# NCEES Practice exam problem 513 question



## Sparky Bill PE (Jan 26, 2020)

Why can't I just do the normal S(xfmr)/Z%, then use Sf to find Isc by doing Sf/sqrt3*VL? Is all of this extra math with bases etc just because they gave me "20 mile" length instead of an "infinite bus" like I"m used to?


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## DLD PE (Jan 26, 2020)

SparkyBill said:


> Why can't I just do the normal S(xfmr)/Z%, then use Sf to find Isc by doing Sf/sqrt3*VL? Is all of this extra math with bases etc just because they gave me "20 mile" length instead of an "infinite bus" like I"m used to?
> 
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Yes the extra line of math is to find the actual impedence of the line, and since it's dependent on the length, you have to convert miles to feet first.


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## Orchid PE (Jan 27, 2020)

I remember a question with units got me during practice. It was calculating something with conductors, and the formula used conductor Radius, but the problem gave conductor Diameter. I missed it and after going through the solution I realized I worked everything out right, but used Diameter instead of Radius in the equation.


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## Zach Stone P.E. (Jan 27, 2020)

SparkyBill said:


> Why can't I just do the normal S(xfmr)/Z%, then use Sf to find Isc by doing Sf/sqrt3*VL? Is all of this extra math with bases etc just because they gave me "20 mile" length instead of an "infinite bus" like I"m used to?
> 
> View attachment 16153
> View attachment 16152


Because the impedance of the line is also being taken into consideration here. 

The approach you are trying: "_Why can't I just do the normal S(xfmr)/Z%, then use Sf to find Isc by doing Sf/sqrt3*VL?_" would be perfectly fine if the line impedance was not being taken into consideration.

You can still use your approach (MVA Method) by finding the fault duty of the transmission line in MVA, and taking the reciprocal sums of the transformer and T line:

Sf = S_XFMR//S_Line

Below is a video example of a Fault Current Analysis practice problem solved using the MVA method with a line impedance in series with a transformer. 

is a little more in-depth than the NCEES practice problem, but it should give you a good idea of how to calculate the fault duty of the transmission line in the NCEES practice problem and then how to solve it using the MVA method (the video should start at 7:46 when the fault duty of the line impedance is being calculated):





The only difference is that the NCEES problem gives you the impedance of the line in ohms/1,000 ft. You will need to multiply the Ω/1,000 ft by the total length of the T-line in order to convert to the total amount of impedance in the line before calculating the fault duty of the line.


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## patkt (Jun 27, 2021)

Using MVA Method:
SfT1 = 7.5 MVA/7.5% = 100 MVA
SfZ = 12kv^2/15.312i = 9.404 MVA

SfTOTAL = SfT1 || SfZ = 100MVA || 9.404 MVA = 8.595 MVA

Isc = SfTOTAL / Sqrt(3)x12KV = 8.595 MVA / (sqrt(3)x12KV) = 413.52A

Nearest Answer is 415A. 

Thanks @Zach Stone P.E.. This is very easy and convenient method to solve the problem


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