Electrical Underground Duct Bank Ampacity Derating (NEC Annex B)

[dbaltazar]

I have seen a lot of electrical designs, typically 480V-3Ph services, where duct bank ampacity derating is completely neglected. I wanted to get some opinions on whether it is necessary to derate ampacity for electrical ducts. If you do, when do you take this into consideration? After a certain number of conduits are installed underground in a duct? Above a certain amperage value? Above or below a certain wire size? Are there any other factors that influence your decision? I appreciate the insight and thank you in advance.

 

[drktmplr12]

I think you will find a reliance on the diagrams referenced by the 310.60(C) tables. Most don't realize those only apply to conductors over 2000V.

 

[ron]

We typically provide either Neher McGrath calculations for all underground feeders, or if the ductbank will fall within the criteria established by the prescriptive tables in Annex B (low voltage) or 310.60 tables (for medium voltage), they can follow the appropriate tables for derate of those feeders.

 

[David Castor]

It's not required by the NEC below 1000 V and generally not done, in my experience. The NEC attempted to mandate this derating be done at low voltage back in the 1980s, but they backed down in the face of overwhelming opposition (and whining) by inspectors and electricians.

In practice, it's almost never a problem for typical commercial and light industrial facilities. This is mainly due, I think, to the extremely conservative NEC load calculation methods and conductor ratings. I would still de-rate for situations where it is expected that the feeder will be heavily loaded for long periods of time.

It's still required by the NEC for medium-voltage.