underground duct cable sizing

[frenchcomp]

Visiting calcware web site, I understood that NEC 2005 ampacities tables (underground duct) are only applicable for cables identical in size and all other characteristics.
Is that true?
May the consideration of B.310.11 "Adjustment factors for... with load diversity" be a solution to consider different cable size in a raceway as factors include 50% load diversity?

 

[charlie b]

Re: underground duct cable sizing

Suppose you have a 2 x 2 array of ducts, per figure Detail 2 of Figure B.310.2. You can install a set of 4/0 in one of the ducts, let it serve one circuit, and look up its ampacity in the Tables. You can install a set of 250 MCM in another duct, let it serve a second circuit, and look up its ampacity in the same Tables. You can install a set of 500 MCM in a third duct, let it serve a third circuit, and look up its ampacity in the same Tables. Is this what you mean?

I have used the Calcware software (?ampcalc?) to assist in the design of this type of underground installation. I cannot endorse that (or any other product) on this Forum, but I can report that I found it to be easy to use.

As I said in response to another of your questions, I would not recommend touching Table B.310.11. Look for the phrase ?load diversity? in other posts in this Forum, and you?ll see the difficulties inherent in trying to define that term.

 

[frenchcomp]

Re: underground duct cable sizing

It is not exactly what I mean.
I have to route many cables in a large range of size.
i.e.: in the same duct, I'd like to route 4/0 AWG + 500kcmil + a set of 8AWG.
Is that acceptable by NEC?
Is that acceptable under calcware?

 

[charlie b]

Re: underground duct cable sizing

Originally posted by frenchcomp: Is that acceptable by NEC?

Yes.

Is that acceptable under calcware?

It?s been a while since I?ve used AmpCalc to model an underground installation. My present company does not own a copy. I don?t remember if the software allows you to model different types of conductors in the same duct. You would have to ask them that question.

 

[frenchcomp]

Re: underground duct cable sizing

Charlie and all,

I received answer from calcware:
AmpCalc uses the Neher-McGrath algorithm which is based on same size phase conductors at any given cable position, whether direct buried or in ducts. The NEC does not forbid multiple circuits in same duct but it is not considered good practice and is rarely done in US practice.

I tried also CYME (CYMCAP) but it was same problem: no different cable size within same duct to run software cable sizing.

The last one is OTI (ETAP). Under demo, it's seems to be possible but I'm still waiting answer to confirm.

As all configurations are not described in NEC, as softwares are not taken multiple size into account, how does Electrical Engineering in North America perform cable sizing specially for Industrial LV cables running in underground ducts?
I believe it's not always possible to run only one cable size per duct.
IEC 60364 states: "A groupe of similar cables is taken to be a group where the current-carrying capacity of all cables is based on the same maximum permissible conductor temperature and where the range of conductor sizes in the group spans not more than three adjacent standard sizes".