Secondary feeder impedance

[Enginerd]

I am trying to find the impedance for the secondary feeder going from the transformer to the customer. I am mainly looking at canter tapped transformers with triplex feeding the load. So far I have found the two modified Carsons Eqtns:

for Self Impedance:
z​

ii = ri + 0.09530 + j0.12134 ln(1/GMRi + 7.93402)


for Mutual Impedance:
z​

ij = 0.09530 + j0.12134 ln(1/Dij + 7.93402)​

I am wondering if the total impedance would just be the addition of these two numbers or if there are further steps that are needed.​

 

[Enginerd]

any help would be great, thanks.

 

[rcwilson]

After spending too much of my evening looking, I couldn't match your equations with ones in my reference books. But my books were are related to three phase overhead power transmission or distribution lines with transposed phase wires and a earth ground return. (Transposition over a long line makes Za=Zb=Zc.) They may not be applicable to two wire +neutral/ground service.

Do you know the units the equations are using? Ohms/mile at 60 Hz, Ohms per foot?

In the past, I came across a manufacturer's table of impedances for single phase 240/120V service wires. The same reference had tables of short circuit currents based on transformer size, wire size and length. I wish I could find it again.

 

[bob]

The is a table about half way down that may give you the information you seek

http://www.cable.alcan.com/NR/rdonl...9041-80CB30546829/0/600VUDALGUARDBrochure.pdf

Let me know if it is of any value.

 

[beanland]

Impedance or Fault Current

Impedance or Fault Current

Do you need to know the impedance of the line or the fault current? There are some great free spreadsheets that calculate fault current using accepted "impedance" values for cable.

http://www.mikeholt.com/freestuff.php?id=freegeneral

I recommend Southwire overhead and underground cable books. The equations are all provided.