3ph 480V 2500a SWBD Feeder Size

[Rpyoung10]

I have a 13.2kv 3ph primary feeding a 2000kva xfmr. I need the # and size of the parallel feeders to a 2500A SWBD 600LF away.
The SWBD has a load in a 3 story office building with 2 200A 277/480v panels and a 200a 120/208 panel on each floor. (Two much info, I know)
I am estimating from the 10,000' view and just need the feeder to the SWBD. Help please??

 

[kingpb]

1 conductor per phase, #2 AWG copper, 15kV cable.

 

[charlie b]

You are talking about a load on the order of 1500 amps. Why are you using a 2500 amp switchboard? Also, why is the transformer so far away from the switchboard? The cost of secondary conductors of the required size and length is much higher than the cost of putting the transformer closer, and having a longer 15kV primary feeder. (That, I think, was kingpb's intended message.)

 

[charlie b]

If I had to run a set of secondary conductors from a 2000 KVA transformer to a 2500 amp, 480V board, I would use 7 sets of 500 KCMIL copper or 9 sets of 500 KCMIL alluminum, with 4" conduit in either case. At a distance of 600 feet, I calculate a voltage drop of 2% for the copper conductors and 3% for the aluminum conductors. Those are within acceptable limits, but I would suggest using a -2.5% primary tap in order to bring the voltage at the switchboard closer to 480 volts.

 

[Julius Right]

I should not be worried about voltage drop only, but about cable ampacity also.
Let's say the cables will run on a cable tray-no sun exposing-at 40oC ambient.
First of all, if we shall follow Anixter Technical Handbook Table 7.9?Phase-to-Phase Voltage Drop Per Amp Per 100 ft. we shall get for 500 MCM copper cable in non-magnetic conduit 0.8 power factor 0.0073 ohm/100 ft.[0.0438 for 600 ft.] and in order to achieve only 2% voltage drop we need 12 parallel cables[at 2500 A total current].
If the cables are laid in groups of 3 phases together, keeping 2.15 diameters clearance between groups then, according to NEC/2014 Art.[Cable Tray] 392.80 Ampacity of Conductors. (A) Ampacity of Cables, Rated 2000 Volts or Less, in Cable Trays/(d), we may use Table 310.15(B)(20) and for 500 MCM copper conductor the ampacity will be 496 A. Only 5 parallel cables will be enough.
However, if they are run in duct bank [20oC Earth RHO 120, 75oC insulation] ampacity will be 192 A only- for 12 parallel cables, 3 [phases]cables per duct.

 

[augie47]

From an inspectors standpoint this is the reason God invented engineers
My first question would be "where are your stamped drawings ? "

 

[JRW 70]

Using MV cable

Using MV cable

Since there are MV cables try to go as far
with it if conditions allow. Even to the transformers.
So it could provide some payback with lesser "Z" in the cable
If this MV reaches the point of use, thus lower billable
cost, due to lower amperage, smaller wire and
less influence of voltage drop because it is taken from
a stronger part of the distribution system.
( the lower billable rate may depend on which class of service
the company's policies dictate )

JR