Impedance question?

[SonofAdam]

I took another swing at the load calc test yesterday and I got this question...

Size the primary and secondary fuse rating of a 75kva transformer. The voltage is 480/ 240/120v single phase, 3 phase secondary 5% impedance.

To me, it was worded weird and what I wrote here is strictly off memory...
I couldn't get any answers to line up with what was given, and i think the impedance has something to do with it. With 4 answers to choose one i just shot from the hip on it.

Any help is appreciated to help me understand this.
Thanks

 

[LarryFine]

The voltage is 480/ 240/120v single phase, 3 phase secondary ...

Non-sequitur.

 

[electrofelon]

Non-sequitur.

Its called a phase converting transformer Larry, duh.....

 

[don_resqcapt19]

Full load primary current is 90.22 and full load secondary current is 180.42 assuming it really is a 240/120 volt secondary...that would be a 4 wire high leg delta voltage and while you can get a transformer with that secondary, they are not near as common as a 208Y/120 secondary.
The impedance does not enter into this question.

 

[LarryFine]

Its called a phase converting transformer Larry, duh.....

He said transformer, not phase converter.

 

[electrofelon]

He said transformer, not phase converter.

....but a transformer with a single phase primary and three phase secondary. I was making a joke

 

[SonofAdam]

some of the answers tho with 125% wern't matching to the next oversize device. I remember that 2 of the choices , one was like both primary and secondary OCPD was 350, another choice was 350 and 400, and i think the other two were about 250,125 and 150,200 .. it was all over the place

On the same test i had a question that the impedance was causing the amperage to be lower on and i had to figure out the actual voltage on the transformer if it was corrected.. i think that is why i got mixed up, but still doesn't explain the weird answers..

even tried doing primary and secondary protection on the 480 side and it just made it the same OCPD as the secondary..

I did write to the state saying the question was worded poorly.

 

[electrofelon]

Note that impedance effects protection in MV transformers, but not LV transformers.

 

[LarryFine]

....but a transformer with a single phase primary and three phase secondary. I was making a joke

Thank goodness!