PE Sample Question - Sizing a Branch Conductor

[Carultch]

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

That is very problematic, when the NEC clearly states that the burden of proof is on the person who desires to use the 75C rating for 100A and less. I would think that the lesson in classrooms and exams is to teach you to go through the process of fulfilling that burden of proof.

 

[chris kennedy]

One could use 90°C connectors (such as Polaris) at a J-box on both ends of the raceway in question to #8.

The question is what it is, nothing more.

 

[Carultch]

One could use 90°C connectors (such as Polaris) at a J-box on both ends of the raceway in question to #8.

The question is what it is, nothing more.

http://iaeimagazine.org/images/2000_03/00bpauley_fig9_811445039.jpg

 

[iwire]

One could use 90°C connectors (such as Polaris) at a J-box on both ends of the raceway in question to #8.

The question is what it is, nothing more.

:thumbsup:

 

[Dennis Alwon]

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

Yes and I don't see how C answers the question. The question wants to know what size wire is required for this install. A #10 is not sufficient unless there are 90C termination. I am confused by what you 2 are thinking. The answer states #10 at 38 amps--This answer is worthless after all the calcs. The wire is noncompliant at 38 amps

 

[jim dungar]

The question is what it is, nothing more.

Taking tests can be skill in itself.
The question asked for the minimum (possible) size for the conductor.
It provided a single temperature reference.

This is a multiple choice test. On the last one I took like this, we answered 200 questions in 4 hours. With only about a minute per question, you do not explore every nuance.
During the essay portion, we spent 1 hour per question and would have been expected to evaluate, and maybe even follow, an optional solution.

 

[iwire]

Yes and I don't see how C answers the question. The question wants to know what size wire is required for this install. A #10 is not sufficient unless there are 90C termination. I am confused by what you 2 are thinking. The answer states #10 at 38 amps--This answer is worthless after all the calcs. The wire is noncompliant at 38 amps

What they are saying, and I clearly missed it with my first response to this thread, :ashamed1: is the question tells us 90C and nothing more.

There are ways to use the 90 C rating even if unconventional.

 

[Dennis Alwon]

What they are saying, and I clearly missed it with my first response to this thread, :ashamed1: is the question tells us 90C and nothing more.

There are ways to use the 90 C rating even if unconventional.

Yes but no matter what the rating #10 could never be used. I get what your saying (TY) and this is why I suck at tests. I did somehow manage an 89 on my NC unlimited exam 35 years ago but I bet there were not questions as archaic as this answer tends to be

 

[Carultch]

Yes but no matter what the rating #10 could never be used. I get what your saying (TY) and this is why I suck at tests. I did somehow manage an 89 on my NC unlimited exam 35 years ago but I bet there were not questions as archaic as this answer tends to be

Right. This is one of those questions where you'd need to be trained how to take the test to answer the question correctly. Just being trained on the NEC concepts in general, and thinking about it as a practical problem with any and all that may apply, can get you the "wrong" answer the way that the author intended.

Now it could be that the question author does intend for you to think about other factors that would ordinarily come in to play, even if unmentioned in the problem statement. As discussed in this example.
https://www.youtube.com/watch?v=k1wPtdpeAE8

Mike Holt's admin says in the comments that what you are supposed to do is unclear:
"I can't speak on what the writer is looking for, so I would do both calculations and hope only one was there. If not, then it's a 50/50 on guessing what the person is looking for. I guess I would then guess the follow the larger of the two calculations. Just a guess."

 

[iwire]

Yes but no matter what the rating #10 could never be used.

Not really true, 10 AWG can be used if you jump through some hoops and it will be NEC compliant.

Not practical but compliant.

 

[JFletcher]

Why is the 37.08A rounded to 38? because the calculated amps are >37, you go to 38? Where is that in the code? Why am I having a flashback to math class and the importance of significant digits?

 

[Carultch]

Why is the 37.08A rounded to 38? because the calculated amps are >37, you go to 38? Where is that in the code? Why am I having a flashback to math class and the importance of significant digits?

That's a good question. The NEC doesn't prescribe any significant digit rounding conventions, so I do all calculations assuming that you never round an answer at intermediate steps.

If you are doing a calculation where the NEC indicates "not exceeding" or "not greater than", or other language that indicates an inequality sign (< or >), then a calculation that is fractions of an ampere would have to round consistently with the inequality sign. For instance, if a breaker sizing calculation gives 50.01A, then a 60A breaker would be required.

In this particular instance, it makes no difference whether it rounds to 37A or 38A. With standard rounding conventions, it would round to 37A.

 

[kwired]

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

It still leaves you guessing what the author of the question intended to ask.

Hopefully when taking an exam you get enough questions answered correctly that you can afford to have one wrong because of misunderstanding of the question, if there are enough poorly written questions it just isn't fair to the person taking the exam.

 

[JFletcher]

That's a good question. The NEC doesn't prescribe any significant digit rounding conventions, so I do all calculations assuming that you never round an answer at intermediate steps.

If you are doing a calculation where the NEC indicates "not exceeding" or "not greater than", or other language that indicates an inequality sign (< or >), then a calculation that is fractions of an ampere would have to round consistently with the inequality sign. For instance, if a breaker sizing calculation gives 50.01A, then a 60A breaker would be required.

In this particular instance, it makes no difference whether it rounds to 37A or 38A. With standard rounding conventions, it would round to 37A.

I do all calculations the same way, only rounding the final answer. Problem is, that final answer has four significant digits, and iirc (I may not) an answer cannot be any more exact than any of the factors to determine it. The 28A part is two significant digits; thus the final answer cant be any more precise than two digits, which it isnt (38). I'm wondering where in the code it requires rounding up in this case when by all usual standards one would round down. Doesnt matter here but it would if the rounding put you in #8 conductor territory.

Can you put a 120V 1440W heated bath fan on #14? NM What about a 1450W unit? What happens to either if the supply voltage is 115V, or 125V? mfg directions do not specify anything other than device must grounded and ground fault protected.

(personally I'd run #12 in all cases an be done with it, but I would have run #8 in the test question too)

 

[kwired]

I do all calculations the same way, only rounding the final answer. Problem is, that final answer has four significant digits, and iirc (I may not) an answer cannot be any more exact than any of the factors to determine it. The 28A part is two significant digits; thus the final answer cant be any more precise than two digits, which it isnt (38). I'm wondering where in the code it requires rounding up in this case when by all usual standards one would round down. Doesnt matter here but it would if the rounding put you in #8 conductor territory.

Can you put a 120V 1440W heated bath fan on #14? NM What about a 1450W unit? What happens to either if the supply voltage is 115V, or 125V? mfg directions do not specify anything other than device must grounded and ground fault protected.

(personally I'd run #12 in all cases an be done with it, but I would have run #8 in the test question too)

With the 1440 vs 1450 watt heater, I am pretty certain most NEC calculations need to be done according to a nominal voltage of 120.

Then whether or not you can put the 1450 watt load on a 15 amp circuit/15 amp conductor comes down to whether it is a continuous load or not.
If not a continuous load you can put 1800 watts on a 15 amp circuit @120 volts as well as on a 15 amp conductor.

100% rated device can handle 1800 watts as well, but you likely don't ever run into that with a typical bath heater/fan application.

 

[JFletcher]

With the 1440 vs 1450 watt heater, I am pretty certain most NEC calculations need to be done according to a nominal voltage of 120.

Then whether or not you can put the 1450 watt load on a 15 amp circuit/15 amp conductor comes down to whether it is a continuous load or not.
If not a continuous load you can put 1800 watts on a 15 amp circuit @120 volts as well as on a 15 amp conductor.

100% rated device can handle 1800 watts as well, but you likely don't ever run into that with a typical bath heater/fan application.

Heated bath fans are fixed heating and require continuous load calcs, yes? If not, run the same numbers with baseboard heaters. My only point really was that with some rounding variations, you can arrive at different practical answers (#14 vs #12). Since I'm probably not going to check the incoming voltage on an install of this nature, or wire a timer into it so it cant run overnight if someone forgets to turn it off, I'd pull #12 for both even tho technically I can pull #14 for the 1440W unit.

 

[jumper]

The NEC states that calcs that result in a number with .05 or less of an amp may be rounded down. 37.8 would round up to 38.

 

[jumper]

The NEC states that calcs that result in a number with .05 or less of an amp may be rounded down. 37.8 would round up to 38.

Oops. It is decimal fractions less than .5.

Not paying attention.:slaphead:

220.5(B)

 

[GoldDigger]

Oops. It is decimal fractions less than .5.

Not paying attention.:slaphead:

220.5(B)

So, instead of a consistent treatment of significant digits, it allows you to round 8.1 down to 8, but not 81 down to 80.
Not uncommon outside scientific and engineering contexts.

 

[Dennis Alwon]

The NEC states that calcs that result in a number with .05 or less of an amp may be rounded down. 37.8 would round up to 38.

You don't have to round up in the rare case where staying put help you.