# Voltage Drop Equation



## shsweet28 (Oct 23, 2013)

Lets say I am calculating Voltage Drop (Vd) and have a load with a lagging PF, then arcos(PF) gives the angle, I make the angle negative since the load is lagging.

So Vd=Vo-(I&lt;-Angle)*(R+j*X).

But Vd=abs(I)*(R*cos(angle)+sin(angle).

Why do I use the positive angle in the second equation and the negative in the first?

Maybe I am missing something simple.


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## shsweet28 (Oct 23, 2013)

the second equation should be Vo-abs(I)*[R*cos(angle)+X*sin(angle)]


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## shsweet28 (Oct 23, 2013)

*I see yet another error so I will start over.*

Lets say I am calculating Voltage Drop (Vd) and have a load with a lagging PF, then arcos(PF) gives the angle, I make the angle negative since the load is lagging.

So,Vd=(I&lt;-Angle)*(R+j*X)

Vd=abs(I)*(R*cos(angle)+X*sin(angle)

Why do I use the positive angle in the second equation and the negative in the first?


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## shsweet28 (Oct 23, 2013)

I found the answer to my question. Please see the image below in case you were curious.


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## MyBeardAndMe (Apr 11, 2015)

Is this accurate? I am having a hard time finding anything online to back up the + for lagging, - for leading.

In the NEC Handbook it only shows Rcos(theta)+Xsin(theta) and doesn't state whether the power factor is leading or lagging.


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## poosri (Apr 13, 2015)

I have the same confusion...but i am planning to use only + because nec doesnt say anything about the sign. Can someone help?


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## dayrongarcia (Apr 13, 2015)

MyBeardAndMe said:


> Is this accurate? I am having a hard time finding anything online to back up the + for lagging, - for leading.
> 
> In the NEC Handbook it only shows Rcos(theta)+Xsin(theta) and doesn't state whether the power factor is leading or lagging.


Yes it is correct and it will also affect the voltage regulation equation.

http://www.nct-tech.edu.lk/Download/Technology%20Zone/Performance%20of%20Transmission%20Lines..pdf


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## poosri (Apr 13, 2015)

If the source is a 1 phase center tap line (V LN= 110V and V L-L = 220V), which voltage would you use to calculate voltage at receiving end?


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## dayrongarcia (Apr 13, 2015)

You always use phase voltage and then multiply by Square root of 3.


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