In what case would 215.2(A)(1)(b) ever be greater than 215.2(A)(1)(a)?
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Originally posted by Xptpcrewx View PostIn what case would 215.2(A)(1)(b) ever be greater than 215.2(A)(1)(a)?
Let's say you had a feeder with a 110A load (100A noncontinuous, 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).

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?
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Originally posted by Xptpcrewx View PostThanks 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?
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.
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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.
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Originally posted by Xptpcrewx View PostDavid,
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.
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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
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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
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Originally posted by Xptpcrewx View PostDavid,
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
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 noncontinuous, 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.
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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.
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So to reemphasize:
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.
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Originally posted by Xptpcrewx View PostOk. 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.
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.
Originally posted by Xptpcrewx View PostSo to reemphasize:
1. The minimum required feeder size current = 138A
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David,
I think we are getting somewhere, but 215.2(A)(1)(b) does not exactly say this:
Originally posted by david luchini View Post215.2(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"?
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Originally posted by Xptpcrewx View PostDavid,
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"?
They've probably changed the wording again because the realize they messed it up with the change they made in 2014.
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