# Voltage Drop Equation confusion



## thammond (Apr 12, 2016)

Hello all,

It is getting down to brass tax, and I have been reviewing material for days now.  I have a question about voltage drop.  in the NCEES sample exam, problem 136, the solution states that the APPROXIMATE Vd=I (X sin theta + R cos theta).  I also seen a Voltage drop equation being Vd = |I| (R cos theta - X sin theta), and another being Vd = |I| ((R/1000)*L* cos theta -(X/1000)*L* sin theta).  I am just trying to understand the differeneces.  The last one I put into my notes a while ago and don't remember the application.  Any help would be appreciated.

Thanks


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## Audi Driver P.E. (Apr 13, 2016)

If you could post the problem, I might be able to help a fellow Coug out.     Just snap a photo of it with your phone or something.


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## thammond (Apr 13, 2016)

Attached is a picture of the problem.  I understand this problem and how using the first equation from above will give you the correct answer per NCEES, however in another sample exam, CI V2 problem 10 has a voltage drop for a single phase system, and they used the second equation above, and then I found in my notes a third equation, (the last one above) that I can't remember or find in any of my text or problems.  I am just trying to understand the differences, and proper use application.  Any help would be appreciated.

Thanks


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## TWJ PE (Apr 13, 2016)

The 3rd equation you are using if you get the resistance and reactance from Table 9 of the NEC (or are told it in the problem).


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## thammond (Apr 13, 2016)

W9TWJ

Thanks, now I remember, I appreciate the help.

do you know why the first equation has + sin and the last two use - sin?


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## knight1fox3 (Apr 13, 2016)

thammond said:


> W9TWJ
> 
> Thanks, now I remember, I appreciate the help.
> 
> do you know why the first equation has + sin and the last two use - sin?


Doesn't that deal with the target circuit being inductive vs. capacitive (i.e. leading or lagging pf)?


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## Audi Driver P.E. (Apr 13, 2016)

I believe it has to do with leading or lagging power factor. Or as Fox put it, inductive vs. capacitive.


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## matt267 PE (May 1, 2016)

You might want to reword your question and remember your agreement with ncees.

@knight1fox3 can help you edit your post.


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## cdiller (May 1, 2016)

Thanks @matt267 PE!  I actually read a post about that a few minutes after submitting my post and I started to freak out.  I flagged my own post lol and hope it gets edited.  Newbie mistake sorry.


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## TWJ PE (May 1, 2016)

cdiller said:


> How did you solve the problems on the exam if they didn't tell you what type of conduit the conductors were run in?  Is the equation aforementioned above correct to use when you have unity power factor and don't know the type of conduit?
> 
> Thanks in advance for your replies!


It's my understanding that if they don't tell you it's in conduit then you are to assume the conductor is in free air.

Just my opinion, but I thought overall those types of questions were fair on the April 2016 exam. I didn't really feel like they tried to throw a curve ball.

So the equation I try to use is (KVL):

Vs - IZ;
where Vs = voltage at the source.

If the question tells me the PF, then I put that angle [cos-1(PF)] (positive/negative depending on leading/lagging) with the magnitude of the current. Then I calculate the reactance.

Now obviously, - IZ could be + IZ depending on what they tell you (i.e. if they give you the voltage at the load and ask for the voltage at the source).

Hope this helps.


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