Wireway derating

[Dave58er]

I'm curious about the application of 376.22.
My first question is how often is this enforced? I don't think I've seen it on a job.
Secondly, and more importantly, how would you go about applying 310.15(B)(2)(a)? Is 376.22 saying that if I have 30 current carrying conductors no problem but if I have 31 I must use 40% ampacity?
That seems a little crazy.
Do you start from the top of the table instead? Maybe 31-33 are @ 80% 34-36 @ 70% and so on? With the number of inspectors in this forum surely someone has enforced it.

 

[infinity]

Derating applies after 30 CCC's. And yes you would start with 40% after you installed the 31 conductor. I agree that enforcement sometimes seems lax. I've seen many installations that have a wireway with 3 or 4 panel nippled to it and many conduits exiting the wireway. IMO most of the time this isn't a problem due to load diversity, however it is a code requirement to derate.

 

[Dave58er]

Thanks for the reply Trevor.
With the ramifications of derating that harsh in wireways I'm suprised it's not planned for more often. Usually if there will be a lot of wire guys will just use larger gutters. But there is no allowence for the size of the gutter that I see...
Seems too scary to leave to chance.
At least voltage drop would be covered after I put those #10's on a 15A breaker.

 

[cpal]

Thanks for the reply Trevor.
With the ramifications of derating that harsh in wireways I'm suprised it's not planned for more often. Usually if there will be a lot of wire guys will just use larger gutters. But there is no allowence for the size of the gutter that I see...
Seems too scary to leave to chance.
At least voltage drop would be covered after I put those #10's on a 15A breaker.

Congratulations
If you are asking the question you are part of the solution.

there are a number of code requirements that are over looked (I believe due to inexperince)as you grow and read, talk, and question; the more percise your performance. I am encouraged by the number of junior members on the forum and the questions that are brought to this forum.

Gutters 366 and wireways 376, 378 do have size requiremnts. The max fill (allowed) is 20% of the cross sectional area. This is without regard to the number of conductors at any cross section.

 

[Bill W]

I'm curious about the application of 376.22.
My first question is how often is this enforced? I don't think I've seen it on a job.
Secondly, and more importantly, how would you go about applying 310.15(B)(2)(a)? Is 376.22 saying that if I have 30 current carrying conductors no problem but if I have 31 I must use 40% ampacity?
That seems a little crazy.
Do you start from the top of the table instead? Maybe 31-33 are @ 80% 34-36 @ 70% and so on? With the number of inspectors in this forum surely someone has enforced it.

You don't state your application here. I was once confused regarding cross-sectional fill allowed in a wireway, and the master electrician on the job said, "what does your book say about auxillary gutters?"

 

[iwire]

I've seen many installations that have a wire way with 3 or 4 panel nippled to it and many conduits exiting the wire way.

Trevor I agree it may be a violation however if the majority of conductors are passing essentially straight through than there may not be more than 30 conductors in the cross sectional area

In other words in your example if all the conductors remain vertical in the wire way it will be OK.

If 30 or more conductors travel horizontal in any particular cross section than you would have a violation.

 

[infinity]

Bob, I thought that too but I don't see those words in 376.22. It mentions the sum of cross sectional area for fill but does not seem to include those words for the derating requirement.

376.22 Number of Conductors.
The sum of the cross-sectional areas of all contained conductors at any cross section of a wireway shall not exceed 20 percent of the interior cross-sectional area of the wireway. The derating factors in 310.15(B)(2)(a) shall be applied only where the number of current-carrying conductors, including neutral conductors classified as current-carrying under the provisions of 310.15(B)(4), exceeds 30. Conductors for signaling circuits or controller conductors between a motor and its starter and used only for starting duty shall not be considered as current-carrying conductors.

 

[iwire]

376.22 Number of Conductors.
The sum of the cross-sectional areas of all contained conductors at any cross section of a wireway shall not exceed 20 percent of the interior cross-sectional area of the wireway. The derating factors in 310.15(B)(2)(a) shall be applied only where the number of current-carrying conductors, including neutral conductors classified as current-carrying under the provisions of 310.15(B)(4), exceeds 30. Conductors for signaling circuits or controller conductors between a motor and its starter and used only for starting duty shall not be considered as current-carrying conductors.

IMO that tells us that it is not the conductor count in the entire raceway but only in certain sections of it.

The only way that can be applied is by the cross sectional area.

IMO the 2002 NEC commentary has the same view.

 

[cpal]

In other words in your example if all the conductors remain vertical in the wire way it will be OK.

If 30 or more conductors travel horizontal in any particular cross section than you would have a violation.

Bob if you mean travel vertical as in a panel nippled to the top of a wireway (which is horizontal) and exits the bottom (as in a straight pull) then the size of that section is subject to 314.28 (straight pulls), but I'm sure you know that. So what am I missing??

Charlie

 

[iwire]

Bob if you mean travel vertical as in a panel nippled to the top of a wireway (which is horizontal) and exits the bottom (as in a straight pull) then the size of that section is subject to 314.28 (straight pulls), but I'm sure you know that. So what am I missing??

Charlie

Charlie I am not talking about pulls at all.

IMO the wire count of 30 before derating starts is not all inclusive.

It only applies to the cross section of the wire way that contains 30 CC conductors.

 

[cpal]

Charlie I am not talking about pulls at all.

IMO the wire count of 30 before derating starts is not all inclusive.

It only applies to the cross section of the wire way that contains 30 CC conductors.

I agree 310.15 (B) is not applicable untill more than 30 (31 +) CCC are present at a CSA

Charlie

 

[Pierre C Belarge]

Bob
The way I read this section is the Cross sectional area portion of the requirement is only for the 20% rule. The 30 conductor/ampacity adjustment is to be applied as a separate requirement from the 20% cross sectional requirement, whether or not there is 20% cross sectional area compliance or not.

 

[iwire]

Bob
The way I read this section is the Cross sectional area portion of the requirement is only for the 20% rule. The 30 conductor/ampacity adjustment is to be applied as a separate requirement from the 20% cross sectional requirement, whether or not there is 20% cross sectional area compliance or not.

Two problems I have with that.

1)It is not logical to derate condutors that are not grouped as in a typcal raceway. Consider a large JB, we do not have to derate at all as the the condutors are not run like they are in a cable or a typical raceway.

2)The wording of the requirement, they specificlly say "only where the number of current-carrying conductors, including neutral conductors classified as current-carrying under the provisions of 310.15(B)(4), exceeds 30"

What is the explaination for that if the requirment applies to every current carrying condutor in the raceway.

 

[infinity]

I still say that the reference to cross section does not apply to the requirement for derating. The use of the word where can mean many things thing. My dictionary list 16 meanings for the word where. In Bob's opinion the word where is the physical part of the wireway that contains the conductors. IMO the use of the word where in this article is when the word is used to mean in a particular circumstance (one of the definitions of the word). That circumstance being when where the wireway contains more than 30 conductors.

 

[iwire]

Yes it could mean that, I do not believe it does.

 

[Dave58er]

You don't state your application here. I was once confused regarding cross-sectional fill allowed in a wireway, and the master electrician on the job said, "what does your book say about auxillary gutters?"

The application that started me thinking about this was a job a few months ago that had 8 panels and 1 lighting contactor mounted under a large gutter.

The gutter wrapped around a corner with 2 panels and the contactor on the south wall and the rest on the east wall.
Most of the home run piping came in on the south wall so you ended up with the wires for the circuits from 6 the panels on the east wall going around the corner to the south wall.
Most of these were 42 circuit panels that were nearly full.

 

[Pierre C Belarge]

Two problems I have with that.

1)It is not logical to derate condutors that are not grouped as in a typcal raceway. Consider a large JB, we do not have to derate at all as the the condutors are not run like they are in a cable or a typical raceway.

2)The wording of the requirement, they specificlly say "only where the number of current-carrying conductors, including neutral conductors classified as current-carrying under the provisions of 310.15(B)(4), exceeds 30"

What is the explaination for that if the requirment applies to every current carrying condutor in the raceway.

Bob
I do not think that logic always plays a role in the NEC.

The gramatical makeup of this paragraph is such that there is more than one requirement in the paragraph, and the period (.) after the first sentence deliniates the two requirements.
When reading this paragraph, one needs to stop at the period and then read the second sentence.


I understand your logic of having a number of conductors traveling vertically through a horizontally installed wireway, but I believe that the wording does not specifically address that concern.

 

[Dave58er]

If they meant that as soon as you have 31 ccc's total in the wireway it should say "when" instead of "only where" IMHO.

After reading the debate in this thread and re-reading 376.22 I'm starting to believe that not only is 310.15(B)(2)(a) applied when the CSA exeeds 30, but also that it is only applied at that point.

For example if you've got an 8' gutter and only 2' has 30+ CCC's in a CSA then the other 6' wouldn't be derated at all.

Please correct me if my thinking is wrong here.