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#86889  12/21/03 11:46 PM
sizing cable for voltage drops


I am going to a electrical tech school. We are now working on calculating wire sizes and such. I want to make sure that I am doing this correctly so I am going to post some sample problems and my answers so you all can check my work. I apprieciate any tips.
1) 25 #12AWG copper cunductors type THHN, in conduit. The ambient temp is 95*F What is the ampacity of each conductor? A:the chart lists #12 copper THHN @30 the correction factor is 0.96 for 95* so 30(0.96)=28.8A then 50% correction for 25 in conduit so 0.96(0.50)=14.4A
2) a single phase load is 2800' from the source and draws a current of 86A, and operates at 480V. Maximum voltage drop must not exceed 3%. What size of Aluminum conductor should be used? A: 480V(3%)=14.4V
R=14.4V/86A R=0.167
CM=(K)L/R CM=(17)2800/0.167 CM=285,030 the closest size is 300Kcmil
3) A 3 phase motor operates on a 480V and is located 1800' from the source. Current is at 235A. Using copper voltage drop not to exceed 6% A: 480V(0.06)=28.8V L= 1800(1.732)=3118
CM=10.4(3118)/0.123 CM= 263,636 all 3 shall be 300Kcmil
Sorry if these formulas are unclear.
Thanks for any help MW



#86890  12/22/03 12:28 PM
Re: sizing cable for voltage drops


Double check on #2; remember that a circuit requires _two_ conductors, and you will have voltage drop in _each_. In other words, the length of wire over which you need a maximum of 3% voltage drop will be 2*2800'.
For #3 it appears that the equations compensate for the return path, by multiplying the length over which you calculate the voltage drop by 1.732 (the square root of 3).
I've never worked with k in units of ohm*foot/cmil, but when I calculated it for aluminium from k given in ohmcm, I got 16.7 at 20C, which agrees with 17. I didn't double
Jon



#86891  12/23/03 12:02 AM
Re: sizing cable for voltage drops


thanks winnie, my instructor pointed out the footage problem in number 2 to me. I understand now.




