Andrew S. answered • 11/04/20
B.S. in Electrical Engineering
Hi Amarya,
for this problem the voltage 120V is fixed, the current 15A is fixed so the maximum resistance in will be fixed by the equation V=IR, or R = V/I. so R = 8 ohms. otherwise if the voltage is fixed the current would not be fixed. This resistance is not the Rline number but the R number.
I am assuming Vdrop is the actual voltage drop and not the % used. Refer to the book in the course or problem to verify this, but I think I am correct.
using the equation equations given Vdrop = 0.03*120 = 3.6 V
so 3.6 = (15 x 2 x 8 x L) / 1000
solve for L and then plug that L value into the R Line equation to solve for R Line.
Repeat process for solving for the voltage now at 240 V and you will find that the L will be larger and so will the R line value as well. Also, I am assuming the equations given and are correct. I would also do a unit analysis of this just to be sure that the L value calculated is in meters or whatever is necessary.
Have a great day!
Andrew S.
not quite right dividing 15 amps by 3.6V will yield 1/R or conductivity not not amps. For conductor sizing, where I am located, we use the National Electric Code and we lower the max current we can use for the system based off of several factors. One factor is heat, this problem does not appear to give any of this information so I would assume we make this decision based on the max current given of 15 amps, I would choose somewhere around 14 AWG, because the length is relatively short. You can likely choose a larger, like 16 or 18 AWG if derating the wire is not necessary. For the last equation, the voltage drop is Vdrop = (15*2*8*15)/1000 which is the 3.6V from earlier.11/05/20
Amarya W.
In the chart I have 15A with an R of .9989Ohms, AWG is 10. where did you get the 8 from? also when i got the L it was in the 3000', it seems wrong?11/05/20
Andrew S.
If their is accompanying charts or diagrams for the problem use those to solve. I got the 8 from dividing 120/15 = 8. This problem seems like more than what can be described with words. I recommend hiring a tutor for this problem or asking the instructor or instructors assistant for further questions.11/05/20
Amarya W.
Is this right? V drop=.03*120V=3.6 V drop I=E/R I=15amps/3.6V=4.17amps R=V/I R=120V/4.17amps=28.78ohms What size conductor? 7 R line = 2 x R x L/1000 3.6V=2*8ohms*15amps*L/1000=15L R line=2*8*15/1000=.24ohms V drop= I x 2 x R x L/1000 V drop=4.17*2*8*15/1000=1.0008V11/04/20