Outside Disconnect Derating

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JAH09190

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Hello all,

Thanks in advance for the assistance.

I am trying to design a cable run that will live completely inside of a disconnect enclosure. This disconnect enclosure will be mounted outside and subject to direct sunlight in California. The cable I hope to use is DLO type flexible cable. The runs will be individual conductors of a 3 phase, 4 wire plus ground. The lengths of these runs will be about 3 feet, maximum.

I have looked through NEC 2017 and other forum posts for a proper derating amount, but have come up empty. My thinking is to figure out the ambient temperature inside the disconnect enclosure and apply the proper derate from there, but I was hoping to find something more concrete.

Any thoughts?
 
There is no derating required by the NEC for conductors simply inside of a disconnect enclosure unless maybe they were considered bundled together.
 
infinity said:
There is no derating required by the NEC for conductors simply inside of a disconnect enclosure unless maybe they were considered bundled together.
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I'm not sure that I understand what you are saying. Single-Insulated conductors in Free Air are subject to ambient temperature correction factors. Why wouldn't conductors in a disconnect enclosure be?
 
If the conductors don't exit the enclosure in raceway or trough or tray, etc, then you will have trouble finding a table for ampacities to use. Table 310.15(B)(16) is for conductors in raceway, cables, or earth.
BTW, DLO is not recognized in the NEC. You will want to choose a conductor insulation from 310.104(A) that also with be listed on the conductor such as possibly RHH or RHW-2
 
ron said:
If the conductors don't exit the enclosure in raceway or trough or tray, etc, then you will have trouble finding a table for ampacities to use. Table 310.15(B)(16) is for conductors in raceway, cables, or earth.
BTW, DLO is not recognized in the NEC. You will want to choose a conductor insulation from 310.104(A) that also with be listed on the conductor such as possibly RHH or RHW-2
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Thanks Ron! Just so I have this right, no derating for simply being in a disconnect, but if the ambient temperature inside the disconnect is different from 30C, then a correction factor would be applied.
 
david luchini said:
I'm not sure that I understand what you are saying. Single-Insulated conductors in Free Air are subject to ambient temperature correction factors. Why wouldn't conductors in a disconnect enclosure be?
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Simply because they're not in a raceway, cable or earth as Rom mentioned.
 
david luchini said:
That doesnt make sense. Single conductors in free air are not in raceway, cable or earth either, but are subject to ambient temperature correction.
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I agree that it might not make sense but what code section would one cite to say that you need to derate conductors within an enclosure?
 
infinity said:
I agree that it might not make sense but what code section would one cite to say that you need to derate conductors within an enclosure?
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310.15(B)(2), assuming the selection of the ampacity of the conductor was from 310.15(B).

Of course if ampacity was selected under engineering supervision, then ambient temperature is already factored in.
 
infinity said:
I agree that it might not make sense but what code section would one cite to say that you need to derate conductors within an enclosure?
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I look at it as a conservative practice. UL test conductors in enclosure be measuring heat rise which includes heat transfer/ventilation. NEC tables, and manufacturers table, for free air would not be applicable because the conductors are enclosed not in free air. About the only references I can think of for conductors in enclosures would be UL508A and NFPA 79.
 
JAH09190 said:
but if the ambient temperature inside the disconnect is different from 30C, then a correction factor would be applied.
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I don't know what correction factor you would use. There is a table but I'm not sure what code section would direct you to Table 310.15(B)(2)(b) in an enclosure.

For example if you were in a raceway or in a cable exposed to sunlight on a roof, you would be directed to that table, but I'm not sure what section applies to you situation inside of an enclosure.
 
ron said:
I don't know what correction factor you would use. There is a table but I'm not sure what code section would direct you to Table 310.15(B)(2)(b) in an enclosure.
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Intuitively, I would assume that if a cable ampacity was selected with 30C ambient temperature in mind, and the temperature inside the enclosure was above that, some derating would have to be done. This is the reason I asked this question, it seems like there has to be something done, however there isn't anything in the code to go by.
 
JAH09190 said:
Intuitively, I would assume that if a cable ampacity was selected with 30C ambient temperature in mind, and the temperature inside the enclosure was above that, some derating would have to be done. This is the reason I asked this question, it seems like there has to be something done, however there isn't anything in the code to go by.
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I think people are getting hung up on the (mistaken) idea that the ambient temperature correction factors only apply to conductors in raceway, cables, or earth. The ambient temperature correction factors apply to ALL of the Ampacity Tables in 310.15(B).

So the question is, how are you determining the ampacity of the cable? 310.15(A)(1) permits you to determine the ampacity either through Engineering Supervision in 310.15(C) which directly factors in the ambient temperature, or through the Ampacity Tables in 310.15(B).

If you use the Ampacity Tables, then whatever the ampacity of the conductor that you come up with at 30degC, you must adjust the ampacity for the ambient temperature within the enclosure.
 
david luchini said:
So the question is, how are you determining the ampacity of the cable? 310.15(A)(1) permits you to determine the ampacity either through Engineering Supervision in 310.15(C) which directly factors in the ambient temperature, or through the Ampacity Tables in 310.15(B).
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For this particular run I was using 310.15(B), since it is such a short run.

Thanks to everyone for the help!
 
david luchini said:
So the question is, how are you determining the ampacity of the cable? 310.15(A)(1) permits you to determine the ampacity either through Engineering Supervision in 310.15(C) which directly factors in the ambient temperature, or through the Ampacity Tables in 310.15(B).
Click to expand...

Since this is such a short run, I was using 310.15(B). After re-reading 310.15(B)(2) it seems to point to using the ambient temp correction across all the tables, although it doesn't explicitly say that. Maybe that's what I am interpreting because it is what I want to hear:D

Thanks to all for the help!
 
JAH09190 said:
Since this is such a short run, I was using 310.15(B). After re-reading 310.15(B)(2) it seems to point to using the ambient temp correction across all the tables, although it doesn't explicitly say that. Maybe that's what I am interpreting because it is what I want to hear:D
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I think 310.15(B)(2) is explicit about using the ambient temperature correction factors for all of the Tables. Each Table also explicitly states to see 310.52(B)(2) for ambient temperatures other than that Table's base ambient temperature.
 
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