Conduit spacing in ductbank

Status
Not open for further replies.
A

anbm

Senior Member
Location
TX
Occupation
Designer
Does anyone know any guide/ code for electrical conduit spacing requirement inside underground ductbank?
In one of the buildings we worked on, in the past, the owner said the conduits stay too close together in ductbank and
because of heat generate, the owner ended up replacing the entire ductbank. Ductbank for MV circuit run.
 
310.60(C)(3) is just a requirement if you are using the ampacities in the tables. If you want to do the calculations you can use a smaller spacing at a reduced ampacity. With heavily loaded circuits often times, even the 7.5 center to center is not enough.

With a reduced spacing, I would require the engineering calculations to show me what the actual ampacity is under the conditions of the installation.

Note that if you have a cover of more than 30" from finished grade to the top of the duct bank, you will also have to do the calculations. The earth is an insulator and more cover equals lower ampacity.
 
It depends on conduit diameter also. For 4" duct [4.5" outside] 4.5+3=7.5".For 6" =6.67+3=9.67".
Minimum concrete it is 3" clearance.
 
Because the smaller the distance between the pipes the temperature of the cables is higher the NEC and also IEEE 835 considers the smallest distance. Of course if the distance is greater the ampacity would be greater--if calculated. But anyway the NEC doesn't allow that. What do you need it is a minimum 3" concrete in order to hold the ducts.
 
Last edited:
Julius Right said:
Because the smaller the distance between the pipes the temperature of the cables is higher the NEC and also IEEE 835 considers the smallest distance. Of course if the distance is greater the ampacity would be greater--if calculated. But anyway the NEC doesn't allow that. What do you need it is a minimum 3" concrete in order to hold the ducts.
Click to expand...
The NEC does allow that. 310.60(B). There is additional information in Annex B, but it appears to only address under 2000 volts.
 
All duct bank construction instruction includes this:
16.7 Minimum concrete encasement shall be 3" on top, bottom and sides of conduit
If using Reinforced concrete, or reinforced cement concrete (RCC) the clearance it is 3"
See for instance:
CONDUIT INSTALLATION GUIDE CD0001U-NVEnergy
or else.
Using PCC concrete-non reinforcing- 1 1/2" it is possible, of course. See, for instance:
CONSTRUCTION STANDARD ELECTRIC OPERATIONS ORGANIZATION NSTAR Electric
 
Status
Not open for further replies.
Top