Medium Voltage concrete encasement

[Zane Bailey]

Can an unlimited amount of concrete be encased and
Placed above a duct bank without derating and engineering supervision.

The details in 310.60 indicates spacing, trench depth, but doesn't indicate how much
concrete cover is permissible. 300.50 requires 2
inch min.

 

[ron]

Below the details in fig 310.60 indciates maximum depth to top of bank is 30". Since generaslly concrete is a better conductor of heat than soil, I would guess you could add concrete above instead of soil, just keep the top of the bank within 30" of grade

 

[Julius Right]

NEC is referring to how to cover a cable trench only:
300.5 Underground Installations.
(A) Minimum Cover Requirements. Direct-buried cable or conduit or other raceways shall be installed to meet the minimum cover requirements of Table 300.5.
For instance for direct buried cable:
In trench below 50-mm (2-in.) 18"
thick concrete or equivalent
No reference to cable ampacity in trench. If the trench is sand or earth filled it may be treated as directly buried. The concrete RHO in IEEE-835 [or Neher&McGrath publication Table IV] =85 dgr.C -cm/watt [in IEC 60287-2 is 1.00[K-m/watt]=100 dgr.C -cm/watt]. For surrounding sand or earth with RHO>100 you may neglect the concrete influence on ampacity.