NCEES #110 vs. #121

[Anasimongirl]

I'm having trouble every time I work problems #110 and #121!

the big picture as I understand is:

S(3phase)=3*V(phase)*I(phase) ------> I=S/V(phase)

or

S(3phase) = sqt*3 *V(L-L) *I(L)-------> I= S/sqt*3 *V(L-L)

so ,on #110: why they calculated Ic which is I(phase) =S/V(L-L)???? without dividing by sqt*3??or using Vphase instead?

while on #121 : they used the right equation!! what is the difference that I'm missing??

 

[user 26087]

On 110 it is a 3-phase system but a single phase load, therefore square root of 3 is not in play. On 121 it is a 3-phase load so it is in play. If you haven't yet I would recommend the Complex Imaginary video on the square root of 3.

 

[Anasimongirl]

I agree with the single load part but why they used the V(L-L) and not the V phase? on #110

thanks!

 

[user 26087]

Because the question is talking about the voltage Vbc. In a wye system that is V(L-L). So that is the phase-to-phase voltage or line voltage. If it said there is a load placed on phase B of a 3-phase, 4wire system then you would use the phase voltage.

I hope this helps!