215.2(A) understanding.

[xptpcrewx]

In what case would 215.2(A)(1)(b) ever be greater than 215.2(A)(1)(a)?

 

[david luchini]

In what case would 215.2(A)(1)(b) ever be greater than 215.2(A)(1)(a)?

When applying adjustment and correction factors, (b) may require a larger conductor than (a), or (a) may require a larger conductor than (b).

Let's say you had a feeder with a 110A load (100A non-continuous, 10A continuous) that is in a conduit with another feeder so that there are 6 current carrying conductors.

215.2(A)(1)(a) says the minimum feeder ampacity shall be 100A + (10A*1.25) = 112.5A

215.2(A)(1)(b) says the minimum feeder ampacity shall be 110A/0.8 = 137.5A.

In this case, (b) is larger than (a).


For another example, let's say you had a feeder with a 100A continuous load in an ambient temperature of 100degF.

215.2(A)(1)(a) says the minimum feeder ampacity shall be 100A*1.25=125A

215.2(A)(1)(b) says the minimum feeder ampacity shall be 100A/0.88=113.6A for a 75deg conductor, or 100A/0.91=109.9A for a 90deg conductor.

In this case, (a) is larger than (b).

 

[xptpcrewx]

Thanks David. I would like to get some clarification on a few things:

1. Where does it say, or is it implied, that you may divide the maximum load current by the correction and/or adjustment factors to obtain minimum allowable ampacity? My interpretation is that the adjustment and correction factors are intended to be applied to the respective table ampacities in Article 310.

2. Couldn't "maximum load to be served" also include overload conditions?

 

[david luchini]

Thanks David. I would like to get some clarification on a few things:

1. Where does it say, or is it implied, that you may divide the maximum load current by the correction and/or adjustment factors to obtain minimum allowable ampacity? My interpretation is that the adjustment and correction factors are intended to be applied to the respective table ampacities in Article 310.

2. Couldn't "maximum load to be served" also include overload conditions?

1. Multiplying the ampacities of conductors in Art 310 by the and dividing the required minimum ampacity required for a conductor are providing the same result in different fashions. The dividing the required ampacity by the correction/adjustment factors just saves steps.

For instance, in my first example, I need a conductor to carry 110A, and I know it must be #2 Awg from 215.2(A)(1)(a). I am going to use a 90deg conductor, so I look at the ampacity of #2 in the 90 deg column, see that it is 130A, and multiply that by the 80% adjustment factor: 130A*0.8=104A. I can see that the #2 conductor is too small for the 110A load. So I then look at the ampacity of #1 Awg from the 90 deg column, see that it is 145A, and multiply that by the 80% adjustment factor: 145A*0.8=116A. I can see that the #1 conductor is adequate for the 110A load.

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.

2. Maximum load to be served would be the calculated load on the feeder from Art. 220 Section III.

 

[xptpcrewx]

David,

I understand your logic however there is something bothering me about the exact code language that doesn't seem to agree.

For example, there are two separate concepts at play:

1. "Minimum Feeder Size" (in terms of ampacity) which is the larger of 215.3(A)(1) or (b); and

2. "Conductor Ampacity " (for a given wire size) which is the smaller of the limiting temperature ampacity or the adjusted/corrected table ampacity.


For the feeder to be sized correctly, you would need to compare two ampacities; ampacity 2 would have to be greater than or equal to ampacity 1.
What are your thoughts. Thanks in advance.

 

[david luchini]

David,

I understand your logic however there is something bothering me about the exact code language that doesn't seem to agree.

For example, there are two separate concepts at play:

1. "Minimum Feeder Size" (in terms of ampacity) which is the larger of 215.3(A)(1) or (b); and

2. "Conductor Ampacity " (for a given wire size) which is the smaller of the limiting temperature ampacity or the adjusted/corrected table ampacity.


For the feeder to be sized correctly, you would need to compare two ampacities; ampacity 2 would have to be greater than or equal to ampacity 1.
What are your thoughts. Thanks in advance.

I'm not sure I follow you. The feeder conductor size is based on ampacity. You have to find the larger required ampacity between (a) and (b), and your minimum feeder conductor size will be based on that.

 

[xptpcrewx]

David,

The point I am trying to make is that there are two different concepts here. It may be easier demonstrated if you answer the following questions with reference to your example:

1. What is the minimum required feeder size current?

2. For the wire size determined in 1., what is the ampacity?

These are not always the same value. Lets start there. Thanks

 

[wwhitney]

In case it helps, I think the overloading of the word "ampacity" may be contributing to some confusion. Per the article 100 definition, the conductor "ampacity" includes any correction or adjustment factors. So I would call the table values the "base ampacity" or "starting ampacity" or "tabular ampacity"

The upshot is that if you know the desired (final) ampacity, and you know the adjustment and correction factors, you can determine the required tabular (starting) ampacity by dividing the desired (final) ampacity by the factors.

Cheers, Wayne

 

[david luchini]

David,

The point I am trying to make is that there are two different concepts here. It may be easier demonstrated if you answer the following questions with reference to your example:

1. What is the minimum required feeder size current?

2. For the wire size determined in 1., what is the ampacity?

These are not always the same value. Lets start there. Thanks

I'm still not following you.

1. The minimum required feeder size current is determined by the load calculation in Art 220. In my first example that is 110A.

2. You haven't determined a wire size in 1, you've determined the current that the feeder is required to carry. Do you want to know the wire size required from 215.2(A)(1) or (b)?


Let me try it this way: In my first example, I have calculated my load in Art 220 as 110A (100A will be non-continuous, 10A will be continuous)

215.2(A)(1) says the feeder conductors shall have an ampacity not less than the larger of (a) or (b).

215.2(A)(1)(a) says the feeder conductors shall have an ampacity of not less than 112.5A

215.2(A)(1)(b) says the feeder conductors shall have an ampacity of not less than 137.5A.

137.5A is larger than 112.5A, therefore my minimum feeder conductor size would have to have an ampacity of not less than 137.A, or #1Awg 90deg conductor, or #1/0Awg 75 deg conductor.


The example D3(a) in Annex D takes you through the entire process, its worth a read.

 

[david luchini]

They changed the wording of this section in the 2014 (I think) Code. Not sure why...it made more sense the way it was previously written.

 

[xptpcrewx]

Ok. According to your example, the minimum feeder conductor size would have to have an ampacity of not less than 138A.

Now if you apply the rules of Article 310, then you will see #1 AWG (90*C) can only carry 116A under the conditions of use. Clearly you cannot use #1 AWG because it does not have the capacity.

 

[xptpcrewx]

So to re-emphasize:

1. The minimum required feeder size current = 138A

2. Ampacity of conductor under consideration (1 AWG, 90*C) = 116A

Two different values where the wire ampacity does not support the minimum feeder size current.

 

[david luchini]

Ok. According to your example, the minimum feeder conductor size would have to have an ampacity of not less than 138A.

Now if you apply the rules of Article 310, then you will see #1 AWG (90*C) can only carry 116A under the conditions of use. Clearly you cannot use #1 AWG because it does not have the capacity.

OK, I see what is confusing you.

215.2(A)(1)(b) says the conductor shall have an ampacity of not less than the load to be served after the application of any adjustment or correction factor.

In my example, that is 110A after the application of any adjustment or correction factor

That means the conductor has to an ampacity of not less than 137.5A BEFORE the application of the 80% adjustment factor. (137.5A*0.8=110A)

#1Awg, 90 deg has an ampacity greater than 137.5A, before the application of the 80% adjustment factor.
#1/0Awg, 75 deg has an ampacity greater than 137.5A, before the application of the 80% adjustment factor.

So to re-emphasize:

1. The minimum required feeder size current = 138A

The minimum required feeder size current is 110A, not 138A. The load is 110A, the conductor has to be able to carry 110A. Ampacity of conductor under consideration (1 AWG, 90*C) = 116A, which is greater than 110A.

 

[xptpcrewx]

David,

I think we are getting somewhere, but 215.2(A)(1)(b) does not exactly say this:

215.2(A)(1)(b) says the conductor shall have an ampacity of not less than the load to be served after the application of any adjustment or correction factor.

215.2(A)(1)(b) - The minimum feeder conductor size shall have an allowable ampacity not less than the maximum load to be served after the application of any adjustment or correction factors.

So does "allowable ampacity" mean "table ampacity"?

 

[david luchini]

David,

I think we are getting somewhere, but 215.2(A)(1)(b) does not exactly say this:




215.2(A)(1)(b) - The minimum feeder conductor size shall have an allowable ampacity not less than the maximum load to be served after the application of any adjustment or correction factors.

So does "allowable ampacity" mean "table ampacity"?

I'm reading the Code on the NFPA website and do not see the word "allowable" in 215.2(A)(1)(b).

They've probably changed the wording again because the realize they messed it up with the change they made in 2014.

 

[xptpcrewx]

I just checked the NFPA website, my handbook and a digital pdf copy... They all say "allowable ampacity". I am referencing NEC 2017.

 

[david luchini]

I just checked the NFPA website, my handbook and a digital pdf copy... They all say "allowable ampacity". I am referencing NEC 2017.

I was reading the 2020 online. Now I'm curious how much the language has changed between the 2014, 2017 and 2020 Codes.

If you can find it, or if you can access it online, read the 2011 version. It was much simpler and straightforward.

They took a section that wasn't broken, and broke it.

 

[xptpcrewx]

I am familiar with 2011, and in my opinion that is very confusing as well. It would be much easier if they just eliminate the language and use equations instead.

Now that we have confirmed that 2017 language is broken, lets discuss intent.

What is the point of 215.2(A)(1)(b)? This seems redundant because of the rules in Article 310 already require that conductors not operate beyond their modified ampacity.

Furthermore, regarding your comment below....

The minimum required feeder size current is 110A, not 138A. The load is 110A, the conductor has to be able to carry 110A. Ampacity of conductor under consideration (1 AWG, 90*C) = 116A, which is greater than 110A.

Suppose the minimum required feeder size current is 110A as you mentioned; again, in what case would this value ever be larger than 215.2(A)(1)(a)?

 

[david luchini]

I am familiar with 2011, and in my opinion that is very confusing as well. It would be much easier if they just eliminate the language and use equations instead.

Now that we have confirmed that 2017 language is broken, lets discuss intent.

What is the point of 215.2(A)(1)(b)? This seems redundant because of the rules in Article 310 already require that conductors not operate beyond their modified ampacity.

The section in 2011 and earlier was simple and straight forward...
1) The conductor has to have an ampacity not less than the load to be served, and
2) The minimum conductor size shall have an allowable ampacity of not less than the non-continuous load plus 125% of the continuous load.

In other words, has to have an ampacity to carry the load, but can't be smaller than this minimum size. Simple.

Suppose the minimum required feeder size current is 110A as you mentioned; again, in what case would this value ever be larger than 215.2(A)(1)(a)?

You're mixing concepts here. The SIZE (cross sectional area) of the conductor, and the AMPACITY of the conductor are different things. This is why I said I believe that had taken a section that wasn't broken, and broke it. As demonstrated by the fact that they are still changing the wording of the section, as I noted earlier.

A conductor needing to carry a current can of 110A can be #2 Awg in the conditions noted in T310.15(B)(16). Or it can be a 1/0Awg, 75 deg conductor when in a raceway with 6 current carrying conductors. Or it can be a 1/0Awg, 90 deg conductor when in a raceway with 6 ccc's in an ambient of 100degF.

The intent of the two sections (a) and (b) in 2014 and later Codes are still the same intent that I listed as (1) and (2) above from the 2011 Code.

Both 215.2(A)(1)(a) and (b) talk about "minimum feeder conductor size." So whichever section requires the larger "minimum size" prevails.

I suppose what is most broken about the current wording is the part of 215.2(A)(1) which says "Conductors shall be sized to carry not less than the larger of 215.2(A)(1)(a) or (b)." 215.2(A)(1)(a) and (b) already specify a conductor size, so 'Conductors shall be sized to carry not less than the larger size' doesn't really make sense.

 

[xptpcrewx]

Thanks for bearing with me David...

You're mixing concepts here. The SIZE (cross sectional area) of the conductor, and the AMPACITY of the conductor are different things.

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.

The intent of the two sections (a) and (b) in 2014 and later Codes are still the same intent that I listed as (1) and (2) above from the 2011 Code.

Both 215.2(A)(1)(a) and (b) talk about "minimum feeder conductor size." So whichever section requires the larger "minimum size" prevails.

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".

I suppose what is most broken about the current wording is the part of 215.2(A)(1) which says "Conductors shall be sized to carry not less than the larger of 215.2(A)(1)(a) or (b)." 215.2(A)(1)(a) and (b) already specify a conductor size, so 'Conductors shall be sized to carry not less than the larger size' doesn't really make sense.

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.

Lastly, (b) is redundant any way you look at it because Article 310 does not permit loading a conductor beyond its ampacity (factoring in conditions of use).

Thanks again. Can we try to be more rigorous here?