the word raceway, and describes them as enclosures, and Article 366 does not use the word raceway. So I would say it is implicit that they are not raceways, rather than explicit.

Anyway, 2023 NEC 366.23(A) on Sheet Metal Auxilliary Gutters says "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 sheet metal auxiliary gutter."

That statement both implies that adjustment factors are required for more than 30 conductors in a cross-section, and not required for 30 or less.. So it is moot whether 310.15(C) itself applies to auxilliary gutters, as 336.23(A) controls.

Interestingly, as auxilliary gutters appear to be enclosures and not raceways, the temperature correction required by 310.15(B) does not apply. As unlike 310.15(C), Article 366 has no reference to 310.15(B).

Cheers, Wayne

P.S. In contrast, the definition of "wireway" refers to them as raceways, so wireways are raceways. That means that 376.22(B) is in the wrong place in the code, as Article 376 can't amend Article 310. The relaxed requirement of 376.22(B) belongs in 310.15(C)(1), just like the relaxed requirement for AC and MC cable in 310.15(C)(1)(d).

Cheers, Wayne

 

[don_resqcapt19]

Hi Don.
...
Would you still consider Table 310.16 the correct reference for ampacity in this case, or is there an alternative approach that could be justified based on NEC definitions and installation conditions?

Thanks again for your insights!

I would use 310.16 as there is no other option. I still tend to doubt that you actually have an auxiliary gutter.

 

[don_resqcapt19]

What's the reference for the explicit such statement? I see the definition in the 2023 NEC does not mention the word raceway, and describes them as enclosures, and Article 366 does not use the word raceway. So I would say it is implicit that they are not raceways, rather than explicit.

Anyway, 2023 NEC 366.23(A) on Sheet Metal Auxilliary Gutters says "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 sheet metal auxiliary gutter."

That statement both implies that adjustment factors are required for more than 30 conductors in a cross-section, and not required for 30 or less.. So it is moot whether 310.15(C) itself applies to auxilliary gutters, as 336.23(A) controls.

Interestingly, as auxilliary gutters appear to be enclosures and not raceways, the temperature correction required by 310.15(B) does not apply. As unlike 310.15(C), Article 366 has no reference to 310.15(B).

Cheers, Wayne

P.S. In contrast, the definition of "wireway" refers to them as raceways, so wireways are raceways. That means that 376.22(B) is in the wrong place in the code, as Article 376 can't amend Article 310. The relaxed requirement of 376.22(B) belongs in 310.15(C)(1), just like the relaxed requirement for AC and MC cable in 310.15(C)(1)(d).

Cheers, Wayne

What limits the temperature correction to raceways? I don't see any reason why 336 would need to reference 310.15(B) for that rule to apply. In my opinion temperature correction is required no matter where conductor is located assuming that the temperature is other than 30°C.

Yet there are tens of rules in Chapters 1-4 that modify other Chapter 1-4 rules.

 

[infinity]

I would use 310.16 as there is no other option. I still tend to doubt that you actually have an auxiliary gutter.

I agree because as Don stated there is no other code option. I think that Emiliano has a good point. If there are no raceways involved for example going from: Cabinet>Auxiliary Gutter>Cable Tray>Cabinet then there is no code section that references the ampacity of the conductors. Once they enter or leave a raceway then 310.16 would apply.

 

[wwhitney]

What limits the temperature correction to raceways?

Good point, a bit of a thinko on my part, as 310.15(C) refers to raceways, cables, etc, while 310.15(B) just refers to conductors.

I was thinking raceway = run of conductor, and enclosure = termination, along with no temperature correction required for termination ampacity checks. But auxilliary gutters are a case where the run of the conductor is in an enclosure, and there's nothing exempting those conductors from 310.15(B).

Cheers, Wayne

 

[Tulsa Electrician]

FYI, I like the upside down question mark.
It caught my attention.
¿¿¿
Learned something new today. Thanks

 

[don_resqcapt19]

PI 3278 for the 2023 code:

366.23 Ampacity of Conductors.
(A) Sheet Metal Auxiliary Gutters.
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 sheet metal auxiliary gutter. 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. The current carried continuously in bare copper bars in sheet metal auxiliary gutters shall not exceed 1.55 amperes/mm2 (1000 amperes/in.⁠2) of cross section of the conductor. For aluminum bars, the current carried continuously shall not exceed 1.09 amperes/mm2 (700 amperes/in.2) of cross section of the conductor.

Where adjustment factors are required to be applied, it shall be permitted to apply the provisions of the Exception to 310.14(A)(2).

Resolution:

The existing language is clear and the proposed text is not necessary. The requirements in 366.23(A) are based on the number of current carrying conductors in a sheet metal auxiliary gutter. These requirements modify the “adjustment factors for more than three current-carrying conductors” where there are 30 or less current carrying conductors in the sheet metal auxiliary gutter. The requirements in 366.23(A) do not modify the adjustment factors where there are more than 30 current carrying conductors. Further, 366.23(A) 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.