derating necessary?

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deano11178

Member
Location
peru indiana
Interesting dilemma i havent seen before... i have 18 conduits with 600mcm in them coming out of a duct bank and into a pull box on the side of a building. They exit out the back through a wall and into the corresponding switch boards that they feed. Pull box is sized for all the pipes and there are no splices in the box but do the wires need to be derated since there are 72- 600mcm wires going through the same box. All though it shouldnt matter the service is 3phase 480v.
 

deano11178

Member
Location
peru indiana
work is just starting. asked the same question of several engineers and electricians and got very different answers from many. figured i would see what a few others thought.
 

kwired

Electron manager
Location
NE Nebraska
What is coming out the back of pull box and what is length before reaching switchboards. Is it single or multiple raceways, wireways, etc? Is conductor spacing maintained?

Though I'm sure this very well may be an enclosure you can actually walk into, I don't think ampacity adjustment factors apply to conductors in a pull box.

but then again from 310.15(B)(3):

(3) Adjustment Factors. (a) More Than Three Current-Carrying Conductors in a Raceway or Cable. Where the number of current-carrying conductors in a raceway or cable exceeds three, or where single conductors or multiconductor cables are installed without maintaining spacing for a continuous length longer than 600 mm (24 in.) and are not installed in raceways, the allowable ampacity of each conductor shall be reduced as shown in Table 310.15(B)(3)(a). Each current-carrying conductor of a paralleled set of conductors shall be counted as a current-carrying conductor.
Where conductors of different systems, as provided in 300.3, are installed in a common raceway or cable, the adjustment factors shown in Table 310.15(B)(3)(a) shall apply only to the number of power and lighting conductors (Articles 210, 215, 220, and 230).

highlighted part is something to think about, as this pull box containing conductors mentioned is very likely larger than 24 inches in any dimension.
 

Smart $

Esteemed Member
Location
Ohio
What is coming out the back of pull box and what is length before reaching switchboards. Is it single or multiple raceways, wireways, etc? Is conductor spacing maintained?

Though I'm sure this very well may be an enclosure you can actually walk into, I don't think ampacity adjustment factors apply to conductors in a pull box.

but then again from 310.15(B)(3):



highlighted part is something to think about, as this pull box containing conductors mentioned is very likely larger than 24 inches in any dimension.
Just to elaborate on that...

314.28 said:
(B) Conductors in Pull or Junction Boxes. In pull boxes
or junction boxes having any dimension over 1.8 m (6 ft),
all conductors shall be cabled or racked up in an approved
manner.
 

deano11178

Member
Location
peru indiana
It seems that there are several ways to look at this as I had gathered from the other gentleman i had posed the question to before. The decision we came up with was that as long as we rack the cables in a way to keep a separation as near as possible to the spacing we will have entering and leaving the box we should not have any need for derating of theses cables. Since it is far from a common instance i appreciate the different points of view expressed by all.
 
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