Duct Bank

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pierre

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I was asked by an electrical contractor today where could he find the official definition for duct bank. An engineer on his job is saying that the underground feeders for his service (4-4inch PVC runs) buried in earth are required to be derated as a duct bank. If someone could help direct me to the definition, I would appreciate it.
Thanks

Pierre
 
Re: Duct Bank

I guess it could be argued that it may not be where it belongs, but the general definition is in 310.60(A). Use 310.60(C)(3) as an auxiliary reference.

Appendix B.310.15(B)(4) for low voltage ducts refers back to 310.60

Edit add: Appendix B is not directly enforceable - but it is good practice.

[ July 20, 2004, 05:51 PM: Message edited by: rbalex ]
 
Re: Duct Bank

Even if the NEC defined ?duct bank? (which it does not), and even if the installation you describe met that definition, it does not follow that the NEC required the ampacities to be de-rated. In my opinion, the NEC is unclear on this issue.

Also in my opinion, if you size the conductors based on a service calculation that was performed in accordance with article 220, and if the ducts are kept at a constant distance from each other (using some type of guide plate), and if they are far enough apart from each other (7.5 inches center-to-center), then the ampacities given in Table 310.16 should be adequate for the installation. But that is a lot of ?ifs.? And I was not the engineer of record for this design, so I cannot pass judgment on the statements made by the engineer to whom your contractor is referring.
 
Re: Duct Bank

He is probably referring to the heating effects of one set of wires on another. Conductor ampacity is based on the installed conditions. If the condition does not match one of the tables then you need to do a calc a la Neher McGrath, which takes a software program to do it right. See 310-15 a,b and c, and appendix B.
 
Re: Duct Bank

310.60.A Definitions
Electrical Duct Bank.

Cables used in 310.16 are used for conductors
in raceway, cable or directly buried. It appears that this table is not for cables in buried electrical conduits encased in concrete.
 
Re: Duct Bank

Thanks for the responses :) .
I have never seen or heard of derating underground conductors that are in multiple raceways in 'earth'. The definition in 310.60 says "embedded in earth or concrete." ...HMMM

I am going to tell the engineer that derating for conductors in multiple raceways (the actual installation has 3 sets of 600kcmil conductors in PVC) will not be required, and I am going to tell the electrical contractor to make sure he keeps at least 7.5 inch separation until he enters the building. - Hows that sound?

Pierre
 
Re: Duct Bank

Section 310.60(A) Defines ?Electrical Ducts? only not ?Electrical Duct Banks;? however, a quick peek at 310.60(C) would reveal that ?Electrical Ducts? refers to what most folks would recognize as ?Electrical Duct Banks.? So ?If it quacks like a Duct?? :D

We don?t know the voltage of the installation in question is so we don?t know if 310.60(A) etc. applies directly or not.

I agree with Charlie that the Code is ambiguous on conductor ampacities in ducts. I believe that ampacities of conductors in ducts generally needs to be determined under engineering supervision.
 
Re: Duct Bank

There are simply too many variables and I don't plan on being the EOR. (This site doesn't pay well enough) But Section 310.60 and following doesn't apply and Appendix B isn't enforceable.

A quick look at Table B.310.7 with its reference to Figure B.310.2, Detail 2 (Second Column) would indicate a single 600kcmil conductor in a 3 conductor/phase configuration has an ampacity of 511A in the best conditions considered by the table. The ampacity would be reduced to 480A for a 4/phase configuration per B.310.(B)(5)(a).

With all other conditions held constant, simply raising the rho to 90 would make 600kcmil unacceptable compared to Table 310.16. On the other hand, Table 310.16 is based on a more conservative ambient temperature (30C) than Table B.310.7 (20C).

As I said above, too many other variables must be considered to determine the "real" ampacity of a conductor in its "condition of use." For example Table B.310.7 is based on a conductor's temperature not conductor temperature rating of 75C. The actual rho, spacing, magnetic vs. non-magnetic ducts, ambient temperature, conductor temperature rating, etc. all must be considered.

Furthermore, the ampacity of a conductor may change as it passes through different conditions of use; e.g., from duct bank to cable tray in direct sunlight to final terminations. Unless the conductor's ampacity can be determined from direct and immediate application of a table or figure,i.e,. it "fits perfectly" with the Table or Figure, I believe a conductor's maximum ampacity should be determined under engineering supervision. JMO
 
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