Cable in Duct Bank De-rating

[Adam1980]

Dear all,

I am aware that this topic has been discussed in many previous threads however none of the threads I saw has indication for de-rating of cables with SI cross-sections i.e. mm.sq.

Any reference? or is it valid to directly transform from AWG\kcmil to mm.sq.

Thank you

 

[MikiBg]

If I understood correctly you're asking about reduction factors for current carrying capacities of cables, with cross section expressed in sq.mm. If so, you can find it in IEC 60364-5-523 standard.
Regards,
Miki

 

[dpc]

Well, the physics doesn't change, but the standards and codes do. You probably need to find the applicable code and use that as a basis. In the US, the original reference is the Neher-McGrath paper on duct bank heating. The concepts and theory behind this are applicable regardless of which system of units you are using.

More info here:

http://www.calcware.com/cwnmcalc2.htm

or see Wikipedia. Note - I have no affiliation with Calcware and have never used it.

dpc

 

[7anoter4]

You can try also:
Sizing conductors and selecting protection devices - Legrand

http://www.legrand.com/files/fck/File/pdf/Power_guide/EX29008.pdf

 

[Adam1980]

Thanks for the reply.

I notice that underground cables in concrete duct-bank are only mentioned in NEC. Is there any reference that considers this method as well. IEC mainly considers ducts in ground or directly buried while from NEC it is concluded that having different configurations of duct banks such as with 6 or 9 ducts leads to different permissible loading level which is not addressed in the IEC or did I miss it somewhere.

P.S: my question is related to MV cables 13.8 kV and 34.5 kV single-core.

Thank you.

 

[dpc]

I don't believe the NEC is limited to concrete-encased duct banks. The sections shown in Annex B indicate "earth or concrete backfill".

 

[Adam1980]

Hello dpc,

I didnt mean that the NEC is limited to concrete encased ductbanks however the de-rating factors for such laying method are only shown in the NEC. Concrete encased ductbank is not shown in IEC unless i missed seeing it.

Thanks.

 

[7anoter4]

You are right, Adam. First, IEC 60364-5-52 it is only for low voltage installation [cables].
Second, Reference D it is destined for buried conduits. NEC [Neher &McGrath] consider concrete RHO=85 [dgr.C*cm/W].In this case if Earth RHO>concrete RHO then the ampacity acc.to IEC will be less. If the shield is grounded only at one end, the difference will be negligible even up to 45 kV cable.

 

[Adam1980]

Thanks all for the input.

So how to calculate the ampacity of cables if we have the input from the manufacturer for directly buried if we are going to have a concrete encased duct bank up to 3 x 3 configuration?

I presume now that NEC is the only available reference apart from using specific programs such as ETAP or CYME.

Thanks

 

[dpc]

Talk with one of the major cable manufacturers. They generally have the software to compute ampacities for duct bank configurations and sometimes will do this gratis for customers. Or you can get the Neher-McGrath paper and calculate manually.