You do get the advantage of being able to not have to dig real deep if you cover the UG conduit in concrete.I don't think that there is any actual definition of a duct bank. It is simply conduits run together in an underground installation. The choice of concrete encasement is most often a design issue and not a code issue.
I agree. I've always understood it to mean concrete or back-fill, it's still a duct bank if it's buried in the ground.I consider a duct bank to be any group of underground conduits more than one. The duct bank may be concrete encased or direct buried. I wouldn't call a single, concrete encased conduit a "duct bank".
There are some ad-hoc definitions out there on the inter-webs. The family of duct bank figures under B.310.(B)(2) are all drawn like concrete, but if you look carefully at the legend, it says Backfill (earth or concrete).
Thank You!The National Electric Safety Code presents different definition of embedded conduit and duct bank.
NESC 2007 Section 32. Underground conduit systemsNOTE 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.
NESC presents also requirements for duct bank construction in 322. Ducts and joints
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.
There are underground electric distribution standards for direct buried duct bank as of JEA [Florida], for instance, in order to comply with NESC.
It really doesn't matter to me, it is still a design issue, and if I have concrete encased raceways, I will still specify a depth, typically of at least 3' feet to help minimize the crushing damage to the conductor insulation from the freeze thaw cycles we have here in Illinois.You do get the advantage of being able to not have to dig real deep if you cover the UG conduit in concrete.