215.2(A) understanding.

[david luchini]

I am not sure why you think I am mixing concepts about cross-sectional area and ampacity. I am trying to limit this discussion to ampacity only.

I think you are mixing concepts because you said "Suppose the minimum feeder size current is 110A"...Sections (a) and (b) talk about minimum conductor size, not minimum current size.

1. How do you know the intent for certain?

2. I disagree. The language does not mention taking the larger of the conductor sizes, it mentions taking the larger of the conductor "ampacities".

1) I think you can find somewhere online the proposal for the language change submitted to the NEC, with the intent given therein.

2). 215.2(A)(1) doesn't actually mention taking the larger of the conductor "ampacities."

Again, I disagree. 215.2(A)(1)(a) and (b) do not already specify conductor sizes, they state that whatever hypothetical conductor size you intend on using must have an "Ampacity" that satisfies each of the conditions in (a) or (b). This supports my main post about applying adjustment/correction factors to table ampacities and not doing the short-cut method of applying adjustment/correction factors to load current. I am still not convinced the short-cut method is legitimately supported in the code

215.2(A)(1)(a) and (b) both use the words "the minimum feeder conductor SIZE..."

As far as doing the short cut method, I'm not sure why this is tripping you up. If you need a conductor to carry a load current of 110A, what size 90deg conductor would you need when you have 6 current carrying conductors, and what size 75 deg conductor would you need when you have 6 current carrying conductors? And how would you arrive at those conductor sizes?

(The fine print notes in 215.2 tell you to see the examples in Annex D. The example D3(a) uses the shortcut that I have described...it divides the load current by the adjustment and correction factors to find the minimum conductor size.)

 

[xptpcrewx]

I appreciate your persistence David.

I think you are mixing concepts because you said "Suppose the minimum feeder size current is 110A"...Sections (a) and (b) talk about minimum conductor size, not minimum current size.

The parent language in 2011, 2014, 2017 and 2020 all start off with the phrase, "Feeder conductors shall have an ampacity not less than....". So I am not convinced the intent is size since it is well known a particular wire size can have different ampacities depending on the conditions of use. Furthermore, parts (a) and (b) state requirements for the minimum feeder conductor size in terms of its ampacity, so once again its about the property of ampacity. Size of wire is arbitrary, ampacity is not since parts (a) and (b) provide quantitative values that must be satisfied. Again, refer to the parent language.

1) I think you can find somewhere online the proposal for the language change submitted to the NEC, with the intent given therein.

Have you actually seen this or are you speculating?

2). 215.2(A)(1) doesn't actually mention taking the larger of the conductor "ampacities."

Refer to NEC 2020 215.2(A)(1) parent language.

215.2(A)(1)(a) and (b) both use the words "the minimum feeder conductor SIZE..."

True but I maintain that it is in reference to the property of ampacity. The size is arbitrary so long as the conductor with the larger ampacity is selected.

As far as doing the short cut method, I'm not sure why this is tripping you up.

(The fine print notes in 215.2 tell you to see the examples in Annex D. The example D3(a) uses the shortcut that I have described...it divides the load current by the adjustment and correction factors to find the minimum conductor size.)

Its tripping me up because Annex D is informative only and its not enforceable. Just like some informative annexes are known to have errors. That by itself is not sufficient to suggest it is allowed, especially when Section 310.15 clearly spells out that the temperature correction/adjustment factors shall be permitted to be applied to the ampacity for the temperature rating of the conductor. Sections 310.15(B)(1) and (C)(1) also elaborate that the purpose of the correction/adjustment factors are to modify the conductor table ampacity. Nothing that I am familiar with suggests applying correction/adjustment factors to load current.

If you need a conductor to carry a load current of 110A, what size 90deg conductor would you need when you have 6 current carrying conductors, and what size 75 deg conductor would you need when you have 6 current carrying conductors? And how would you arrive at those conductor sizes?

You would select a hypothetical conductor, use its table ampacity in conjunction with the correction/adjustment factors to determine its modified ampacity, then compare the modified ampacity of the hypothetical conductor selected to the requirements of 215.2; If the modified ampacity satisfies the requirements of 215.2, you may select that conductor for use.

For 110A load consisting of 100A non-continuous and 10A continuous 6 current carrying conductors, my interpretation is as follows:

215.2(A)(1)(a) requirement - Ia = [100A + 1.25*(10A)] = 112.5A or Ia = 113A

215.2(A)(1)(b) requirement - Ib = 110A

so Ia > Ib (always the case), therefore the minimum conductor size must have an ampacity of no less than 113A

Ampacity of 1/0 AWG 75*C wire = [150A*(0.80)] = 120A
Ampacity of 1/0 AWG 90*C wire = [170A*(0.80)] = 136A (but limited to 75*C column ampacity of 120A so you cant use this value)

Ampacity of #1 AWG 75*C wire = [130A*(0.80)] = 104A (cannot use this value since it 104A<113A)
Ampacity of #1 AWG 90*C wire = [145A*(0.80)] = 116A (limited to 75*C column ampacity of 130A, but since 116A<130A you must use 116A per 310.14(A)(2) and 310.15(A))

Therefore #1 AWG 90*C wire is the minimum feeder conductor size for this particular case since 116A >= 113A.

So the complete answer is a #1 AWG 90*C conductor with an ampacity of 116A is the minimum feeder conductor size permitted to carry a load of 110A under the conditions of use. You could also use a 1/0 AWG 75*C conductor with an ampacity of 120A to carry a load of 110A, but this is not the minimum feeder conductor size you could get away with so we disregard it for purposes of this discussion.

 

[david luchini]

The parent language in 2011, 2014, 2017 and 2020 all start off with the phrase, "Feeder conductors shall have an ampacity not less than....". So I am not convinced the intent is size since it is well known a particular wire size can have different ampacities depending on the conditions of use. Furthermore, parts (a) and (b) state requirements for the minimum feeder conductor size in terms of its ampacity, so once again its about the property of ampacity. Size of wire is arbitrary, ampacity is not since parts (a) and (b) provide quantitative values that must be satisfied. Again, refer to the parent language.

The language says the "Feeder conductors shall have an ampacity not less than..." But you are ignoring that to try to find some other ampacity to apply to the feeder sizing. Just follow what it says in the parent language.

Have you actually seen this or are you speculating?

On the NFPA website, under the "Free Acess" section, there is Current & Prior Editions - Archived Revision Information.

True but I maintain that it is in reference to the property of ampacity. The size is arbitrary so long as the conductor with the larger ampacity is selected.

The size specified is a minimum size. That is not arbitrary. The conductor can be larger, but not smaller.

Its tripping me up because Annex D is informative only and its not enforceable.

The information material in Annex D is put there by the Code writers. They tell you that they put it there for Informational Purposes. They point you to that informational material in section 215.2. The reason they put that informational material there is to help people understand the Code sections when there is confusion about them, as in this case. Why would you reject the informational material that the Code writers included to help you get past any confusion is beyond me.

You would select a hypothetical conductor, use its table ampacity in conjunction with the correction/adjustment factors to determine its modified ampacity, then compare the modified ampacity of the hypothetical conductor selected to the requirements of 215.2; If the modified ampacity satisfies the requirements of 215.2, you may select that conductor for use.

For 110A load consisting of 100A non-continuous and 10A continuous 6 current carrying conductors, my interpretation is as follows:

215.2(A)(1)(a) requirement - Ia = [100A + 1.25*(10A)] = 112.5A or Ia = 113A

215.2(A)(1)(b) requirement - Ib = 110A

so Ia > Ib (always the case), therefore the minimum conductor size must have an ampacity of no less than 113A

Ampacity of 1/0 AWG 75*C wire = [150A*(0.80)] = 120A
Ampacity of 1/0 AWG 90*C wire = [170A*(0.80)] = 136A (but limited to 75*C column ampacity of 120A so you cant use this value)

Ampacity of #1 AWG 75*C wire = [130A*(0.80)] = 104A (cannot use this value since it 104A<113A)
Ampacity of #1 AWG 90*C wire = [145A*(0.80)] = 116A (limited to 75*C column ampacity of 130A, but since 116A<130A you must use 116A per 310.14(A)(2) and 310.15(A))

Therefore #1 AWG 90*C wire is the minimum feeder conductor size for this particular case since 116A >= 113A.

So the complete answer is a #1 AWG 90*C conductor with an ampacity of 116A is the minimum feeder conductor size permitted to carry a load of 110A under the conditions of use. You could also use a 1/0 AWG 75*C conductor with an ampacity of 120A to carry a load of 110A, but this is not the minimum feeder conductor size you could get away with so we disregard it for purposes of this discussion.

It took you all that effort to get to the answer of #1awg 90deg conductor, or a #1/0 75deg conductor, whereas it took me this long in post #4...

If I use the divide the required ampacity by the adjustment factor method, I get 110A/0.8=137.5A. I can then look at Table 310.15(B)(16) and see that a #1awg, 90 deg conductor or a #1/0awg, 75 deg conductor has sufficient ampacity. I don't have to apply the adjustment factor to multiple conductor sizes to find the right one. Just saves you some steps.

And you were trying to find a conductor with an ampacity of 116, when you only needed a conductor with an ampacity of 110.


As I mentioned earlier, it's pretty clear that they messed up the language as evidenced by the fact that they have changed it again in 2020 NEC (and still don't have it right, IMHO.) Looking at the revision information that I noted on the website, they have made it clear that the revision from 2011 to 2014 was only to clarify language and not to change the intent of the Code. So go back to the 2011 and work it in that Code version...

NEC 2011: 215.2(A)(1)
"Feeder conductors shall have an ampacity not less than required to supply the load..." The load is 110A, A #1awg 90deg conductor with a 80% adjustment factor has an ampacity of 116, so that meets the first requirement of this section.

"The minimum feeder-circuit conductor size, before the application of any adjustment or correction factors, shall have an allowable ampacity not less than the noncontinuous load plus 125% of the continuous load." 100+(10*1.25)=112.5A. A #2 conductor would be the minimum conductor size to meet this section, but the #2 will not have sufficient ampacity for the load, so the #1 conductor is required.

 

[wwhitney]

From the 2020 NEC per nfpa.org:

215.2(A)(1) . . .

(a) Where a feeder supplies continuous loads or any combination of continuous and noncontinuous loads, the minimum feeder conductor size shall have an ampacity not less than the noncontinuous load plus 125 percent of the continuous load.

Exceptions . . .

(b) The minimum feeder conductor size shall have an ampacity not less than the maximum load to be served after the application of any adjustment or correction factors in accordance with 310.14.

It seems to me the intent here is that by including the phrase "after the application of any adjustment or correction factors" in part (b), and not including it in part (a), the comparison in part (a) is to be done based on unadjusted and uncorrected ampacity values. The difficulty is that the definition of ampacity in Article 100 indicates that the term ampacity already includes adjustment and correction factors

So 215.2(A)(1)(a) is poorly written in 2020 and should be fixed. The current text could be explicitly followed by a phrase like "before the application of any adjustment or correction factors in accordance with 310.14." Or the word ampacity in part (a) could be modified to be, e.g., " tabular ampacity per Section 310."

Cheers, Wayne

 

[david luchini]

So 215.2(A)(1)(a) is poorly written in 2020 and should be fixed. The current text could be explicitly followed by a phrase like "before the application of any adjustment or correction factors in accordance with 310.14." Or the word ampacity in part (a) could be modified to be, e.g., " tabular ampacity per Section 310."

This 2020 change was a "fix" to the 2014/2017 language, which was a "fix" to the 2011 language.

Interestingly, the "fix" in 2014 was to remove the "before the application of any adjustment or correction factors" because that was supposedly "misleading."

A proposal to add "before the application of any adjustment or correction factors" back into the language in the 2017 Code was rejected.

 

[wwhitney]

Well, I just submitted a PI to add the phrase "from Tables 310.16 through 310.21" to part (a) after the word "ampacity". So we'll see what happens. The committees are not always good at logic.

Cheers, Wayne

 

[xptpcrewx]

From the 2020 NEC per nfpa.org:



It seems to me the intent here is that by including the phrase "after the application of any adjustment or correction factors" in part (b), and not including it in part (a), the comparison in part (a) is to be done based on unadjusted and uncorrected ampacity values. The difficulty is that the definition of ampacity in Article 100 indicates that the term ampacity already includes adjustment and correction factors

So 215.2(A)(1)(a) is poorly written in 2020 and should be fixed. The current text could be explicitly followed by a phrase like "before the application of any adjustment or correction factors in accordance with 310.14." Or the word ampacity in part (a) could be modified to be, e.g., " tabular ampacity per Section 310."

Thanks Wwhitney, I could not agree more. As it stands, the way the language is written, 215.2(A)(1)(a) will always be greater than 215.2(A)(1)(b) because as you mention, the term "ampacity" already includes adjustment and correction factors; and yes, the intent for 215.2(A)(1)(a) is probably the unadjusted and uncorrected table ampacity (even though it does not say that). Unfortunately, I would be inclined to with the exact language (even if it doesn't make sense) because this is what would hold up in a court of law.

 

[xptpcrewx]

The language says the "Feeder conductors shall have an ampacity not less than..." But you are ignoring that to try to find some other ampacity to apply to the feeder sizing. Just follow what it says in the parent language.

Sorry David, I respectfully disagree.

On the NFPA website, under the "Free Acess" section, there is Current & Prior Editions - Archived Revision Information.

I have not had any luck searching for this. If you have actually seen it, would you please provide the reference/link?

The size specified is a minimum size. That is not arbitrary. The conductor can be larger, but not smaller.

Sorry David, I respectfully disagree once more and defer to my initial comment about ampacity being the parameter of importance while size is arbitrary since that can vary under different conditions of use.

The information material in Annex D is put there by the Code writers. They tell you that they put it there for Informational Purposes. They point you to that informational material in section 215.2. The reason they put that informational material there is to help people understand the Code sections when there is confusion about them, as in this case. Why would you reject the informational material that the Code writers included to help you get past any confusion is beyond me.

I am not necessarily rejecting it, but Annex D is not a code rule, is not enforceable and has been known to contain mistakes. Simply put, I would prefer not to put all my faith into Annex D, especially when it contradicts exact verbiage about how to use correction factors. I will reference my previous point again:

Section 310.15 clearly spells out that the temperature correction/adjustment factors shall be permitted to be applied to the ampacity for the temperature rating of the conductor. Sections 310.15(B)(1) and (C)(1) also elaborate that the purpose of the correction/adjustment factors are to modify the conductor table ampacity.

It took you all that effort to get to the answer of #1awg 90deg conductor, or a #1/0 75deg conductor, whereas it took me this long in post #4...

Your method is quick, but mine provides a more complete answer.

And you were trying to find a conductor with an ampacity of 116, when you only needed a conductor with an ampacity of 110.

I disagree, if you follow my steps above, you can see that we need a conductor with an ampacity of 113A. My method tells you that you have an additional capacity of 3A (if you ever needed) by selecting a #1 AWG 90*C wire.

 

[xptpcrewx]

David,

Would you mind showing me how you would size the feeder conductors for the following situation?

1. 3 Phase, 3 Wire, 20A Continuous Load
2. Ambient Temperature = 51.6*C

Thanks in advance.

 

[david luchini]

Sorry David, I respectfully disagree once more and defer to my initial comment about ampacity being the parameter of importance while size is arbitrary since that can vary under different conditions of use.

Conductor size is not arbitrary. 215.2(A) is literally telling you what the minimum size conductor must be.

Section 310.15 clearly spells out that the temperature correction/adjustment factors shall be permitted to be applied to the ampacity for the temperature rating of the conductor. Sections 310.15(B)(1) and (C)(1) also elaborate that the purpose of the correction/adjustment factors are to modify the conductor table ampacity.

Multiplication and division are inverse operations. If multiplying x by 0.8 gives you y, then dividing y by 0.8 gives you x. It's elementary arithmetic, not voodoo.

Your method is quick, but mine provides a more complete answer.

We both came up with a #1 conductor being required...but somehow your #1 conductor is "more complete" than my #1 conductor. You would have to explain your "logic" for that one.

if you follow my steps above, you can see that we need a conductor with an ampacity of 113A. My method tells you that you have an additional capacity of 3A (if you ever needed) by selecting a #1 AWG 90*C wire.

This is where your steps are wrong. If the load is 110A, you need a conductor that has an ampacity of 110A. What possible reason would you need to have an additional capacity of 3A?

You are taking something simple and trying to make it complicated. I don't know how else to explain what should be straight forward and simple to understand. I would suggest you visit the NFPA website and go to the "Ask a Technical Question" section. Get it from the horse's mouth.

 

[david luchini]

David,

Would you mind showing me how you would size the feeder conductors for the following situation?

1. 3 Phase, 3 Wire, 20A Continuous Load
2. Ambient Temperature = 51.6*C

Thanks in advance.

1) 215.2(A)(1)(a): 20*1.25=25. The minimum conductor size is #12AWG

2) 215.2(A)(1)(b): 20/0.76=26.32. The minimum 90deg conductor size is #12AWG

Therefore the feeder conductor size for that feeder would be #10AWG per 240.4(D),

 

[xptpcrewx]

1) 215.2(A)(1)(a): 20*1.25=25. The minimum conductor size is #12AWG

2) 215.2(A)(1)(b): 20/0.76=26.32. The minimum 90deg conductor size is #12AWG

Therefore the feeder conductor size for that feeder would be #10AWG per 240.4(D),

Now what is the final ampacity of the wire?

 

[david luchini]

Now what is the final ampacity of the wire?

The final ampacity of a #10 THHN in 51.6*C ambient would be 30.4A.

You could also use a #10 THWN with a final ampacity in 51.6*C ambient of 23.5A.

Both are above the load current of 20A.