# NCEES Problem #136



## jd5191 (Jan 7, 2021)

In this problem about Voltage Drop, they give you impedance and current and ask to solve for voltage drop using the below equation which exists in the handbook




My question, why is it wrong to use ohm's law to calculate voltage drop in this situation? They've given you Z and |I|. If I plug in different phase angles for the I, am I not calculating the voltage drop at different power factors using V=IZ?

View attachment 7854


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## COJeff (Jan 7, 2021)

Sorry Where do you see "Z"  The current, and circuit length dont really matter.  Plug in the values of A,B,C,D, and the voltage drop can be quantified.


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## COJeff (Jan 7, 2021)

Edit Above, look at A and the equation for the voltage drop.


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## COJeff (Jan 7, 2021)

I think I understand your question Z=sqrt(r^2+x^2), but this is not how the equation works.  your principle would work for single phase load but would have to account for the supply and return.


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## jd5191 (Jan 9, 2021)

COJeff said:


> I think I understand your question Z=sqrt(r^2+x^2), but this is not how the equation works.  your principle would work for single phase load but would have to account for the supply and return.


Is your answer in relation to the Voltage Drop equation that the answer uses? My question was why can't you use ohm's law equation _instead_? And If I shouldn't use ohm's law, then why not.


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## RedRaider2020 (Jan 17, 2021)

This looks like a max power transfer theorem question to me with no calculation required.


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