NCEES #121

[mhelms_2]

The answer requires the following equation:

I= S/(V*sqrt3)

Why and when is the sqrt 3 used?

In the problem, you have the line voltage, so if anything, I would assume you would divide V by the sqrt 3 to get phasze voltage.

Any help is appreciated.

 

[mhelms_2]

Okay... think I answered my own question.

For three phase S=VI * sqrt 3, which in turn gives I=S/(V*sqrt 3).

I guess I was thinking in terms of single phase.

 

[gte636i]

There are two ways to work out any three phase power problem. Per phase and multiply be three, or use the total three phase power equation. The NCEES solution uses the total three phase power formula S(total 3-phase) = Sqrt3 * VL-L * IL the alternative is the per phase method S(total 3-phase) = 3 * VL-N * IL either way works out the same.

If you did like you said and divide voltage by the sqrt of 3 to get phase voltage, you would need to divide S by 3 to get the VA per phase, your equation would then look like IL= (S(total 3-phase)/3) / (VL-L/sqrt3). If you punch it in the answer comes out the same.