PE Sample Question - Sizing a Branch Conductor

[gray.one]

My question is the last step of solution. The answer they give is #10 AWG, but I think that I need to use the 60 degree C column because. the circuit is rated less than 100A. Table 310.15 shows #10AWG as 30 A. The answer should be #8AWG.


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[iwire]

Another test question with insufficient information. They really should have included the termination ratings.

But they are asking for the minimum and if the terminals and equipment are marked 75C we can size the circuit for that even when under 100 amps and remain in NEC compliance.

 

[iwire]

I believe 110.14(C)(1)(a)(3) is the relevant code section






2011 NEC

110.14 Electrical Connections.

(C) Temperature Limitations.

(1) Equipment Provisions.

(a) Termination provisions of equipment for circuits
rated 100 amperes or less, or marked for 14 AWG through
1 AWG conductors, shall be used only for one of the following:

(3) Conductors with higher temperature ratings if the equipment
is listed and identified for use with such conductors.

 

[chris kennedy]

I get C. Granted crappy question.

Didn't read past trident. They gave you the answer.

 

[infinity]

Even at 75° C #10 THHN is only rated for 35 amps.

 

[Carultch]

Another test question with insufficient information. They really should have included the termination ratings.

But they are asking for the minimum and if the terminals and equipment are marked 75C we can size the circuit for that even when under 100 amps and remain in NEC compliance.

I thought if it isn't given, then for 100A and less, you cannot assume that terminations are listed and marked otherwise for 75C.

As I understand it, for 100A and less, the default is that it is rated for 60C, albeit rare to find unmarked terminals in practice.

 

[Dennis Alwon]

I agree the answer should be #8 not only because they didn't give the temperature ratings but you cannot use overcurrent protective device at the 90C rating. The 90C is used for de-rating. Thus you would need to be at #8

 

[Dennis Alwon]

I might add that most questions today assume 90C wire and 75C ratings

 

[chris kennedy]

Even at 75° C

and 75C ratings

Question states "what size 90°C THHN conductors".

We all preach not to read too much into a question or assume.

 

[Carultch]

I agree the answer should be #8 not only because they didn't give the temperature ratings but you cannot use overcurrent protective device at the 90C rating. The 90C is used for de-rating. Thus you would need to be at #8

So in an exam, when you have a question with given information that is focused on only one part of the sizing algorithm, but you know that other unmentioned parts of the sizing algorithm must also apply, does this mean that the correct answer should also have those other unmentioned parts applying as well?

Another example would be a question like "given a continuous 120V single phase load of 40 amps that is a 35 ft conduit run from the source, what size copper conductor is needed to limit voltage drop to not more than 3%?". If you calculate this out, you'll find the answer is #10. However, we also know that #10 wire is insufficient for this load in the first place, because #8 is what is required prior to considering voltage drop. This question leads you to focus on voltage drop, even though it is a red herring in this particular example.

 

[Carultch]

I might add that most questions today assume 90C wire and 75C ratings

Unless there is a statement in the question, or master statement for the whole exam indicating this assumption, requiring you to assume 75C terminations is incorrect per 110.14(C).

 

[gray.one]

Another test question with insufficient information. They really should have included the termination ratings.

But they are asking for the minimum and if the terminals and equipment are marked 75C we can size the circuit for that even when under 100 amps and remain in NEC compliance.

So, I'm not crazy. To get C, one must assume that the equipment is rated for 75 C terminations. However, if there is no rating we generally go to 60 C based on 110.14C

 

[Dennis Alwon]

Question states "what size 90°C THHN conductors".

We all preach not to read too much into a question or assume.

yes but you still have to use the 75C for the OCPD

 

[gray.one]

So in an exam, when you have a question with given information that is focused on only one part of the sizing algorithm, but you know that other unmentioned parts of the sizing algorithm must also apply, does this mean that the correct answer should also have those other unmentioned parts applying as well?

Another example would be a question like "given a continuous 120V single phase load of 40 amps that is a 35 ft conduit run from the source, what size copper conductor is needed to limit voltage drop to not more than 3%?". If you calculate this out, you'll find the answer is #10. However, we also know that #10 wire is insufficient for this load in the first place, because #8 is what is required prior to considering voltage drop. This question leads you to focus on voltage drop, even though it is a red herring in this particular example.

I feel like a lot of these sample questions are either laughably easy or worded to trick to reader. Neither seem an appropriate way to determine the capability or understand to be a licensed engineer.

 

[jumper]

Question states "what size 90°C THHN conductors".

We all preach not to read too much into a question or assume.

Yep.:thumbsup:

 

[jumper]

yes but you still have to use the 75C for the OCPD

Nothing in the question about OCPD.

 

[Carultch]

Yep.:thumbsup:

So should you be thinking about 110.14(C), or should you be ignoring it, when given a question like this on an exam?

Because the 90C answer can be correct, although it is a very rare scenario when it is correct in practice. You'd probably have to do a work-around, as shown in this graphic, in most cases.
http://iaeimagazine.org/images/2000_03/00bpauley_fig9_811445039.jpg

 

[Dennis Alwon]

Yes the question states 90C wire but the temperature rating of the terminations have to come into play-- NO.....Otherwise what is the point of the question to answer it half way. I agree if we ignore termination then a 90C rated #10 wire is good for 40 amps but you cannot use it to supply power to these units

 

[Dennis Alwon]

Unless there is a statement in the question, or master statement for the whole exam indicating this assumption, requiring you to assume 75C terminations is incorrect per 110.14(C).

Yes but that is in the field where the terminals are not rated but not necessarily on exams or in classrooms. On many mike holt videos he states we assume 75C rated terminal unless it's given at 60C

 

[jumper]

Yes the question states 90C wire but the temperature rating of the terminations have to come into play-- NO.....Otherwise what is the point of the question to answer it half way. I agree if we ignore termination then a 90C rated #10 wire is good for 40 amps but you cannot use it to supply power to these units

Dennis, Chris and I are saying that one should answer the question asked, and nothing more.