15kV/3000A Distribution cables

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EEngineer007

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TN, USA
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Engineer
Hello,

I have a 15kV incoming cable at a distribution transformer, and we are running underground concrete encased duct bank.
Do we require space below the distribution transformer in the range of 10' high?

Also, I also want to know what happens to number of cables once they transition from Underground to Open air. For e.g. if there are 6 cables in the underground duct bank, and then it comes in the building (open air) and further goes to another substation which is 1500ft away.
Do we have to carry 6 Cables to the substation which is equivalent to the ampacity of the cable when underground, or shall we reduce the number of cables based on open air ampacity? Probably to reduce the number of cables a transition box will be required.
 
Please update your profile to show a little more accurate location, at least the state you are in.
 
EEngineer007 said:
there are 6 cables in the underground duct bank, and then it comes in the building (open air) and further goes to another substation which is 1500ft away.
Click to expand...
Is this a service or a feeder?
Presuming its a feeder per 225.52 and then 225.32 you need a disconnect when you enter a building, then a chapter 3 wiring method.
Id probably look into cable tray and MV-90
You cant use open wiring on insulators for systems over 1000V, even in industrial.
 
The need for a vault under the transformer is depending on the type of primary switching (if there is any) at the transformer, as some flavors of GIS Switchgear require the vault to be able to terminate the conductors with plugs into the bottom, which would mean a person needs to fit under there to make the termination.

If the conductors just terminate to lugs in the transformer primary compartment, then as long as you can make the sweep from horizontal to vertical (based on the depth and radius of elbow), you wouldn't require a vault.

Because the 15kV cables terminate in an enclosure, it would not be free air, although you might not need to derate as much as underground for the 2nd segment (if it is aboveground), just use conductors in conduit ampacities found in 310.60
 
Let's say each cable it is single core copper conductor 1250 kcml 15 kV [XLPE insulated] one cable per duct 6*3=18 ducts [45"*22.5" duct bank cross section] ampacity per cable/phase=530 A [at 20oC Earth]. Overall diameter for unarmored cable it is 2".
Cables in cable tray. Ampacity per cable/phase[NEC Art 392] Art. 392.80 Ampacity of Conductors. (B) Ampacity of Type MV and Type MC Cables (2001 Volts or Over) in Cable Trays.
(2) Single-Conductor Cables (2001 Volts or Over).
(3) Where single conductors are installed in a triangular or
square configuration in uncovered cable trays, ampacity it is as per Table 310.60(C)(67). [for 1000 kmcl-and more-it is 850 A].
Tray width 2.15*2*6=25.8" minimum
Art.310.60 Conductors Rated 2001 to 35,000 Volts.
(A) Ampacities of Conductors Rated 2001 to 35,000 Volts.
(1) Selection of Ampacity. Where more than one calculated
or tabulated ampacity could apply for a given circuit length,
the lowest value shall be used.
Then the entire cable ampacity will be 530 A.
 
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