Which do you prefer and why?

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Dsg319

Dsg319

Senior Member
Location
West Virginia
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Wv Master “lectrician”
What is your favorite way to calculate voltage drop of feeders/branch circuits.

I’ve always used 2 x k x I x D/KCMIL. Or a variant of to find max distance, size of conductor, or simply the voltage drop.
But as of recent I have just noticed that VD=IxR comes out to be just a tad bit higher of voltage drop. As an example using 2/0 copper , 200ampere load at 186’. VD=IxR came out 7.5vd compared to another calculation at 7.2Vd. I know it’s not enough to worry about.

And Obviously VD calculations are not based on the size of OCPD but rather the calculated load. I just used 200amp as an example for a single family dwelling voltage drop at worst case scenario.

Are there any reasons one calculation is more accurate than the other?

Also where do you start your distance for feeder/service conductors calculation. At the POCO transformer or “our service equipment”.
I’m going to make a guess and say our service equipment.
 
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Dsg319 said:
What is your favorite way to calculate voltage drop of feeders/branch circuits.

I’ve always used 2 x k x I x D/KCMIL. Or a variant of to find max distance, size of conductor, or simply the voltage drop.
But as of recent I have just noticed that VD=IxR comes out to be just a tad bit higher of voltage drop. As an example using 2/0 copper , 200ampere load at 186’. VD=IxR came out 7.5vd compared to another calculation at 7.2Vd. I know it’s not enough to worry about.

And Obviously VD calculations are not based on the size of OCPD but rather the calculated load. I just used 200amp as an example for a single family dwelling voltage drop at worst case scenario.

Are there any reasons one calculation is more accurate than the other?

Also where do you start your distance for feeder/service conductors calculation. At the POCO transformer or “our service equipment”.
I’m going to make a guess and say our service equipment.
Click to expand...
VD=I x R

What is R? It is 2 x k x D/KCMIL isn't it? So you basically still using same formula, your difference in results is probably in that k factor.

BTW with three phase you substitute 1.732 where you had 2
 
kwired said:
VD=I x R

What is R? It is 2 x k x D/KCMIL isn't it? So you basically still using same formula, your difference in results is probably in that k factor.

BTW with three phase you substitute 1.732 where you had 2
Click to expand...
R-Resistance. As listed for conductors sizes in chapter 9.
 
Dsg319 said:
R-Resistance. As listed for conductors sizes in chapter 9.
Click to expand...
Like I said the k factor is the difference. If using table 8 resistance values those are at 75C (167F) temperature. If they would adjust the table values to 30C, which is common base for the k value in many general use calculation situations, you likely get the same result.

R is resistance of the conductor, I was trying to point out all the more complex looking formula is doing is figuring out what R should be instead of plugging it in from a different calculation.

The 2 in the calculation is there to account for the return path- presuming same size and length of conductor, and the 1,73 I mentioned applies to three phase because that is how the return path current divides up if balanced current on all three phases.
 
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I typically use an online calculator for voltage drop. The one I use is


That gives the 3% or 7.2 volts when I set it for a DC circuit, but gives 3.4% or 8.16 volts for a single phase AC circuit. The difference is the use of just resistance for the DC calculation and the use of impedance for the AC calculation. Note, in both cases the conductor temperature was set at 75°C.
 
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