De-Rating 480V Underground Feeder in Ductbanks using Annex B

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mull982

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I am looking at an application where (6) 600MCM cables will be routed in an underground ductbank consisting of (6) conduits routed in a concrete encased ductbank. I have read some previous posts on this forumn relating to the same topic and was hoping to gain some further clarification here.

From what I have gathered with it comes to de-rating cables for temperature in the NEC you have options in both 310.15(B) or Annex B depending on the particular application. Because I am looking at an underground ductbank I don't think any of the tables in 310.15(B) apply for the use of adjustment factors although I have read from other posts that some have used the correction factors in these tables for de-rating underground ductbanks.

I understand the use of the tables in Annex B requires "Engineering Supervision" With that said I am looking to find a way to use the correction factor in the Annex B tables for the particular duct configuration that I am looking at. In looking at Figure B310.15(B)(2)(2) the duct configuration for my (6) conduit ductbank matches that shown in Detail 3. Because each of the conduits in the ductbank will have (3) single conductor 600MCM cables I believe that Table B.310.15(B)(2)(7) is the table that I would use for determining correction factor.

After identifying the Duct Detail, and Table I need to determine a RHO and LF. Because the Ductbank is concrete and the Soil surrounding the Duct is located in a costal area, I am going to use a RHO of 60 for this application.

The part that I am unclear about is determining the LF and using it for the tables. I understand that Load factor is the relationiship between maximum demand and average demand and can be determined through measurement or some other means. How is this ususally done for new installations where actual operating conditions cannont be measured? If I do not have a LF value of 50 or 100 as shown in the Table then must I use Figure B.310.15(B)(2)(1) to interpolate a correction factor based on a given LF? For instance lets say I determine that I have a LF of 80%, then can I use Figure B.310.15(B)(2)(1) with a given RHO in order to determine the correction factor to be used for the values shown in Table B.310.15(B)(2)(7)?


Can the tables in 310.15(B) be used for temperature correction of underground ductbanks if enough information (such as LF) is not avalaible for the use of tables in Annex B?

I appreciate any insight.
 

Julius Right

Senior Member
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Electrical Engineer Power Station Physical Design Retired
In NEC Annex B CH. B.310.15(B)(2) Typical Applications Covered by Tables. it is recommended also:
IEEE Standard 835-1994, Standard Power Cable Ampacity Tables.
In this standard there are tables for 6 ducts single conductor cables [type f]
ambient Earth temperature: 25 oC ;LF 100,75;insulation rated 90,75 oC;RHO 60,90,120.
There is a problem: in tables it is no 600 MCM cable [only 500 and 750 MCM].:roll:
 

Julius Right

Senior Member
Occupation
Electrical Engineer Power Station Physical Design Retired
First of all in NEC also- art.310.60 tables- it is no 600 MCM conductor cross-section. So, in my opinion, for 600 MCM, one has to use 500 MCM ampacity only.:weeping:
 

Julius Right

Senior Member
Occupation
Electrical Engineer Power Station Physical Design Retired
Maybe under engineering supervision?
You are right. This will be a better way, of course. However, IEEE 835/1994 indicates how to calculate it- according to Neher & McGrath and a lot of precisions-adding some examples too. :happyyes:
 
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