NCEES Morning #134

[wilheldp_PE]

In the solution to problem 134 in the NCEES practice test, I don't understand where the power, P, of 21.7 comes from in the calculation of Qnew. Also, when I do 21.7[tan(cos-10.9)], I get ~10.5, not 10.3. Likewise, when I calculate Qnew=21.7[tan(cos-10.75)], I get 19.1 instead of 18.7 from the book. I would still guess the same answer (10.5-19.1 = -8.6, instead of -8.4), but I wonder why my numbers are off.

 

[Flyer_PE]

The value in the book is a typo. The correct value for P is 21.2. It is arrived at by the following:

S=sqrt3*VLLIL=28.27kVA

P=pf*S=0.75*28.27=21.2kW

If you put 21.2 in place of 21.7 you will get their value of 10.3 kVA for QNew.

 

[wilheldp_PE]

Awesome. Thanks for the help, IFR.