Concrete encased ductbank requirements

[designer82]

Sec. 300.50(A)(3) says that "other nonshielded cables... shall be encased in 3in. concrete"


Most of the time, I see the utility primary (and even secondary) U/G power service cables installed in concrete encased ductbank. It doesn't seem the NEC requires this, the only section I've found is the one shown above.

Are utility primary cables unshielded most of the time? In that case it would make sense.


thank you

 

[ron]

The utility doesn't follow the NEC. They follow the NESC which doesn't have such requirements.

 

[Julius Right]

In my opinion, concrete embedded duct bank it is a convenient solution for a long run of lot of different cables:
medium voltage, low voltage, control and instrument cables. Usually it is done even before the supplied equipment is located.
A number of manholes has to be located all 100-150 ft. in order to facilitate heavy cable pulling.
If you have to traverse an area where heavy vehicle could circulate it could be a solution too.
Since the concrete may be reinforced only at duct bank outer envelope and using plastic pipes the magnetic losses on steel pipe is avoided.
You don't need an embedded duct for a few cables connecting between existing buildings and roads.