New Power NCEES practice problem #110

[Gerbera]

I thought that I had a pretty good understanding of when to use sqrt 3 and when to not use it. However, I was working problem #110 in the new NCEES practice Power problems which reads: The only load on a 3 phase 4 wire system is place between Phase B and Phase C. The phase to phase voltage is 13.2kV. The load is 500 kVA at .85 power factor lagging. What is the magnitude of the line current in Phase C?

I thought that kVA, or S = sqrt 3 * V * I. Therefore, I = S/sqrt 3 * V. However the solution is simply I = S/V. Could someone help me to understand why the sqrt 3 was omitted in this case? Thanks in advance for the help!

 

[KSU-ARE]

Gerbera,

The reason you do not use sqrt 3 on this problem is though the system is 3 phase, the load is not. As stated in the problem, "Load is placed between Phase B and C", not all three phases. Just make sure to remember that the kVA or S you get is for combined Phase B and C. If you need to know what is the kVA per phase, you will need to divide the load by 2. Hope that helped.

 

[Gerbera]

KSU-ARE, thank you so much for the response! Yes, that is very helpful and makes complete sense. Thanks again!

 

[tonyb]

So for this problem.

Phase 'A' makes a loop from source 'A' to load neutral back to source neutral. And phase 'B' makes a loop from source B through load to C and then back to Source 'C'?

 

[tonyb]

So for this problem.

Phase 'A' makes a loop from source 'A' to load neutral back to source neutral. And phase 'B' makes a loop from source B through load to C and then back to Source 'C'?

I should put this sketch up so my misunderstanding is more clear.

 

[Flyer_PE]

^Phase A doesn't go anywhere, it isn't hooked to anything. You only have a single-phase load connected to B and C. They give you the line (phase to phase) voltage. The fact that the source is a 4-wire system is just extra information you don't need. It doesn't matter if the source is wye or delta.