Ductbank Derating

[lyla1711]

I know this topic has been discussed a lot here, but I have a situation with a twist that I haven't seen discussed elsewhere.

For the overwhelming majority of the run I'm concerned with, it's a textbook three-conduit system directly out of the examples in NFPA 70. For about ten feet before it terminates in the gear, it's part of a 15-conductor ductbank system (the feeder I am specifically concerned with is entering the gear, and the other conduits it is in parallel with are feeding downstream loads; i attached a crude sktech of this). My question is this - do I have to take the derated value of the conductor at that 15-conductor ductbank section?

I'm using the SKM module for the derating, and it is giving me a deration of 68% compared to the value for the feeder when it's outside of this 15-conduit ductbank system.

 

[7anoter4]

NEC Art.
310.14 Ampacities for Conductors Rated 0 Volts - 2000 Volts.
(2) Selection of Ampacity. Where more than one ampacity
applies for a given circuit length, the lowest value shall be used.
See Exception
So, if the entire circuit length of the cable is more than 100 ft and the duct bank length is not more than 10 feet then you may use the ampacity of the location outside duct bank.

 

[lyla1711]

I see that in the 0-2000V section, but I do not see a similar paragraph in the 2001-35kV section.

 

[lyla1711]

Oh man, I just read it as soon as I posed this... Thank you, 7anoter4.

310.60 Conductors Rated 2001 to 35,000 Volts.
(A) Ampacities of Conductors Rated 2001 to 35,000 Volts.
Ampacities for solid dielectric-insulated conductors shall be permitted to be determined by tables or under engineering supervision, as provided in 310.60(B) and (C).
(1) Selection of Ampacity.
Where more than one calculated or tabulated ampacity could apply for a given circuit length, the lowest value shall be used.
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 3.0M (10 ft) or 10 percent of the circuit length calculated at the higher ampacity, whichever is less.