Duct Bank Definition

Merry Christmas

PWigman

New User
Location
Michigan
Occupation
Electrical Project Manager
Can anyone identify a resource that can identify the difference between an underground conduit duct and an underground conduit duct bank? The issue comes up on a project that doesn't identify a group of conduits running underground within the same trench as duct bank. There is a separate plan detailing another area with duct banks that includes typical details for spacing and concrete. The group of conduits within the same trench are identified as underground conduits but the GC is trying to hold them to the standards of conduit duct bank. Any help you could provide would be much appreciated.
 

Hv&Lv

Senior Member
Location
-
Occupation
Engineer/Technician
This should be good…
Several locations and groups say a duct bank is a protected (usually by concrete) group(more than 2) of ducts (conduit being used interchangeably)

2017 NEC 310.60 has a picture that shows a duck bank with one conduit and earth or concrete backfilled.
There used to be a definition in 310.60, I guess it was removed.
 

Julius Right

Senior Member
Occupation
Electrical Engineer Power Station Physical Design Retired
NEC 310.2 Definitions.
Electrical Ducts. Electrical conduits, or other raceways round in cross section, that are suitable for use underground or embedded in concrete.
A definition in National Electric Safety Code:
NESC Section 32. Underground conduit systems
NOTE 1: While it is often the practice to use duct and conduit interchangeably, duct, as used herein, is a single enclosed raceway for conductors or cable; conduit is a structure containing one or more ducts; and conduit system is the combination of conduit, conduits, manholes, handholes, and/or vaults joined to form an integrated whole.
In my opinion a duct is a bigger conduit . A duct bank it is something else.
Nor NEC neither IEEE 835 presents any ampacity for buried conduits-only for direct buried cables and in duct bank.
However, maintaining a controlled distance between ducts and use a reduced thermal resistance concrete it is possible to increase the cables ampacity [4-6%]. For instance:
Cable 3*500 MCM 75oC copper conductor 3 ducts in a bank NEC Table B.310.15(B)(2)(6) [RHO 90] 320 A ampacity.
Calculated according to Neher and McGrath in duct bank 319A
Calculated according to Neher and McGrath in buried conduits at same distances 317 A
Calculated according to Neher and McGrath in buried conduits with no clearance 307 A


 

GoldDigger

Moderator
Staff member
Location
Placerville, CA, USA
Occupation
Retired PV System Designer
To me a duct bank will have a constant geometric relationship among the individual ducts. Calculations about heating and mutual inductance can be done as needed.
A single conduit and a duct bank with only one element are essentially the same thing.
Individually laid conduits in same trench with no effort to maintain a constant geometry will cause problems if heating is an issue with wire ampacity specification.
 
Top