Concrete Encased Duct Bank

Status
Not open for further replies.
T

tkman85

Member
Location
FL
Occupation
EE PE
Working on a project for an industrial client. Large facility with multiple low voltage services. All greater than 2000A at 277/480V 3PH. Multiple duct banks running around in parking lot with heavy truck traffic for deliveries and pickups. I specified concrete encased rebar reinforced duct bank. Client is demanding removal of this on the drawings. Looking for thoughts/opinions please. Local utility does not require concrete for the duct banks. I am of the opinion as EOR all underneath the paved way should be concrete encased. Thanks
 
You probably need assistance from a structural and/or geotech engineer. And the appropriate weight load (H20, HS20, etc). Reinforced duct bank may be appropriate, but you need more data.
 
By the way, in order to avoid the rebar heating due to induction from near by heavy currents the rebar has to be only in outside concrete envelope and no rebar shall be use between ducts.
 
The concrete duct-bank is required in many situations. If you don't know what cable will run underground and if you don't know if the entire routing surface will be free for inspection, for adding other cables and in order to keep distances and segregation you have to employ conduits. If the conduits are plastics according to NEC- for instance- a 2 inch concrete around the outside surface has to be done or to use steel ducts or conduits instead. If they are many different cable as medium voltage and low voltage power cables running parallel along with control instrument or communication cable you have to maintain segregation and distance so you have to hold all the conduit in a stable frame.
If you run underground cable simply buried you have to keep the depth up to 42 feet-it depends on location and voltage. The top of the concrete duct bank could be at only 5 feet depth.
 
There is no general code requirement for concrete encased for that application, but there may be specifics that you can use to convince the owner that it is recommended. Such as soils, heaving, etc
 
Need to make sure that someone runs electrical engineering software on the duct design, conductor size, and loads. It is easy to overheat conductors in heavily loaded underground duct banks. The ampacity will often be much less than what you see in Table 310.16.
 
tkman85 said:
Working on a project for an industrial client. Large facility with multiple low voltage services. All greater than 2000A at 277/480V 3PH. Multiple duct banks running around in parking lot with heavy truck traffic for deliveries and pickups. I specified concrete encased rebar reinforced duct bank. Client is demanding removal of this on the drawings. Looking for thoughts/opinions please. Local utility does not require concrete for the duct banks. I am of the opinion as EOR all underneath the paved way should be concrete encased. Thanks
Click to expand...
First off, the voltage correctly stated is 480Y/277V. Although many use it interchangeably, 277/480V is actually incorrect according to ANSI/IEEE standards. Second, sounds like you do not have a lot of experience in duct banks, however, and this is without knowing other details, you are of the correct opinion in assuming concrete encased is appropriate for this location. If client is demanding approval, simply ask them for the data they are using to make this determination.

For the design, many factors need to be included to obtain the proper cable sizing. Heating is an issue and compounded by being concrete encased. You will need to, among other things, know the concrete and earth thermal resistivity[RHO]. I will guess you don't have the earth RHO unless you had a soils report done that specifically called for it. Concrete or not, duct bank design is not a simple process.
In my experience, I have not seen one under a roadway that wasn't reinforced concrete.
 
See Julius Right, he gave you your answer. Use plastic spacers to support the ducts. Suggest duct size 4 inch schedule 40 PVC.
 
Duct banks are often essential. But as you know the ampacity derating factors for power conductors and the limitation in their numbers , including the limited number of pvc ducts for power cables in the ductbank is very restrictive. The NEC is also limited in the design support.
Can you provide more details on your physical design. I can help if i can review your design details.
 
SKM has an optional calculation module for ampacity of underground ducts but of course it's pricey.
 
tkman85 said:
Local utility does not require concrete for the duct banks.
Click to expand...
something to consider .. sometimes engineering something beyond the need is wasteful and or unnecessary.... just sayin.
I know .. your name is on is, although a copper budget many times supersedes a gold design.
 
Status
Not open for further replies.
Top