Medium Voltage Underground Duct Bank

[zemingduan]

We have a job that has 13.2KV campus distribution system. Between buildings' pad mount transformers and the site MV switch gear, we need to run (4) 5"C conduit with concrete encased duct bank for the MV feeder.

1) What is the spacing requirements between the 5" MV conduits in the duct bank? Does it fall under NEC and should follow figure 310.60 (C) (3)? Or does it fall under NESC with other requirements?

2) What are the MV duct bank burial depth requirements? The NEC figure 310.60(C) (3) has the note in below screen shot. It says minimum burial depth per 300.50 and maximum 30" burial depth to the top of the duct banks. But I see many engineer electrical drawings specify Min. 36" burial depth to top of the MV electrical duct bank. Is it allowed and Why? If you MV duct bank burial Min. 36" , should NEC 310.60(B)(2) (b) derating factor apply?



Thanks!

 

[Julius Right]

• What is the spacing requirements between the 5" MV conduits in the duct bank?
• What are the MV duct bank burial depth requirements?
• Does it fall under NEC and should follow figure 310.60 (C) (3)?

NEC does not state the duct diameter, only the center-line-to-center line distance. The duct diameter may be 4 to 6".
The duct bank depth is state 30", the RHO 90 and the Earth temperature 20oC in order to define the ampacity. If you intend to calculate the ampacity
as per NEC Art. 315.60 B Engineering Supervision, you may use other dimensions.
However, there are some limitations. For instance, the minimum distance [clearance] between duct is 3" -mechanical resistance,
and the minimum depth as per NEC Art.300.5 [I think 18 to 24" usually].

 

[zemingduan]

However, there are some limitations. For instance, the minimum distance [clearance] between duct is 3" -mechanical resistance,

I also see some electrical engineer drawings specify minimum 3" between ducts. Could you educate me where this requirement comes from? I only see Min. 7.5" center to center in NE.

Thanks,

 

[ron]

Many Engineers do application specific Neher McGrath calculations, which result in unique depths and spacing. If you are following the prescriptive methods shown in the NEC, then you follow those spacing / depths.
If these ductbanks are on the owners side of the meter, then the NEC is what to follow. The NESC is for the utility side of the meter.