Duct Bank Derating - 9 Conduits

[bcorbin]

I want to install 9 sets of a 3-wire+neutral feed in a 9-conduit ductbank (3x3). It's a 3,000 amp, 480Y/277V feed.

Figure B.310.2 shows this configuration, but Table B.310.5, the only table to refer to Figure B.310.2, calls out one conductor per conduit.

I will be using a 0.8 factor for more than 3 CCC's, and a soil impedance rho of 0.9, but I'm at a loss on calculating the derating factor for ambient temperature, since it doesn't seem to fit the table.

Any help is greatly appreciated.

 

[ron]

In the 2011 NEC, Table B.310.15(B)(2)(7) gets you to (6) ducts. To do anything more, you need to do the Neher McGrath seperately for a "unique" situation.

 

[kingpb]

There are programs that can do it quite simply and save you a lot of headache.

 

[Julius Right]

First of all, I agree with ron: you better calculate the conductor temperature as per: The Calculation of the Temperature Rise and Load Capability of Cable Systems, by J.H. Neher and M. H. McGrath.
Nevertheless, NEC Informative Annex B Application Information for Ampacity Calculation recommends:
?For additional information concerning the application of these ampacities, see
IEEE/ICEA Standard S-135/P-46-426, Power Cable Ampacities, and IEEE Standard 835-1994, Standard Power Cable Ampacity Tables.?
According to IEEE-835/1994 page 7 for 9 circuits of 3*1/c copper conductors, ins.90oC, RHO=90 25dgr.C earth, 100 LF: 1000 MCM 340 A allowed.
The standard indicates also, how to calculate the temperature according to J.H.Neher and M.H.McGrath.

 

[bcorbin]

There are programs that can do it quite simply and save you a lot of headache.

The pricetag on that software would cause me a lot of heartache too. :happysad:

Can I use the RHO "60" column if I encase the duct bank in concrete?

 

[ron]

Can I use the RHO "60" column if I encase the duct bank in concrete?

Yes, if the concrete has a rho of 60 and you fill the ductbank and space above the bank with that concrete.