Ampacity question
- Thread starter Dan120
- Start date
- Location
- New Jersey
- Occupation
- Journeyman Electrician
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.
Elect117
Senior Member
- Location
- California
- Occupation
- Engineer E.E. P.E.
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).
In the real world, most breakers and wire are 75°C rated. When the temperature ratings are unknown, you default to 110.14(C).
- Location
- New Jersey
- Occupation
- Journeyman Electrician
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.
ggunn
PE (Electrical), NABCEP certified
- Location
- Austin, TX, USA
- Occupation
- Consulting Electrical Engineer - Photovoltaic Systems
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.
Elect117
Senior Member
- Location
- California
- Occupation
- Engineer E.E. P.E.
"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.
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.
ggunn
PE (Electrical), NABCEP certified
- Location
- Austin, TX, USA
- Occupation
- Consulting Electrical Engineer - Photovoltaic Systems
Note that it wasn't a test question but a "what if" inquiry by the OP.
Elect117
Senior Member
- Location
- California
- Occupation
- Engineer E.E. P.E.
Ya and I took that more literally than needed lol.
I answered it as if that was the question.
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.
and if not given the specific conductor type then the question would be answered based on 110.14(C)
Elect117
Senior Member
- Location
- California
- Occupation
- Engineer E.E. P.E.
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.
Like, THHN/THWN in PVC running underground to a light post would have the THWN rating.
- Location
- New Jersey
- Occupation
- Journeyman Electrician
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.
wwhitney
Senior Member
- Location
- Berkeley, CA
- Occupation
- Retired
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
- Location
- New Jersey
- Occupation
- Journeyman Electrician
I agree but the OP never mentions an insulation type so should we assume that it's a 90° C conductor?
wwhitney
Senior Member
- Location
- Berkeley, CA
- Occupation
- Retired
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
- Location
- New Jersey
- Occupation
- Journeyman Electrician
I agree with that too and would say that to keep it simple without an insulation type you cannot answer the question.