Underground duct bank

I recalculate the ampacity for 3*4/0 copper XHHW 2*7 conduits of 2”
First, the maximum temperature in duct may be 90oC if in the air will be as per Table 310.16 for 75oC.
So, for 5.5" distance center line-to-center line, 60% load factor, the ampacity will be 200 A.
 
wwhitney said:
I don't follow how ampacity can depend on load factor, but I'm not particularly familiar with duct bank calculations, can you elaborate?

Per the NEC ampacity is a continuous current rating. So I would think the ampacity of the conductors in the duct bank would be determined by finding the continuous current that all the conductors could carry such the steady state temperature is equal to the conductor insulation rating.

Obviously for other periodic loading profiles, the peak current at which the peak periodic steady state temperature is the maximum allowable will differ from that value. For example, if only 50% of the conductors are carrying current. Or if the conductors are loaded only 50% of the time, and carry no current the other 50% of the time.

But I wouldn't think that ampacity is an appropriate term to refer to those peak allowable currents.

Thanks,
Wayne
Click to expand...
The extent of derating is related to load factor. The derated ampacity from the 90deg column would be compared to the 75deg column and the lower of the two gets applied to the design.
 
If the load factor is 100% then the ampacity is 150 A. You may set the protection according to Art.240.4 B
 
ron said:
The extent of derating is related to load factor.
Click to expand...
That's fine, I was just objecting to using the term "ampacity" for any situation other than 100% load factor. You can calculate a current rating for a lower load factor, but that situation doesn't conform with the NEC definition of "ampacity."

Cheers, Wayne
 
In NEC Art.319.14 B is recommended -instead of tables-Engineering Supervision.
In Annex B is written:
Where the load factor is less than 100% and can be verified by measurement or calculation, the ampacity of electrical duct bank installations can be modified as shown.
So, NEC considered the load factor less than 100% if the ampacity is
calculated as per Neher McGrath basic ampacity paper.
Bytheway, IEEE STD 835 recommended by NEC, also presents ampacity for different load factors.
For instance, in pg., 0.6 to 5 kV Unshielded Single Conductor Extruded Dielectric Cable in Underground Duct Bank - Triplexed - Nine Circuits for 4/0 awg, 90 Rho, load factor 100% the ampacity is 146 and for 75% it is 173A.
 
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