Next Size Up Rule?

[Tulsa Electrician]

Guys - Please assume it DOES need a derate for having 4 CCCs so we can address the core issue as to why I asked for help.

. (1)Please read the original post for what I'm asking but in summary, shouldn't the ampacity required be satisfied FIRST,
(2) and then if needed, use a next higher up OCPD to protect the wire?

Ok I will look at that.

Here is what I see:
"you are asked to add a new 100 ampere subpanel, fed with a 100A 3 phase 4 wires plus ground feeder from the main service equipment"

What I take from that is 100 amp wire. (1)

So I will based it on that.
95 amps do not meet that requirement. So you move up in size.

Now how did I base my decision that? Next part of question.
" what is the minimum THWN copper wire size"

So I will see what thwn copper has a value of 100 amps. I will choose based on no other consideration.

#3 then cu is at 100 amps.

Based on that (C) is the answer per your request.

This question reminds me of an old Arkansas J test. They wanted to be sure you could find the wire size for a 100 amp panel with no other consideration.

Now for (2).
You can not consider that based on the 100 amp wire minimum. Again 95 is to low. I would say round up rule can not be considered as we are not considering any other rules as well. If we did than as mentioned more info required.

Now if I was an inspector and a what if. If I noticed an undersized conductor for a fixed breaker size termination I would be asking some questions.

What does this feed
Where does this feeder go.
What is the load
What is the termination temp of the equipment.

Then I would decide what is compliant based on the rules in the NEC.

This is one down side to a Basic J test. A poorly written question and the knowledge of people here that guides one thru the process. Sometimes it's a simple answer to a simple question. Most J test I have took were simple with out the bla bla bla. On a Master exam I would expect more.
An example; What is the minimum size thwn cu for 100 amps.
100 amp thwn cu wire= #3

For what it worth.
Yes I agree, the wire must carry the load. Now being the first step? In this case, yes. A 100 amp load.

" You are given no details of what the load will be. Without applying any exceptions or special conditions requirements, what is the minimum THWN copper wire size that you can use for the current carrying conductors?"

Can not use the round up rule as part of the question says you can not.

 

[Tulsa Electrician]

If you choose to use derate than use you load of 100 amps and a wire size based on qty of 3 thwn cu for derate. Now no derate required.

I base this on on the requirements in the table. Under the table it says, you shall use 5 and 6. This is not an exception rather a requirement.

Still #3 thwn cu. (c)

 

[jaggedben]

Guys - Please assume it DOES need a derate for having 4 CCCs so we can address the core issue as to why I asked for help. Whether or not the question clearly indicates 3 or 4 CCCs can be for another discussion. My problem is that they allowed a lower wire ampacity than what's needed due to the derate b/c of how they applied the Next Size Up rule and I don't think it applies it in the manner they used it . Please read the original post for what I'm asking but in summary, shouldn't the ampacity required be satisfied FIRST, and then if needed, use a next higher up OCPD to protect the wire?

On this aspect you are correct. If you're told to install a 100A rated feeder you shouldn't install a 92A feeder just because the breaker can be 100A.

(I mean, in the real world I do that sort of thing all the time tbh. But for a test question it's not a good answer.)

 

[jaggedben]

It is a perfect example as to why test questions are often so difficult for folks dealing with "real life" installations.
We are so accustomed to 90° conductors and 75° terminations that anything else seems unreasonable......................

Actually it's a good example in favor for banning test questions that refer to 75C single conductors nowadays.