When applying adjustment and correction factors, (b) may require a larger conductor than (a), or (a) may require a larger conductor than (b).In what case would 215.2(A)(1)(b) ever be greater than 215.2(A)(1)(a)?
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.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?
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.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 still not following you.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
OK, I see what is confusing you.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.
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.So to re-emphasize:
1. The minimum required feeder size current = 138A
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.
I'm reading the Code on the NFPA website and do not see the word "allowable" in 215.2(A)(1)(b).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 was reading the 2020 online. Now I'm curious how much the language has changed between the 2014, 2017 and 2020 Codes.I just checked the NFPA website, my handbook and a digital pdf copy... They all say "allowable ampacity". I am referencing NEC 2017.
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)?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.
The section in 2011 and earlier was simple and straight forward...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.
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.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)?
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.You're mixing concepts here. The SIZE (cross sectional area) of the conductor, and the AMPACITY of the conductor are different things.
1. How do you know the intent for certain?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.
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.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.