Secondary service conductors - Underground ductbank

[designer82]

So I know primary underground service conductors are always run in concrete ductbank since this is a poco requirement. However, on every project I see the secondary service conductors are always run in underground ductbank as well.

The NEC doesn't require this for the secondary service conductors, from what I understand it allows just direct buried conduit. So how come almost all projects are showing ductbank for these?

Thanks

 

[beanland]

The NEC does not "require" encasement but many utilities and owners prefer it for mechanical protection of the secondary conduits. There are also depth-of-cover issues that must be considered where it is cheaper to cover the ducts in concrete than dig deeper. Be careful though because the commonly used flowable air-entrained concrete has a poorer thermal resistance than all the cable ampacity tables assume.

 

[EmagSamurai]

..... Be careful though because the commonly used flowable air-entrained concrete has a poorer thermal resistance than all the cable ampacity tables assume.

This is a good point. We typically use Fluidized Thermal Backfill (FTB) for our ductbank as it has a known resistivity, and we can calculate the ampacity based on the specific geometry.