Difference between TABLE 310.15(B)(16) and (ANNEX B) TABLE B.310.15(B)(2)(1)

TABLE 310.15(B)(16) address up to three current carrying conductors in raceway, cable, or directly buried. TABLE B.310.15(B)(2)(1) covers two or three insulated conductors with an overall covering in raceway in free air. Both are based on the same ambient temperature of 30 deg C.

If I had a 3/C #8 AWG cable rated 75 deg THWN, each conductor is insulated with an overall PVC jacket, I would expect to use TABLE 310.15(B)(16) for ampacity determination routing this cable in conduit outside above ground.

While there are many references to the other tables and figures in ANNEX B, I can't find any guidance on using TABLE B.310.15(B)(2)(1) as those ampacities are lower.

Does anyone use TABLE B.310.15(B)(2)(1) for a specific application? This is the only table in this section that applies to free air installation as opposed to underground.

Am I misunderstanding terminology between "three current carrying conductors in a ...cable...being the same as "three insulated conductors with an overall covering"? Are they not both considered multiconductor cable at that point?

Thank you!
 

kwired

Electron manager
Location
NE Nebraska
Free air around the conductor will allow more heat dissipation and will result in higher current current carrying capability before insulation is compromised by high temp.

Please note that when sizing a conductor though you still have to have a minimum size that accounts for the termination temperature rating, but you can use the higher insulation ampacity rating as a starting point before making ampacity adjustments.
 

Dennis Alwon

Moderator
Staff member
Location
Chapel Hill, NC
Occupation
Electrical Contractor
correct typo

correct typo

Annex B should not be used unless under supervision of an engineer. Basically we ignore it. Of course you are an engineer so............just saw that. :D
 
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Annex B should not be used unless under supervision of an engineer. Basically we ignore it. Of course you are an engineer so............just saw that. :D
In that lies my problem I guess, I am an engineer, and I am trying to understand why ANNEX B - TABLE B.310.15(B)(2)(1) would be used (ever). Though in my other part question, I don't quite follow in some parts of the code a multiconductor cable is described as "three conductors in a cable" and in the parts such as ANNEX B, a multiconductor cable is described as multiple insulated conductors with an overall jacket. What's the difference? Can you refer to a cable manufactuer cable datasheet that shows the difference of these "different" multiconductor cables?

While certainly good that it was mentioned in the post, I am aware of termperature considerations for termination points. My concern is really focused on the the usefullness of ANNEX B and specifically TABLE B.310.15(B)(2)(1) as I have typically used 310.15(B)(16) or 310.15(C).

So no one uses TABLE B.310.15(B)(2)(1) for any ampacity determinations then and simply refer to 310.15(B)(16) for above ground installations of multiconductor cable?

Thanks!
 
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infinity

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New Jersey
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Am I misunderstanding terminology between "three current carrying conductors in a ...cable...being the same as "three insulated conductors with an overall covering"? Are they not both considered multiconductor cable at that point?
Three current carrying conductors in a cable is not the same as three insulated conductors with an overall covering. In the first example you could have a 5 condcutor cable and still only have 3CCC's. In the latter example if you had the same cable you would have 5 insulated conductors.
 
Three current carrying conductors in a cable is not the same as three insulated conductors with an overall covering. In the first example you could have a 5 condcutor cable and still only have 3CCC's. In the latter example if you had the same cable you would have 5 insulated conductors.

Guess that's a perfect example of how word usage is very important being that current carrying is not used in the other description. Rob - can you weigh in whether or not you have used ANNEX B TABLE B.310.15(B)(2)(1) and if so, for what purpose, and if not, why do you think it is not used?

Thanks!
 

infinity

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New Jersey
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Journeyman Electrician
Guess that's a perfect example of how word usage is very important being that current carrying is not used in the other description. Rob - can you weigh in whether or not you have used ANNEX B TABLE B.310.15(B)(2)(1) and if so, for what purpose, and if not, why do you think it is not used?

Thanks!

Hmm let me think about this. Maybe someone else will chime in.
 

Dennis Alwon

Moderator
Staff member
Location
Chapel Hill, NC
Occupation
Electrical Contractor
I bet there is a safe factor in using the standard tables. I am not sure what it takes to use Appendix B but my guess is most engineers don't want to risk using a smaller conductor. There may be a situation where there is an existing conduit and the proper size conductor will not fit so an engineer can do some calculations that may show a smaller conductor will work for that given situation.

I don't do much commercial work but I have never seen appendix B used and I bet it is not used often. Where are our engineers to weigh in here.
 
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