Pls help with 4000A 480V duct bank configuration

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fandi

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Los Angeles
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Electrical Engineer
Hi,
I'm trying to figure out a 30 ft 4000A ductbank to connect a 2.5MVA transformer of utility company (I secondary = 3000A) to a new 4000A 480V switchboard. Usually the utility company do this but this time the owner is responsible because the meter is on the utility transformer primary side.

The transformer room is underground and the new 4000A switchboard will be installed on grade outdoors.
I can't use Table 310.15 (NEC 2011) because of the heat generated between the conductors and I don't have a computer based Neher Mc Grath program to calculate.

I know there are many configurations of ductbank but I just want to know a general idea of:
- Is that a concrete encased ductbank?
- typical rows of conduits.
- typical wire size to be used? Is that 500kcmil?

Thanks a lot.
 
fandi said:
Hi,
I'm trying to figure out a 30 ft 4000A ductbank to connect a 2.5MVA transformer of utility company (I secondary = 3000A) to a new 4000A 480V switchboard. Usually the utility company do this but this time the owner is responsible because the meter is on the utility transformer primary side.

The transformer room is underground and the new 4000A switchboard will be installed on grade outdoors.
I can't use Table 310.15 (NEC 2011) because of the heat generated between the conductors and I don't have a computer based Neher Mc Grath program to calculate.

I know there are many configurations of ductbank but I just want to know a general idea of:
- Is that a concrete encased ductbank?
- typical rows of conduits.
- typical wire size to be used? Is that 500kcmil?

Thanks a lot.
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It appears as though you have limited experience in this area. I would get an engineering service business whom is familiar with this type of instalation to help in the design purchase and instalation of this busway. Don't mess with it yourself.
 
I agree with templdl. It is not so simply. First of all you have to collect a lot of data - for instance: soil temperature and RHO, concrete RHO, concrete depth, available space [width and height], maximum permissible temperature of conductor, conductor material [copper or aluminum],insulation material and other.
For instance, if earth temperature is 20oC, rho=90, depth of top of concrete 2.5 ft., concrete rho=100, insulation temperature 90oC [75oC conductor temperature], single-core copper cable of 1000 MCM one cable per duct [6" pvc duct at 10" center line to center line] you will need 10 cables per phase[30 ducts] ,concrete duct bank dimensions : 8.5 ft. length 2.5 ft. height.
 
You wouldn't be able to get to 3000A with the typical configurations shown, but you can get an idea. Take a look in the Annex B, and depending on the arrangement, you might be able to start from Table B.310.7. For example with (6) raceway, a soil rho of 90 and load factor of 100, 600kcmil CU is good for 296A per set.
 
bob said:
Could you consider 4000 amp buss duct instead?
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It sounds like it is underground.

If it is overhead I would look at cable bus long before I would look at bus duct for an outside installation.

I have seen too many connection failures where wet location rated bus duct was installed in a wet location. I am working on a project right now to replace 10 bus duct runs with cable bus. The runs are 1,600 amp, 2,500 amp and 3,200 amp.
 
don_resqcapt19 said:
It sounds like it is underground.

If it is overhead I would look at cable bus long before I would look at bus duct for an outside installation.

I have seen too many connection failures where wet location rated bus duct was installed in a wet location. I am working on a project right now to replace 10 bus duct runs with cable bus. The runs are 1,600 amp, 2,500 amp and 3,200 amp.
Click to expand...

that was my first thought... cable bus. it'd be an engineered solution, just put
it in, and away you go. even lacing it up is pretty easy, if you set up the spool
of cable next to the install, and rent a crane and pull up legs into the air, and then
let them down into the blocks.
 
If I plan to run (16) sets of 4"C-3#600Kcmil (4x4 ductbank), then from the top concrete to grade level, can it be 2ft instead of 3ft? There's no heavy vehicular traffic in this 30ft ductbank run.
assume load factor = 100, earth RHO =90, dirt RHO factor = 90, each set of 600kcmil can provide 225A. 16 x 225 = 3600A. To me 3600A is enough because the Isec = 3000A max from the utility transformer.
 
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Julius Right said:
I agree with templdl. It is not so simply. First of all you have to collect a lot of data - for instance: soil temperature and RHO, concrete RHO, concrete depth, available space [width and height], maximum permissible temperature of conductor, conductor material [copper or aluminum],insulation material and other.
For instance, if earth temperature is 20oC, rho=90, depth of top of concrete 2.5 ft., concrete rho=100, insulation temperature 90oC [75oC conductor temperature], single-core copper cable of 1000 MCM one cable per duct [6" pvc duct at 10" center line to center line] you will need 10 cables per phase[30 ducts] ,concrete duct bank dimensions : 8.5 ft. length 2.5 ft. height.
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Also I need to add ground wires into each conduit, right?
 
Per NEC, temperature of each conductor needs to stay below 75 degree C or 90 degree C (given the fact that each temp rating of conductor is 105 degree C)?
 
NESC Rule 352 D requested only 2 ft. cover depth for [unprotected, direct buried] cable up to 600 V, I think.
In my opinion, 3 single-core cables 600 MCM copper/XLPE [pvc sheathed] 600 V per each 4" pvc duct [90 rho earth and concrete] 4*4 ducts, at 25oC ambient will withstand 225 A[ if the connected equipment can withstand 90oC].For 75oC conductor maximum temperature only 202 A it will be the allowable 100% continuous current.
 
i try to find terminals in nema enclosure (or panelboard insulation) that i can transform 90c underground cable (cable was calculated per 90c underground) to transform to 75c installation. this could be a good solution for underground installation. does someone know such terminals?*

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For the connection from the transformer to the new switchboard, with the Y configuration with neutral wire, should it be 4"C-4#600kcmil + EGC per phase instead of 4"C-3#600kcmil + EGC?
 
fandi said:
For the connection from the transformer to the new switchboard, with the Y configuration with neutral wire, should it be 4"C-4#600kcmil + EGC per phase instead of 4"C-3#600kcmil + EGC?
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It would be the three phase conductors, the grounded conductor and a supply side bonding jumper. You don't have an EGC until after the first OCPD.
Note, that assumes that you are not permitted to bond the grounded conductor at both the transformer and the first OCPD. There are cases where the code would permit you to bond in both locations, and in those cases, you do not need the supply side bonding jumper.
 
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