Ampacity question

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Dan120

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Location
Florida
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Journeyman Electrician
Hello everyone, if I’m taking a test and they asked me what is the ampacity for a 12 gauge wire should I answer base on the 90° column?
 
Unless they tell you the insulation type you cannot answer the question. If you assume a modern conductor like THHN then you would use the 90 degree ampacity to answer.
 
no. It a question based on 110.14(C). Anything less than 100A you should use the 60°C column.

In the real world, most breakers and wire are 75°C rated. When the temperature ratings are unknown, you default to 110.14(C).
 
Elect117 said:
no. It a question based on 110.14(C). Anything less than 100A you should use the 60°C column.

In the real world, most breakers and wire are 75°C rated. When the temperature ratings are unknown, you default to 110.14(C).
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I don't think that you can make that assumption based on the Article 100 definition of ampacity. You would need to know the insulation type to know its temperature rating.

Ampacity.
The maximum current, in amperes, that a conductor can carry continuously under the conditions of use without exceeding its temperature rating.
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Dan120 said:
Hello everyone, if I’m taking a test and they asked me what is the ampacity for a 12 gauge wire should I answer base on the 90° column?
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A question on an exam won't be simply that, or at least it shouldn't be. Without providing more information it is an unanswerable question.
 
"The temperature rating associated with the ampacity of a conductor shall be selected and coordinated so as not to exceed the lowest temperature rating of any connected termination, conductor, or device."

It is commonly used on tests to limit the conductor ampacity based on the termination rating.

If the question was worded like. "16 current carrying conductors of #12 copper THHN", I would agree with you. Pretty easy to know which ampacity to use on derating. etc.

If the question was literally just "what is the ampacity of #12" and didn't say copper or insulation type, then my guess is going to be that it is a question based on 110.14(C) and copper.

Either way it isn't a great question but it is not wrong to default to 110.14(C) when info is missing on a test.
 
Elect117 said:
"The temperature rating associated with the ampacity of a conductor shall be selected and coordinated so as not to exceed the lowest temperature rating of any connected termination, conductor, or device."

It is commonly used on tests to limit the conductor ampacity based on the termination rating.

If the question was worded like. "16 current carrying conductors of #12 copper THHN", I would agree with you. Pretty easy to know which ampacity to use on derating. etc.

If the question was literally just "what is the ampacity of #12" and didn't say copper or insulation type, then my guess is going to be that it is a question based on 110.14(C) and copper.

Either way it isn't a great question but it is not wrong to default to 110.14(C) when info is missing on a test.
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Note that it wasn't a test question but a "what if" inquiry by the OP.
 
OK, yeah, it was more of a “what if question”. So what I’ve gather is that most likely I won’t be asked a question as simple as this, but if asked about a specific conductor type like THWN-2 and the question is a simple as that, I could answer then with the 90° column, or whatever column relate to the conductors rating.
 
Dan120 said:
OK, yeah, it was more of a “what if question”. So what I’ve gather is that most likely I won’t be asked a question as simple as this, but if asked about a specific conductor type like THWN-2 and the question is a simple as that, I could answer then with the 90° column, or whatever column relate to the conductors rating.
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and if not given the specific conductor type then the question would be answered based on 110.14(C)
 
You should get an insulation type or temperature. It would be a very poorly written question without it. They usually use insulation type because the wet rating vs dry rating might play into the question.

Like, THHN/THWN in PVC running underground to a light post would have the THWN rating.
 
Dan120 said:
and if not given the specific conductor type then the question would be answered based on 110.14(C)
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That would be an extremely poorly written question. If the question uses the word ampacity then the Article 100 definition would still need to be applied.
 
Elect117 said:
no. It a question based on 110.14(C). Anything less than 100A you should use the 60°C column.
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110.14(C) only applies once you terminate the conductor. If the question is literally "what is the ampacity of #12 Cu THHN in a dry location?" then the answer is 30A. Terminate the conductor on a 60C rated termination, and the termination has an ampacity of only 20A.

[I like to think of the wire as still having an ampacity of 30A but the circuit being limited to the minimum of the constituent ampacities, namely the 20A termination ampacity. But the wording of 110.14(C) is not fully compatible with that point of view.]

Cheers, Wayne
 
wwhitney said:
I'm not saying we should. I was just objecting to the idea that because of 110.14(C) we should assume a 60C termination.

Cheers, Wayne
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I agree with that too and would say that to keep it simple without an insulation type you cannot answer the question.
 
Dan120 said:
OK, yeah, it was more of a “what if question”. So what I’ve gather is that most likely I won’t be asked a question as simple as this, but if asked about a specific conductor type like THWN-2 and the question is a simple as that, I could answer then with the 90° column, or whatever column relate to the conductors rating.
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The test should also give the ambient temp of the place the wire will be installed... One question i have, I know guys at my shop that derate for temps and multiple current carrying conductors based off of the 90c ampacity of the wire but size the wire off the the load ampacity at 75c to match the breaker. IE 20 A load gets #12s due to the 25A rating at 75c. But for derating it to go in a pipe with 3 other circuits they use the 30A rating of the conductor at 90c. Is that legal?
 
I would provide an answer then how you came to that.
It could be CCA for all we know.

Generic question, generic answer. 20 something maybe.

Let's see what AI has to say about it.
What do you know I was right.
 

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Hermie17 said:
The test should also give the ambient temp of the place the wire will be installed... One question i have, I know guys at my shop that derate for temps and multiple current carrying conductors based off of the 90c ampacity of the wire but size the wire off the the load ampacity at 75c to match the breaker. IE 20 A load gets #12s due to the 25A rating at 75c. But for derating it to go in a pipe with 3 other circuits they use the 30A rating of the conductor at 90c. Is that legal?
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#12 is 20A since you have to worry about 240.4.

But yes. You can use the 90°C column for derating conductors.

310.15(A) says:
"The temperature correction and adjustment factors shall be permitted to be applied to the ampacity for the temperature rating of the conductor, if the corrected and adjusted ampacity does not exceed the ampacity for the temperature rating of the termination in accordance with the provisions of 110.14(C)."
 
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