Wireway Conductor Derating

[Shrek]

I have a wireway with more than 41 Current carrying conductors (but less than the 20% fill by cross sectional area). One of the loads is a 40HP fan drawing 52 Amps. Assuming an ambient temperature of 30?C (86?F), I have calculated that a #1 cable would be required (#1 THHN at 90?C (194?F) is rated at 150 Amps multiplied by 35 percent = 52.5 Amps) Is this correct? It would appear that significantly larger conductors are required when using a wireway as opposed to an individual conduit. Does this not largely offset the advantages of using a wireway?

 

[Bob NH]

I have a wireway with more than 41 Current carrying conductors (but less than the 20% fill by cross sectional area). One of the loads is a 40HP fan drawing 52 Amps. Assuming an ambient temperature of 30’C (86’F), I have calculated that a #1 cable would be required (#1 THHN at 90’C (194’F) is rated at 150 Amps multiplied by 35 percent = 52.5 Amps) Is this correct? It would appear that significantly larger conductors are required when using a wireway as opposed to an individual conduit. Does this not largely offset the advantages of using a wireway?

It indicates that perhaps the person who designed the installation didn't properly consider the derating requirements.

Metallic wireways don't require derating of conductors unless the number of current carrying conductors exceeds 30.

Extensive use of multiwire branch circuits can reduce the number of current carrying conductors. Large numbers of control circuits that carry little current can be installed in a wireway without significant penalty because they often carry less than the fully derated ampacity of the minimum size wire. Power circuits where there is a derating penalty can be installed in another wireway. Effective use of subpanels can reduce the number of conductors required in a wireway.

 

[TommyC]

While we are on the subject of wireways I have a question too. I have two panels side by side one is LP the other PP. Each has a number of MC cables that enter an auxiliary gutter above each panel and are then routed down into the panels through 2" EMT. For the purpose of running the grounding conductor it would be convenient to run a 1/0 ground into one panel and then go from the LP gutter to the PP gutter and down into the panel. Supposing I did this would that in effect make the two gutters 1? or can I still bring 30 conductors into each gutter before derating?

 

[Smart $]

...Supposing I did this would that in effect make the two gutters 1? or can I still bring 30 conductors into each gutter before derating?

Sounds like you have one auxiliary gutter to begin with, but you are keeping "sides" and calling it two gutters :roll:

The 30 conductors (meaning current-carrying conductors herein) are counted by the cross section location, same as wire fill. If there are more than 30 conductors in a particular cross section location, all conductors running through that location need to be derated. In locations where there are 30 or less, the conductors which do not pass through a derated location elsewhere do not need derated.

 

[TommyC]

Actually there are 4 end caps so I think that makes it 2 gutters The question is basically this: If these two gutters were to be mechanically joined by a 1/2" conduit, or even by a 1/0 EGC choked as it enters each gutter would I then have to consider these TWO gutters (no kidding they are now 2) one?

 

[Pierre C Belarge]

Tommy
No, the conductor entered into both of the wireways does not make them one wireway.

I am not too sure if I follow your method though???

Are you trying to use the equipment ground conductor from one panel up to one wireway and then into the other wireway?

BTW: welcome to the forum.

 

[infinity]

Sounds like you have one auxiliary gutter to begin with, but you are keeping "sides" and calling it two gutters :roll:

The 30 conductors (meaning current-carrying conductors herein) are counted by the cross section location, same as wire fill. If there are more than 30 conductors in a particular cross section location, all conductors running through that location need to be derated. In locations where there are 30 or less, the conductors which do not pass through a derated location elsewhere do not need derated.

Smart where does it say that the 30 CCC are counted at a particular cross section of the wireway?

 

[Smart $]

Actually there are 4 end caps so I think that makes it 2 gutters

OK, I believe you. I meant no offense. It's just that typical instalations that I've witnessed and experienced use one wireway over all panels instead of dividing the space up with several wireways.

The question is basically this: If these two gutters were to be mechanically joined by a 1/2" conduit, or even by a 1/0 EGC choked as it enters each gutter would I then have to consider these TWO gutters (no kidding they are now 2) one?

No the two "gutters" will not be considered as one.

 

[Smart $]

Smart where does it say that the 30 CCC are counted at a particular cross section of the wireway?

Well it doesn't, specifically.

It, meaning 376.22, does say in the second sentence, "The derating factors in 310.15(B)(2)(a) shall be applied only where the number... exceeds 30."

 

[infinity]

Well it doesn't, specifically.

It, meaning 376.22, does say in the second sentence, "The derating factors in 310.15(B)(2)(a) shall be applied only where the number... exceeds 30."

I agree that logically your idea should apply, but I don't see the words of the article supporting it. IMO if there are more than 30 CCC's within any wireway then the derating applies.

 

[TommyC]

Hey Smart$ no offense taken, I was just having some fun:grin:
Pierre,to answer your question, this is not something I intend to do now it was something I was thinking about doing on a project I recently finished. In fact it was one that you inspected. I was just asking for future situations that might come up.

 

[Smart $]

I agree that logically your idea should apply, but I don't see the words of the article supporting it. IMO if there are more than 30 CCC's within any wireway then the derating applies.

The words, "...only where the number of [ccc's] exceeds 30" is good enough for me... but I'm open to debate on the matter.

As an example, in the following depiction, I do not believe the 20 ccc's going into LL1 need to be derated. Whereas all the other conductors need to be derated.

PS: The preceding image is for illustrative purposes only. Given the option, I would not run conduits where the conductors would criss-cross to this degree.

 

[Bob NH]

The words, "...only where the number of [ccc's] exceeds 30" is good enough for me... but I'm open to debate on the matter.

As an example, in the following depiction, I do not believe the 20 ccc's going into LL1 need to be derated. Whereas all the other conductors need to be derated.

PS: The preceding image is for illustrative purposes only. Given the option, I would not run conduits where the conductors would criss-cross to this degree.

Suggest you look at 310.15(A)(2) Exception: Where two different ampacities apply to adjacent portions of a circuit, the higher ampacity shall be permitted to be used beyond the point of transition, a distance equal to 3.0 m (10 ft.) or 10 percent of the circuit length figured at the higher ampacity, whichever is less.

The overlapping sections between LP1 and LP2 need not be derated if the overlapping lengths are less than 10 percent of the circuit length.

I will be interesting to see others debate what is the circuit length, but it would be at least from termination to termination, and could be construed to be from OCPD to load.

 

[Pierre C Belarge]

Bob
I am not sure if I agree with your statement. In reading 376.22, it specifically addresses 310.15(B)(2)(a) and how to apply it.

I would need more substantiation to be able to apply 310.15(A)(2) Ex.

 

[Bob NH]

Bob
I am not sure if I agree with your statement. In reading 376.22, it specifically addresses 310.15(B)(2)(a) and how to apply it.

I would need more substantiation to be able to apply 310.15(A)(2) Ex.

376.22 says that derating factors of 310.15(B)(2)(a) shall be applied (required) only when CCCs exceed 30.

310.15(A)(2) Exception says that after you determine the ampacity for two adjacent portions of a circuit, presumably using the derating factors that apply, the higher ampacity may be used for the lesser of 10 ft or 10% of that portion of the circuit figured at the higher ampacity.

For example:
I have a circuit with conductors that are 40 ft from termination to termination. Thirty-seven ft are in a raceway with only the three conductors, but 3 ft pass through a section of raceway (say a conduit off a wireway) where they are with 3 other CCCs going to a panel. Six conductors in the conduit would ordinarily require derating to 80% of the basic rating, but because the length in the conduit with a total of six conductors is only 3 ft (3/37=0.081 = 8.1%), the 310.15(A)(2) Exception permits use of the higher ampacity corresponding to 3 conductors in a raceway.