Power Cable Impedance 1

[backer86]

I am trying to find the impedance of power cables for fault current calculation purposes. I am trying to solve a question just like the one in chapter 4 of IEEE red book. I am using the tables provided at the end of the chapter but I don't know whether to use tables A4-3 and A4-6 or to use table A4-7 ??

My cables are:

300 feet
(1) 3-conductor
# 1/0 copper
1/2 inch conductor separation
nonshielded
nonmagnatic

60 feet
(3) 1-conductor
# 4/0 copper
thw insulation
steel conduit

Any help is appreciated. Thanks

 

[7anoter4]

For the #1/0 copper cable Table A4-3 is indicated for XA and A4-6 for XB
If the cable is THW as per NEC the maximum temperature will be 75oC.
R=0.115 ohm/1000ft XA=0.103 ohm/1000ft from Table A4-3.
But since in the A4-3 table the resistance is only for 50Oc you have to multiply the resistance by k=1+.00393*(75-50)= 1.1 approx.
XB=-0.0729 ohm/1000 ft for 2/4=1/2 inches from Table A4-6
Then for 300 ft length Z=0.115*1.1*300/1000+j*(0.103-0.0729)*300/1000 Z=0.03795+j*0.00903 ohm
For the #4/0 copper cable Table A4-7 is indicated if all 3 single core cables are in the same steel conduit.
R= 0.064 ohm/1000 ft X= 0.0497 ohm/1000 ft
And for 60 ft Z=0.064*60/1000+j*0.0497*60/1000
Z= =0.00384+j*0.002982

 

[backer86]

Thank you for your answer. but I am still confused why did you use tables A4-3 and A4-6 for the first cable and a4-7 for the second cable!!!!

Also could you help me with this cable ??

500 feet
(2) 3-conductor
# 3/0 copper
1/2 inch conductor separation
nonshielded nonmagnatic conduit

Thank you. I really appreciate it.

 

[backer86]

About previous cable, the separation is 3/4 inch not 1/2. Thanks

 

[7anoter4]

If the single core cables are laid in a horizontal line side by side and separated by a certain distance you have to use Table A4-3 and A4-6.
If the cables are laid in a steel conduit [all three] you have to use Table A4-7a.
For 3*3/0 awg cable you have to use Table A4-7b.
R=0.0766 ohm/1000 ft Table A4-7b for non magnetic ducts[if it is not a steel conduit].
X=0.0346 ohm/1000
If there are 2 cables of 3*#3/0 copper conductors supplied from the same source, parallel, in conduits you can take one cable impedance and divide by 2.
So, for 2 parallel cables 500 ft R=0.0766*500/1000/2 =0.01915 ohm X=0.0346 *500/1000/2=0.00865 ohm
Note: if the cables are laid in conduits the separation does not matter.

 

[backer86]

Thank you very much. But I still don't get it. How did you know which ones are laid side by side and which are in conduit ??

about the following cables:

300 feet
(1) 3-conductor
# 1/0 copper
1/2 inch conductor separation
nonshielded nonmagnatic conduit

500 feet
(2) 3-conductor
# 3/0 copper
1/2 inch conductor separation
nonshielded nonmagnatic conduit

You used tables A4-3 and A4-6 for the first cable and table A4-7 for the second one... but I don't see the difference between them !!!! could you explain some more ? Thanks

 

[7anoter4]

From the beginning the open posted data was not so clear. The number included in parentheses means-now I understand- the item no. I supposed it is the number of cables-e.g. 3*1*conductor the meaning usually is 3 single core cables and 1*3*conductors the meaning is 1 threephase cable.
For threecore cable the space between cables is useless in order to state the impedance as the magnetic influence between
conductors of different cables is negligible.
The three conductors in threecore cable are twisted together so each conductor change all the time the position resulting a
total 0 average outside magnetic field. Only for singlecore cable the distance between phases is important.
But you don’t need to mention distances in a conduit as the cables are close one to another.
Now let's make an order in the above data:
Pos (1) cable is it a #1/0 threecore cable? then, the distance of 1/2 inch it is the distance between this cable and another?
It is the core separation one phase with respect to another? Nevertheless as you see you don't need to mention the separation if it is threecore cable and you have to use Table A4-7b for nonmagnetic duct[it is not important if the cable is in a duct or not].
Pos (2) one threecore cable in a steel conduit, isn't it? You can use table A4-7b for magnetic conduit.
Pos (3) there are 3 singlecore #4/0 cables all together in a steel conduit.
None of these cables need to mention the separation. So, I am sorry, no one need to use A4-3 and A4-6.These tables are for 3 single core cable laid side by side in the same plane.
Now let's revise the calculation.
Item (1) one threecore cable THWN 75oC copper #1/0 conductor nonshielded nonmagnetic 300 ft length
From Table A4-7b for nonmagnetic duct and 600-5 kV nonshielded:
R=0.127 ; X=0.0360 ohm/1000 ft. and for 300 ft R=0.127*300/1000 =0.0381 ohm and X=0.0108 ohm.
Item (2) one threecore cable THWN 75oC copper #3/0 conductor nonshielded nonmagnetic 500 ft length
From Table A4-7b for nonmagnetic duct and 600-5 kV nonshielded:
R=0.0766 ; X=0.0346 ohm/1000 ft and for 500 ft:
R=0.0383 X=0.0173 ohm.
Item(3) 3 singlecore cables THWN 75oC copper #4/0 conductor nonshielded in steel conduit[magnetic] 60 ft
From Table A4-7a in magnetic duct and 600-5 kV nonshielded cable:
R=0.0640 ; X=0.0497 ohm/1000 ft and for 60 ft:
R=0.00384 X=0.002982 ohm.
I think now it is clear. Isn't it?