# Annex B Ampacity Table B.310.15(B)(2)(3)



## KennyW (Aug 31, 2013)

Hi Guys, I noticed a manufacturer's ampacity rating for a cable was based on an Annex B table:

Table B.310.15(B)(2)(3) Ampacities of Multiconductor Cables with Not More Than Three Insulated Conductors, Rated 0 Through 2000 Volts, in Free Air Based on Ambient Air Temperature of 40°C (104°F) (for Types TC, MC, MI, UF, and USE Cables)*


I am a Canadian and only use the NEC from time to time, we do not have an equivalent table to this so I am interested to understand its application.

On average a 3c/500MCM cable has about 130amps more ampacity using this table than a "standard table". 

Is the intent that it assigns a higher rating to TC/MC type cables, and this is the normal table to use for tray rated cables, or what's the deal? It makes a huge difference. 


Any insight? Do you use this table? 

Cheers!


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## McClary’s Electrical (Feb 21, 2009)

Make sure you're looking at the 75 degree chart whenbusing that table.


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## don_resqcapt19 (Jul 18, 2010)

Even if you can use the ampacities from that table, 110.14(C)(1) specifies ampacities from Table 310.15(B)(16) for terminations.


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## KennyW (Aug 31, 2013)

mcclary's electrical said:


> Make sure you're looking at the 75 degree chart whenbusing that table.


Still works out way higher. Did you actually look at it? 


Example:
500MCM from this table is 416Amps (75 degree rating, at 40 degree ambient)

Bottom of the table then says:

*Refer to 310.15(B)(2) for the ampacity correction factors where the ambient temperature is other than 40°C (104°F).

Per that table, to correct the value to 30c, you multiple by 1.1, which gives:

416 * 1.1 = 447A (75c conductor temp at 30c ambient)


500MCM, 75c column from 310.15(B)(16) is on 380A 

70 Amp difference?


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## KennyW (Aug 31, 2013)

don_resqcapt19 said:


> Even if you can use the ampacities from that table, 110.14(C)(1) specifies ampacities from Table 310.15(B)(16) for terminations.


That section is for specifying equipment provisions, not selecting cables.

What they are saying is that if you are building a piece of equipment, the termination points need to accommodate cable sizes based on 310.15(B)(16). It doesn't mean the end user needs to use that table. 

The reason they say is is because that table is worst case. The lugs/terminals, or whatever need to be large enough to accommodate a cable based on that table with the appropriate derating.


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## KennyW (Aug 31, 2013)

Ok I spent the time to actually read the whole Annex (again I don't normally work with the NEC). 

The Annex seems to be a technical guideline for use by engineers who are doing ampacity calculations. That annex table can only be used if an engineer performs and stamps the calculations justifying it. A published ampacity rating from the cable manufacturer falls into this realm. 

The appropriate temp column still needs to be used for the termination of course.


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## don_resqcapt19 (Jul 18, 2010)

KennyW said:


> That section is for specifying equipment provisions, not selecting cables.
> 
> What they are saying is that if you are building a piece of equipment, the termination points need to accommodate cable sizes based on 310.15(B)(16). It doesn't mean the end user needs to use that table.
> 
> The reason they say is is because that table is worst case. The lugs/terminals, or whatever need to be large enough to accommodate a cable based on that table with the appropriate derating.


It means you cannot use a smaller cable at a higher ampacity. In the example in this thread, the maximum ampacity of a 500 kcmil on the termination would be 380 amps. Using a 500 kcmil conductor at a higher current or a smaller conductor at the 380 amps results in a higher temperature at the termination and is a violation.


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## KennyW (Aug 31, 2013)

don_resqcapt19 said:


> It means you cannot use a smaller cable at a higher ampacity. In the example in this thread, the maximum ampacity of a 500 kcmil on the termination would be 380 amps. Using a 500 kcmil conductor at a higher current or a smaller conductor at the 380 amps results in a higher temperature at the termination and is a violation.


No it does not. If you read the whole annex it makes sense. 

An engineer can assign whatever ampacity to a cable at whatever temperature they choose, provided they have the calculations to back it up. The Annex table is a guideline for engineers. 

Within the scope of a project or product that is not under the review/supervision of an engineer, you are correct. The Annex does not apply in that situation and the code does indeed explain that fact.


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## don_resqcapt19 (Jul 18, 2010)

KennyW said:


> No it does not. If you read the whole annex it makes sense.
> 
> An engineer can assign whatever ampacity to a cable at whatever temperature they choose, provided they have the calculations to back it up. The Annex table is a guideline for engineers.
> 
> Within the scope of a project or product that is not under the review/supervision of an engineer, you are correct. The Annex does not apply in that situation and the code does indeed explain that fact.


Nothing the engineer can do will change the requirement of 110.14(C).

110.14(C) prohibits the connection of a conductor to equipment where that conductor is used at a higher current than shown in Table 310.15(B)(16). 
The following is from the commentary in the 2014 NEC Handbook.


> When equipment of 600 volts or less is evaluated, conductors sized according to Table 310.15(B)(16) are required to be used. UL Guide Information for Electrical Equipment — The White Book clearly indicates that the 60°C and 75°C termination temperature ratings for equipment have been determined using conductors from NEC Table 310.15(B)(16). *However, installers or designers who are unaware of the UL guide information might attempt to select conductors based on a table other than Table 310.15(B)(16),* especially if a wiring method is used that allows the use of ampacities such as those in Table 310.15(B)(17). That use can result in overheated terminations at the equipment. The ampacities shown
> in other tables [such as Table 310.15(B)(17)] could be used for various conditions to which the wiring method is subject (such as ambient or ampacity correction conditions), *but the conductor size at the termination must be based on ampacities from Table 310.15(B)(16)*.


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## KennyW (Aug 31, 2013)

I really would think that is to decide what conductor size range is needed for the equipment termination itself when you the manufacturer is designing it. 

Are you telling me that if you had piece of gear that needed 1500A, you could not run three sets of 500MCM single conductors using the 75C column of 310.15(B)(17) 
(75C rating is 620A, assuming cables are spaced 1xD and 30C ambient).

You would still use 310.15(B)(16), even if running single conductors? No offense but that makes no sense to me. Then you would need 3 sets of single conductor 1000MCM for a 1500A service. That is flat out insane, nobody I know would ever spec that. 

Just... seems... not quite right.


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## don_resqcapt19 (Jul 18, 2010)

KennyW said:


> I really would think that is to decide what conductor size range is needed for the equipment termination itself when you the manufacturer is designing it.
> 
> Are you telling me that if you had piece of gear that needed 1500A, you could not run three sets of 500MCM single conductors using the 75C column of 310.15(B)(17)
> (75C rating is 620A, assuming cables are spaced 1xD and 30C ambient).


That is exactly what I am telling you. That would produce more heat at the termination point then the equipment was designed and tested for.



> You would still use 310.15(B)(16), even if running single conductors? No offense but that makes no sense to me. Then you would need 3 sets of single conductor 1000MCM for a 1500A service. That is flat out insane, nobody I know would ever spec that.
> 
> Just... seems... not quite right.


There are a lot of things in the NEC that don't seem "quite right".


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## KennyW (Aug 31, 2013)

Thanks for your patience, Don. 

That's pretty wild. 

In CEC we have this:


>  4-006 Temperature limitations (see Appendix B)
> (1) Where equipment is marked with a maximum conductor termination temperature, the *maximum allowable ampacity of the conductor shall be based on the corresponding temperature column from Table 1, 2, 3, or 4.*
> (2) Where equipment is not marked with a maximum conductor termination temperature, *90 °C shall be used by default.*


Table 1 is the equivalent to 310.15(B)(17). Table 2 is the equivalent to 310.15(B)(16)


To me this makes logical sense- use the appropriate temp column from the table relevant to the cable you are using- 75C column is a 75C column. We do have to derate a breaker size/uprate single conductors relative to the protection device they are connected to, which does something similar, but you only take 15% hit in ampacity which is still not as bad as have to use the 3c ampacity table for a 1c cable. 

That must be depressing. 

Seems like maybe it's some kind of residential based rule whose relevance is really lost when put in the context of large industrial equipment.


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## KennyW (Aug 31, 2013)

Check this out:

http://www.belden.com/docs/upload/A...nificantly_Enhance_Performance_and_Safety.pdf


Belden do a cable selection example and actually use the annex B table. 

The manufacturer I saw reference it originally was TPC wire.


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## don_resqcapt19 (Jul 18, 2010)

KennyW said:


> Check this out:
> 
> http://www.belden.com/docs/upload/A...nificantly_Enhance_Performance_and_Safety.pdf
> 
> ...


It is my opinion that the Belden paper is incorrect. 

There is no provision in the NEC that lets you use an ampacity greater than what is shown in Table 310.15(B)(16) at an equipment termination.


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