Voltage drop with Power Factor

E

ElecBeginner

Member
Location
Hawaii
Occupation
Engineer
Hello All, I've been looking into my local voltage drop calculator and various online calculators online and spent the better half of the day trying to understand whats going on.

I am under the assumption
VD(single phase) = I x Z x L/1000 x 2
I = VA/V = Watts/(V*PF)
PF = Cos(theta) = Watts/VA
Z = Rcos(theta) + Xcos(theta)

With the above ive been trying to recreate the results i see in various VD calculators and getting a result that is off by a factor of PF.
Example:
120V, 1ph, 12000 watts, 0.9PF, 10ft, #12 copper wire in PVC (numbers arbitrarily chosen, the result ends up the same for different values)
using the equation above i get
VD = 12000watts / (120V x 0.9) x (2 x cos(25.8) + 0.054 x sin(25.8) ) x 10ft/1000 x 2
= 4.07V
Using the Calculator i get 3.65V (which is 4.07 x 0.9), this includes other calculators i find including southwire, the one recommended by mike holt, and other miscellaneous calculators.

Not sure how to add an image but ive used excel to compare and its generally always off by a factor of PF, so the question is am I missing a variable somewhere or converting real vs apparent parent wrong somewhere or something else altogether? Any help would be appreciated.
 
So in the Southwire calculator, given i have a load of 12kW for the example above i calculated a current of 111.11A (using .9 power factor), which when i put into the calculator (at 75deg wire) i get a ~3.34VD or 4V and i also tried 100A resulting in ~3% VD or 3.65V, which doesnt jive with my hand calculated value of 4.7. Do you input a different current than I when using the calculator given a 12kW, 120V, .9 power factor load?
 
Last edited:
ElecBeginner said:
So in the Southwire calculator, given i have a load of 12kW for the example above i calculated a current of 111.11A (using .9 power factor), which when i put into the calculator (at 75deg wire) i get a ~3.34VD or 4V and i also tried 100A resulting in ~3% VD or 3.65V, which doesnt jive with my hand calculated value of 4.7. Do you input a different current than I when using the calculator given a 12kW, 120V, .9 power factor load?
Click to expand...
The difference in your calculation cannot be explained by power factor. Southwire would give the same result using a load of 111.11A at unity PF.
 
Just to clarify, how do you input the values into Southwire given 12000watts, 120v, 0.9 power factor? Is it different to what is shown below, feels like i would be applying PF twice if i specify 0.9 PF and upsize the current by PF? I'm not sure if im using the calculator correctly.
southwire.PNG
 
First, I quickly pounded your situation using my red book formula on a hand calculator and got ~ 4.05 volts drop.

You should read this thread from a bit ago on the Southwire calculator.

Is the Southwire Voltage drop calculator wrong?

One example here, and maybe someone can just point out where my math's wrong but I cannot for the life of me get the same answer as Southwire's calculator #2 Aluminum 480V run of 510 feet for 90 amps. Setting Southwire to power factor of 1 and commercial 75, it gives an answer of 5.61%. 5.61%...
forums.mikeholt.com

I'm sure you realize 100A on #12 wire is an issue but I understand that really isn't the point. My advice is to take any on-line whiz bang calculator with a small grain of salt, especially if you are operating near a boundary condition, e.g. will a motor start under these conditions... and my second piece of advice is code up the red book formula in excel and familiarize yourself with the phasor diagram understanding that even that formula is an approximation (although usually a good approximation).
 
dkarst said:
First, I quickly pounded your situation using my red book formula on a hand calculator and got ~ 4.05 volts drop.

You should read this thread from a bit ago on the Southwire calculator.

Is the Southwire Voltage drop calculator wrong?

One example here, and maybe someone can just point out where my math's wrong but I cannot for the life of me get the same answer as Southwire's calculator #2 Aluminum 480V run of 510 feet for 90 amps. Setting Southwire to power factor of 1 and commercial 75, it gives an answer of 5.61%. 5.61%...
forums.mikeholt.com

I'm sure you realize 100A on #12 wire is an issue but I understand that really isn't the point. My advice is to take any on-line whiz bang calculator with a small grain of salt, especially if you are operating near a boundary condition, e.g. will a motor start under these conditions... and my second piece of advice is code up the red book formula in excel and familiarize yourself with the phasor diagram understanding that even that formula is an approximation (although usually a good approximation).
Click to expand...
Thank you! seems your calculated value is similar to mine. I think i just needed some reassurance of my work. I think i've been using the online calculators wrong and wasnt properly scaling the input current to the chosen PF and seems the one we use inhouse is just plain wrong (you dont wanna know where i work).

I will be sure to look into those resources and try it out and yes the values i chose is nowhere near practical but wanted easier numbers to visualize the situation.
 
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