question - transformer and voltage

[dshaff3]

Anyone knows the answer to this one.
I think it could be either B or D
the right answer according to ballast is D. Which i understand why it can be D but i don't understand why it can't be B.

Ballast Question.JPG

[corbismyhomeboy]

I think it's that the wye transformer is used with 120V (or lower voltages) while delta transformers are used for higher voltages like 277V. The delta is typically what is used by the power company to bring in power to an area (so it can handle higher voltages) while the wye is what is used for winding power down to lower voltages like 120V.

At least this is my understanding of it!

[exams4ever]

I had the same doubt while solving the problem. Both B and D seemed correct. Didn't think about the wye and delta connection. Still not very clear understanding on those connections. If your reasoning is correct, then thanks for it!

[gbalaka]

I am also struggling with this one. But it certainly has to do with the Y or Delta system on the left hand side.

I've just accepted that the answer to this question is 277/480V and that if the system was a Delta, then it'd be 120/208V

But certainly not 120/240 as that'd be a single phase power delivery system.

[gbalaka]

I also found this video that sort of clarified it for me.

You use the Y type of connection for high voltages and the Delta for low voltages. You can watch the whole video, but the link jumps to the part that talks about the Y and Delta connections.

https://youtu.be/vh_aCAHThTQ?t=3m44s

[danio1011]

Gbalaka, FWIW my understanding is Wye is useful for supplying 110v off any three legs due to the neutral being present...so it is used in more small commercial settings where low voltage is required. Delta is better for factories etc because it can supply 240 or 480.

[danio1011]

Also I think the reason it can't be 'B' is because the diagram shows a Wye, which gets 208 on its legs, not 270. Only Delta gets 270 in each leg as far as I can tell. Not an expert, just my current (heh heh) understanding.

[gbalaka]

....so confusing. That's why i say "leave it to the electrical engineer"