Next Size Up Rule?

[hitehm]

Hey All - Referring to the test question below, I'm not sure their answer is correct and could use some help. They are allowing the ampacity of the conductor to be lower than the adjusted ampacity based on the OC protection following the next size up rule. But shouldn't the ampacity requirement be satisfied FIRST and THEN the next size up rule can apply? Meaning, the 80% derate for 4 or more conductors says the THWNs in their question need to have an adjusted ampacity of 125A minimum to compensate for the additional wire on the possible total LOAD of 100A from the feeder: 125 * .8 = 100A when derated. You still need 125A 75deg rated wire to carry the adjusted current. So doesn't the next size up rule only apply ONCE the ampacity for the wires is satisfied?

A simple example to my point: You have a non continuous appliance that has a FLA of 85A. You select #4 THWN wire, which has an ampacity of exactly 85A. The ampacity is now satisfied. However, 85 is NOT a standard OCPD so you can next size up to a 90A breaker. The 90A OCPD is protecting a wire with an ampacity of 85A per the next size up rule, which is OK b/c the #4 wire is capable of carrying the 85A needed current. Again, the ampacity requirement has been satisfied first. I don't see this applying to their question though.

Test Question:

18.

In the interior of a finished commercial facility, 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, which is sitting 50' away. Both the new subpanel and the existing main are surface mount units, and you can install surface mounted EMT conduit. 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? a. #4 THWN b. #2 THWN (the marked correct answer) c. #3 THWN d. #1 THWN (my answer) 310.15(B)(3)(a) requires an ampacity adjustment for more than 3 current carrying conductors, and since you do not know what the loads are, you must count the neutral. Table 310.15(B)(3)(a) shows an 80% adjustment factor needing to be applied, Table 310.15(B)(16) shows #2 THWN having an ampacity of 115A: 115A x 80% = 92A. 240.4(B) allows you to use the next size up if ampacity is not equal to a standard size, therefore this wire can be connected to a 100 breaker.

 

[infinity]

IMO the answer is C. I don't know of any section in the NEC where it is required to assume that a load is non-linear so the neutral is not counted as a CCC.

 

[Little Bill]

Hey All - Referring to the test question below, I'm not sure their answer is correct and could use some help. They are allowing the ampacity of the conductor to be lower than the adjusted ampacity based on the OC protection following the next size up rule. But shouldn't the ampacity requirement be satisfied FIRST and THEN the next size up rule can apply? Meaning, the 80% derate for 4 or more conductors says the THWNs in their question need to have an adjusted ampacity of 125A minimum to compensate for the additional wire on the possible total LOAD of 100A from the feeder: 125 * .8 = 100A when derated. You still need 125A 75deg rated wire to carry the adjusted current. So doesn't the next size up rule only apply ONCE the ampacity for the wires is satisfied?

A simple example to my point: You have a non continuous appliance that has a FLA of 85A. You select #4 THWN wire, which has an ampacity of exactly 85A. The ampacity is now satisfied. However, 85 is NOT a standard OCPD so you can next size up to a 90A breaker. The 90A OCPD is protecting a wire with an ampacity of 85A per the next size up rule, which is OK b/c the #4 wire is capable of carrying the 85A needed current. Again, the ampacity requirement has been satisfied first. I don't see this applying to their question though.

Test Question:

18.

In the interior of a finished commercial facility, 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, which is sitting 50' away. Both the new subpanel and the existing main are surface mount units, and you can install surface mounted EMT conduit. 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? a. #4 THWN b. #2 THWN (the marked correct answer) c. #3 THWN d. #1 THWN (my answer) 310.15(B)(3)(a) requires an ampacity adjustment for more than 3 current carrying conductors, and since you do not know what the loads are, you must count the neutral. Table 310.15(B)(3)(a) shows an 80% adjustment factor needing to be applied, Table 310.15(B)(16) shows #2 THWN having an ampacity of 115A: 115A x 80% = 92A. 240.4(B) allows you to use the next size up if ampacity is not equal to a standard size, therefore this wire can be connected to a 100 breaker.

You derate from the 90 deg column. You used the 75 deg column.
#2 = 130A. 130A x 0.80 = 104A
You are good with #2 with 4A to spare

 

[infinity]

You derate from the 90 deg column. You used the 75 deg column.
#2 = 130A. 130A x 0.80 = 104A
You are good with #2 with 4A to spare

THWN is a 75° C conductor not a 90°.

 

[Tulsa Electrician]

The test questions was thwn Not thwn-2
So 90 c column can not be used.

It also said "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?"

I choose (C) #3 thwn

 

[Tulsa Electrician]

Dang it beat me again. I was looking up the NEC on my phone and getting screen shots.
Happy Thanksgiving every one.

 

[augie47]

I agree #3, nothing indicates derating applies

 

[Little Bill]

THWN is a 75° C conductor not a 90°.

The test questions was thwn Not thwn-2
So 90 c column can not be used.

It also said "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?"

I choose (C) #3 thwn

So only NM can use the 90 deg for derating?

 

[winnie]

So only NM can use the 90 deg for derating?

Only 90C rated conductors can use 90C for derating.

THWN (which I don't think is even available anymore, and I don't think was ever available as straight THWN) is a 75C conductor. Kind of nit-picky if the test was basing the number on the 75C rating of THWN, since real wire would almost certainly have been THHN/THWN and have a 90C rating in dry conditions!

-Jon

 

[Tulsa Electrician]

So only NM can use the 90 deg for derating?

No, there are others. For the question posted i would say you start at 75c in lieu of 90c for any derating / termination calculations.

I would also say no derating required based on the question context.

So the correct choice would be C or is it.

If I would take a look at 110.14. 60 c termination as the question said nothing about 75c equipment. So I could also assume based on the questions context a #1 would be the correct answer.

"No exception or special conditions"

The #1 if derated at .80 would also be compliant.
B, still not the correct ansawer the way I see it.

C, based on field work.
D, for test
B, ok
A, Red tag

 

[hitehm]

The test questions was thwn Not thwn-2
So 90 c column can not be used.

It also said "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?"

I choose (C)

 

[hitehm]

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?

 

[infinity]

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 .

Assuming that the derating is correct why wouldn't the next size up rule apply?

If I would take a look at 110.14. 60 c termination as the question said nothing about 75c equipment. So I could also assume based on the questions context a #1 would be the correct answer.

Looking at this again IMO Tulsa had the correct answer {110.14(C)(1)(a)} and it has nothing to do with the neutral being a CCC. Since it's 100 amps or less you would assume a 60° C terminal which means that:

#2 AWG @ 60° C=95 amps, next size up 100 amps.

 

[winnie]

. 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?

First, I think we can all agree that the question is poorly worded because of all the unstated assumptions.

IMHO the author of the question used the common practice of saying '100A panel' to mean a panel protected by a 100A breaker. But the unstated assumption is that the load itself is 95A or less.

I believe you are correct on the issue of needingto know the actual load priorto applying the next size up rule. A 100A load cannot be supplied by 92A conductors fed by a 100A breaker. And based on the question you might have a true 100A load.

Jon

 

[infinity]

I agree with Jon a poorly worded question and the OP may be on to something because the next size up rule makes an assumption that the load is actually less than 100 amps and we don't know what the load actually is.

So could be rewrite the question based on:
1) 60° C for 100 amps or less and
2) 100 amps of calculated load
3) IMO there is no reason to assume 4 ccc's.

 

[winnie]

On the point of counting the neutral:

As the code is written, for a balanced 3 phase wye system, you do not count the neutral _unless_ there is a special condition of having lots of non-linear loads. So even though you don't know what the load is, and therefore it might be majority non-linear loads, IMHO as the question is written you shouldn't count the neutral.

-Jon

 

[infinity]

On the point of counting the neutral:

As the code is written, for a balanced 3 phase wye system, you do not count the neutral _unless_ there is a special condition of having lots of non-linear loads. So even though you don't know what the load is, and therefore it might be majority non-linear loads, IMHO as the question is written you shouldn't count the neutral.

-Jon

I agree and my response to not counting the neutral as a CCC was focused on the no special conditions aspect of the question. In my estimation 99.9% of all feeders are linear in nature.

 

[augie47]

Assuming that the derating is correct why wouldn't the next size up rule apply?


Looking at this again IMO Tulsa had the correct answer {110.14(C)(1)(a)} and it has nothing to do with the neutral being a CCC. Since it's 100 amps or less you would assume a 60° C terminal which means that:

#2 AWG @ 60° C=95 amps, next size up 100 amps.

Since we don't know the actual load would we not have to consider it at 100 amps and since there was no mention of 75° termination we would need to consider 60°. In that case would the #2 not be too small ?

 

[infinity]

Since we don't know the actual load would we not have to consider it at 100 amps and since there was no mention of 75° termination we would need to consider 60°. In that case would the #2 not be too small ?

Yes I revised my thinking in post #15 making the #2 at 95 amps indeed too small.

 

[augie47]

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......................