Debating of ductbanks

[mshields]

Figure 310.60 and its related tables gives ampacities based on the number of circuits being run in a MV ductbank. To my knowledge there is no equivalent to that for Low voltage. Is that accurate. If not, how is it covered?

 

[rbalex]

See Annex B.4 and following.

 

[Coppersmith]

So we are debating about derating? I'm for it.

 

[Julius Right]

No low voltage cable in ductbank table it is in art.310 [only for more than 2000 V rated].
Let's take Detail 3 [6 ducts 3 CABLES/DUCT] cable 750 mcm copper 90 oC I[A]=365 A
In Annex B [for information] Table B.310.15(B)(2)(7) detail 3
[6 ducts 3 CABLES/DUCT] cable 750 mcm copper 75 oC I[A]=330 A.
The explanation is simple: conductor maximum temperature it is only 75oC while in 2-5 kV case it is 90oC:slaphead:

 

[ron]

As pointed out, the LV tables are in Annex B, but Annex B is not enforceable.

If you were just taking the prescriptive information from the code, what code requirement would make you derate a LV ductbank? The MV ductbank tables are right there in 310, so that is clear that you need to follow, but I don't know of a claer path for derating the LV ductbanks.

It is not obvious to a reader that an UG ductbank would perform any different than a trapeze mounted ductbank or one installed in a wall cavity.

 

[Julius Right]

Referring to derating of cable in duct bank ampacity :
According to NEC 2017 Art.310.60 (C) Tables.
Informational Note No. 1: For ampacities calculated in accordance
with 310.60(A), reference IEEE 835-1994, Standard Power Cable Ampacity Tables, and the references therein for availability of all factors and constants.
IEEE 835/1994
3.4 Adjustments for change in parameter
3.4.1 Adjust for changes in ambient temperature
Ampacities may be corrected for different ambient temperatures using the following equation:
I'=√[(Tc-Ta')/(Tc-Ta)]
3.4.2 Adjustment for change in maximum conductor temperature or temperature due to dielectric loss
I'=√ [(Tc'-Ta-∆T'd)/(Tc-Ta-∆Td)*(τc+Tc)/(τc+T'c)]-for copper conductor τc=234.5 for aluminum 228.
NEC 2017 Art. 310.60(B) Informational Note: The dielectric loss temperature rise (ΔTd )
is negligible for single circuit extruded dielectric cables rated below 46 kV.
No adjustment recommended for more than 3 conductors per duct.