# 1 phase loads connected with 3 phase load



## cruzy (Oct 25, 2010)

I got a question i'm trying to solve. There is a 3 phase system with a delta motor load and three single phase loads. Part of the question asks to find the line current (so the line current of one of the phase motors plus the current from one of the single phase loads). To find the single phase load current (second wire connected to neutral) i know you would take the kW of the single phase load and divide by 277. But in the book they don't take into account a 30 degree phase shift from going from line-line to line-neutral voltage. Is that because this 30 degree phase shift is only for 3 phase loads? Even though each of these single phase loads are connected to one of the 3 phase system conductors?


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## cableguy (Oct 25, 2010)

Putting this in the perspective of the PE exam...

If you are given phase rotation and they nail down a reference (stick a nail in Vab=480 angle 0 for example), and you see that the answers below the question have the angles with them, then yes, it's important.

Odds are good that for most problems, you won't have to worry about it.

Basically, the 30 degree phase shift happens because they've nailed down the reference and the rotation. Then you need to start carrying things out. If they don't... you're working with magnitudes, and don't have to be as "neat" with the phase shifts.

And keep in mind, a lot of times operations are done with a shift in and out... line, to phase, and back to line.

Try not to read too much in to the problem. If they nail it down, then you need to be detailed about it.


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## cruzy (Oct 25, 2010)

cableguy said:


> Putting this in the perspective of the PE exam...
> If you are given phase rotation and they nail down a reference (stick a nail in Vab=480 angle 0 for example), and you see that the answers below the question have the angles with them, then yes, it's important.
> 
> Odds are good that for most problems, you won't have to worry about it.
> ...


What about for questions like NCEES #129, where they don't give any reference angle, but when they start doing the math they include the -30 degree phase shift when doing 480/sqrt 3 to then subtract the voltage from the feeder to find the final volts at the load?


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## cableguy (Oct 26, 2010)

I think they did fine on this one with the assumption that you're stamping Van at zero degrees. They did this because you've got to account for the angle of the current. We're given the current magnitude and a power factor (current angle), so it's "simplest" to work the problem with Van = 0, because the angle of Ia depends on the angle of Van.


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