transformer secondary feeder

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bsh

Senior Member
I have a 1500kva 120/208 volt 3 phase 4 wire pad mount transformer. Using EDSA (a commercial software package)and IEEE std 399-1990 the derating required for the secondary feeder running in an underground duct bank will require 30 - 500kcm cables per pahse due to the insulating properties of the ground. Though the number of cables required seems excessive it is correct. The secondary current of the transformer is 4164 amps and have it terminated at a 5000 amp fusible switch in a switchboard. I am trying to get some confirmation or other ideas from others who have had a similar application.
 

bob

Senior Member
Location
Alabama
bsh said:
I have a 1500kva 120/208 volt 3 phase 4 wire pad mount transformer. Using EDSA (a commercial software package)and IEEE std 399-1990 the derating required for the secondary feeder running in an underground duct bank will require 30 - 500kcm cables per pahse due to the insulating properties of the ground. Though the number of cables required seems excessive it is correct. The secondary current of the transformer is 4164 amps and have it terminated at a 5000 amp fusible switch in a switchboard. I am trying to get some confirmation or other ideas from others who have had a similar application.
You are rating 500 kcm at 167 amps. That seem excessive to me.
 

bsh

Senior Member
The cable ampacity is based on the output of EDSA and IEEE 399. I agree the ampacity seems low and varies depending on the location of the duct in the duct bank. I'm wondering if others have had a similar application and what their experience has been.
 

kingpb

Senior Member
Location
SE USA as far as you can go
Occupation
Engineer, Registered
You should be able to make it work with 10 -750KCMIL or 100 KCMIL conductors per phase, in underground concrete encased ductbank, using 5" pvc conduit.

You will probably have to twiddle around with spacing of conduits etc,

Check soil temp, Rho, etc. Since it's concrete encased try to get it as close to the surface as possible, this will allow the use of 750KCMIL instead of 1000Kcmil.

All in all, it should work as far as cable size, but typically padmounts can only accept 8 conductors per phase, you better make sure you can get one that will accept 10 per phase.
 

iwire

Moderator
Staff member
Location
Massachusetts
kingpb said:
but typically padmounts can only accept 8 conductors per phase, you better make sure you can get one that will accept 10 per phase.

I saw you mention that before and have never run into that.

Is it a local power company spec?

Every 4000 amp service I run into seems to have 10 sets of 600 CU supplying it from a padmount.
 

W6SJK

Senior Member
What configuration of duct bank (#rows/columns)? 2 rows/ 15 columns? If not, keep the conductors on the outside edge of the duct bank, i.e. don't use a 6 x 5 duct bank arrangement.

I sure hope EDSA works better than it did at my former employer in the 90's! It would be interesting if someone here had AMPCALC so we could see a comparison.
 

bsh

Senior Member
You should be able to make it work with 10 -750KCMIL or 100 KCMIL conductors per phase, in underground concrete encased ductbank, using 5" pvc conduit.

I checked IEEE 390 and found rhat 10-750's have a capacity of 3190 amps and 10-1000's have a capacity of 3345.
I will terminate in a swbd on a 5000 amp sw, per NEC 240.21 I need feeder cable ampacity of 5000 amps.

I appreciate the input from everyone
 

bob

Senior Member
Location
Alabama
bsh said:
You should be able to make it work with 10 -750KCMIL or 100 KCMIL conductors per phase, in underground concrete encased ductbank, using 5" pvc conduit.

I checked IEEE 390 and found rhat 10-750's have a capacity of 3190 amps and 10-1000's have a capacity of 3345.
I will terminate in a swbd on a 5000 amp sw, per NEC 240.21 I need feeder cable ampacity of 5000 amps.

I appreciate the input from everyone
I assume you are concerned about the ampacity of underground cable in duct banks. The code has ampacity for different duct arrangements in the back of the book. Before you derate the cable, I would suggest that you ask someone who has the software to run the caculations using the Neher Mcgrath method. This has been discussed here many time before.
 

kingpb

Senior Member
Location
SE USA as far as you can go
Occupation
Engineer, Registered
If your parameters are causing you to derate the ampacity as you describe, then I would suggest you start looking for a location for the transformer that will allow use of above ground installation of conduit, or is close in proximity so you can use above ground non-segregated bus duct.

More then 10 cables per phase is an impractical solution for any installation.
 

W6SJK

Senior Member
bob said:
Before you derate the cable, I would suggest that you ask someone who has the software to run the caculations using the Neher Mcgrath method.

That's what he's doing with EDSA.
 
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