Derating and 310.15(A)(2)

Status
Not open for further replies.
C

Canton

Senior Member
Location
Virginia
Occupation
Electrician
Just curious on everyone's take on the installation in this picture. The guys on this job are trying to take advantage of 310.15(A)(2) for derating. They have 6-12 CCC #10 THHN in each pipe. All are going on 20 amp breakers. They would essentially have (3) different calculated ampacities for each circuit. They would have a calculated ampacity in the 1" EMT (6-12 CCC's), the Trough (well over 100 CCC's), and the 4" Nipples (20-40 CCC's) going down to the panels (Nipple lengths are over 24").

Also, the branch circuit lengths as they go out into the field range from 40'-200'.
I guess the transition is less than 10', any comments?
 

Attachments

  • 2014-11-25 14.41.54.jpg
    2014-11-25 14.41.54.jpg
    149.3 KB · Views: 9
IMO, the ampacity of the field conductors in the 1" EMT should be permitted to be used as the circuit ampacity. Unfortunately, under the current Exception, only the adjacent portion is permitted to count toward the lesser of 10' or 10%.

With respect to the conductors in the 4" EMT, the adjacent portion is the the wireway, which range in length from a few inches to not over 48" (guessing)... so at best, you could only extend the higher ampacity values of the wireway into the 4" EMT at most 4.8".

Now if we use the conductor ampacity in the 1" EMT beyond the point of transition, you say the field conductors range from 40' - 200'. What if a 40' circuit has 48" in the wireway and another 24" in a 4" EMT. That's 72" (6') beyond the point of transition. (40'- 6'):6' is less than 10:1.

FWIW, I submitted a Public Input hoping to remedy this exact type of dilemma.

BTW, derating for over 30 conductors in the wireway applies only to conductors within a cross section. There could be well over 100 conductors within a wireway and none have to be derated as long as the number in any cross section is 30 or less.
 
Smart $ said:
IMO, the ampacity of the field conductors in the 1" EMT should be permitted to be used as the circuit ampacity. Unfortunately, under the current Exception, only the adjacent portion is permitted to count toward the lesser of 10' or 10%.

With respect to the conductors in the 4" EMT, the adjacent portion is the the wireway, which range in length from a few inches to not over 48" (guessing)... so at best, you could only extend the higher ampacity values of the wireway into the 4" EMT at most 4.8".

Now if we use the conductor ampacity in the 1" EMT beyond the point of transition, you say the field conductors range from 40' - 200'. What if a 40' circuit has 48" in the wireway and another 24" in a 4" EMT. That's 72" (6') beyond the point of transition. (40'- 6'):6' is less than 10:1.

FWIW, I submitted a Public Input hoping to remedy this exact type of dilemma.

BTW, derating for over 30 conductors in the wireway applies only to conductors within a cross section. There could be well over 100 conductors within a wireway and none have to be derated as long as the number in any cross section is 30 or less.
Click to expand...

Yes, I agree with you. This section needs some clarification.

I would have to read the article again, but I thought it was 30 CCC and you had to derate for a metal wire way (trough) regardless?
 
Last edited by a moderator:
As Smart stated the wireway only requires derating when there are more than 30 CCC's at any cross section. If the larger nipples were 24" or less and no cross section had more than 30 CCC's the installation would likely be code compliant because the 6-12 CCC's in the 1" EMT would have a maximum derating of 50% (40*50%=20 amps).
 
Canton,

I always struggled with 376.22(B) and the 30 conductor rule. That it meant only for a given cross section was hard for me to see clearly. The 2014 cleared that up and states it plainly now.

Bold is new.

(B) Adjustment Factors. The adjustment factors in
310.15(B)(3)(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)(5), exceeds 30 at any cross section of the wireway.
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.
 
infinity said:
As Smart stated the wireway only requires derating when there are more than 30 CCC's at any cross section. If the larger nipples were 24" or less and no cross section had more than 30 CCC's the installation would likely be code compliant because the 6-12 CCC's in the 1" EMT would have a maximum derating of 50% (40*50%=20 amps).
Click to expand...
Judging from the picture, the wireway could have been substantially lower, so the 4" EMT nipples would have been 24" or less. The only reason I fathom for the wireway being installed at the height it is in the picture is because there will be a suspended ceiling installed later on.
 
Last edited:
Smart $ said:
Judging from the picture, the wireway could have been substantially lower, so the 4" EMT nipples would have been 24" or less. The only reason I fathom for the wireway being installed at the height it is in the picture is because there will be a suspended ceiling installed later on.
Click to expand...

Could be, had a little forethought and planning gone into the nipple length it would have eliminated the EMT derating problem.
 
Smart $ said:
Judging from the picture, the wireway could have been substantially lower, so the 4" EMT nipples would have been 24" or less. The only reason I fathom for the wireway being installed at the height it is in the picture is because there will be a suspended ceiling installed later on.
Click to expand...

The reason the trough was put up higher is they only wanted to use a 10' stick of one inch pipe. They did not want a one or two foot nipple and couplings added.

So how do you go about calculating if there are 30 CCC conductors at any "cross section"? How do you calculate a section of the trough?
 
Canton said:
So how do you go about calculating if there are 30 CCC conductors at any "cross section"? How do you calculate a section of the trough?
Click to expand...

There is no calculation, you just look for any section with more than 30 conductors.

In the case of your photo you would be looking for any area with more than 30 conductors running horizontally through the wireway.
 
iwire said:
There is no calculation, you just look for any section with more than 30 conductors.

In the case of your photo you would be looking for any area with more than 30 conductors running horizontally through the wireway.
Click to expand...

So if I had a 12" x12" x 8' trough with 300 CCC passing through top to bottom no derating would apply.

But if I had the same trough and had conductors entering top to bottom again but 30 CCC traveled to the right 3' then exited out the bottom I would have to derate?
 
Canton said:
So if I had a 12" x12" x 8' trough with 300 CCC passing through top to bottom no derating would apply.

But if I had the same trough and had conductors entering top to bottom again but 30 CCC traveled to the right 3' then exited out the bottom I would have to derate?
Click to expand...
An interesting analysis!
My first thought was to say that the limit should apply to any cross section, horizontal, vertical, or front to back.
The code does not specify the direction.
But common sense tells me that what is really important is a cross section perpendicular to the longest dimension of the trough.
So my answers to your questions are yes and yes....
 
Canton said:
So if I had a 12" x12" x 8' trough with 300 CCC passing through top to bottom no derating would apply.

But if I had the same trough and had conductors entering top to bottom again but 30 CCC traveled to the right 3' then exited out the bottom I would have to derate?
Click to expand...

Yes and yes.
 
GoldDigger said:
An interesting analysis!
My first thought was to say that the limit should apply to any cross section, horizontal, vertical, or front to back.
The code does not specify the direction.
But common sense tells me that what is really important is a cross section perpendicular to the longest dimension of the trough.
So my answers to your questions are yes and yes....
Click to expand...

I always look forward to reading the knowledge that everyone possesses and shares on this site. The Funny thing is i always look and hope to see a response from a select few. Interestingly everyone with the exception of one person in that elite group I hold so highly has responded and shared a little knowledge and guidance. GoldDigger, Infinity, Iwire, Dennis, Jumper, Smart, and the one that is missing is Augie. I always appreciate and read everyone's perspective, opinion and views on a certain subject, but i always hold your group a little higher.

Just wanted to say thank you for everything you all have shared! & Happy Thanksgiving!
 
Status
Not open for further replies.
Top