Derating factors

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naji

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hi
i have 6 MV cables ( 240 sq mm) in one circuit (2 per phase), it was installed in duct bank (PVC pipe in the ground) but the contractor install each phase in one duct (conduit) for example phase A two cables in one conduit also B and C in flat form
therefore, i need to know how can i calculat the derating factor in this case

thanks
 
naji said:
hi
i have 6 MV cables ( 240 sq mm) in one circuit (2 per phase), it was installed in duct bank (PVC pipe in the ground) but the contractor install each phase in one duct (conduit) for example phase A two cables in one conduit also B and C in flat form
therefore, i need to know how can i calculat the derating factor in this case

thanks
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It will have no effect on the calculated derating factor for the duct bank (as in Annex B, for example). And as long as the phase currents are reasonably balanced, it will have no effect on the actual temperatures in the conduits either.
Underground duct meeting certain conditions is an exception to the rule that all phase conductors in each parallel set have to travel in the same raceway.
 
GoldDigger said:
No derate for two conductors in one duct, but possibly for the duct bank?
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The derate for the duct blank is often overlooked. Most of the time, as long as the conductors have been sized per the load calculations in Article 220, it is not a problem. When more real engineering load calculations are made and used for the conductor sizing the derating for multiple raceways in a duct bank becomes a real issue.
 
Derating conductors in duct banks

Derating conductors in duct banks

The main concern in duct banks is the heating of heavily loaded conduits/ducts. The ducts nearest the center will run hotter. Therefore all the conductors in the subject feeder must be derated so that the center duct stays at 90 deg C or lower (if the cable is 90 deg stuff).
Etap has a program that will do this simulation.
 
I don't know from which standard ampacity you intend to derate.:?
If one follows NEC Annex B recommendations ?Table B310.15(2)(7) for 3 single-core cable the ampacity for RHO 90 [oC.cm/w], LF 100[%] Det. 2[three ducts per duct bank] I=341A for 500 mcm[253 sqr.mm] 75 oC insulation.
According to Table B310.15(2)(5) for only 1 cable per duct will be 511 A.
No indication for 2 cables/duct. Also this is stated for low-voltage cable-never-the-less, for underground cable it is not big difference.
Let's say for 2 cables/duct it will be the average (341+511)/2=426 A:roll:
IEC 60364-5-52-also for low-voltage cable-for 2*3 cables and 0.250 m distance between ducts the derating factor is 0.8.[for RHO=250 oC.cm/w and only 0.7 m duct-bank depth].But for RHO=100 a factor of 1.18 has to be used. For XLPE insulated cable [90oC maximum temperature] 2 loaded cable/duct 419 A-no indication for single cable per duct.
Finally, I=1.18*419*.8=395.5 A. However, each single-core cable runs in separate pvc duct and all 3 ducts are in trefoil formation.:happysad:
Calculated [according to IEC60287] for 2*1*240/25 copper , 20/12 kV XLPE cable in 3 PVC ducts 250mm apart[center-to-center], the ampacity will be 437 A.
 
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