Practice exam ?? [balancing loads]

[bamit]

I'm trying to understand why balancing a load one way is better than the other? Here's the exam question.

Balance a 120/240v delta 3 phase panelboard in kva for the following loads: One 18 kva 3 phase, two 5 kva 1 phase 240v, and three 2 kva 120v.

ex 1:

a= 6.0 kva + 2.5 kva + 2 kva + 2 kva = 12.5 kva
b= 6.0 kva + 2.5 kva + 2.5 kva = 11.0 kva
c= 6.0 kva + 2.5 kva + 2.0 kva = 10.5 kva

ex 2:

a= 6.0 kva + 2.5 kva + 2.5 kva = 11.0 kva
b= 6.0 kva + 2.5 kva + 2.5 kva = 11.0 kva
c= 6.0 kva + 2.0 kva + 2.0 kva + 2.0 kva = 12 kva

ex 3:

a= 6.0 kva + 2.5 kva + 2.5 kva + 2.0 kva = 13.0 kva
b= 6.0 kva + 2.5 kva + 2.0 kva = 10.5 kva
c= 6.0 kva + 2.5 kva + 2.0 kva = 10.5 kva


ex 1 = the answer given.
ex 2 = my 1st answer. [neutral not balanced]
ex 3 = my 2nd answer. [neutral balanced]

If my thinkings right, I need to know the rules for BALANCING ???

 

[charlie b]

I don't think one is right and the other two are wrong. All are reasonably balanced, in my opinion. I don't think a test question should be worded in such a way as to require any one of these answers as the "correct solution."

I don't understand what you meant by talking about neutral being balanced or not balanced. This is a delta system, so it has no neutral.

 

[Cavie]

:-? The collage boys are at it again!!!!:smile:

 

[Cavie]

I don't think one is right and the other two are wrong. All are reasonably balanced, in my opinion. I don't think a test question should be worded in such a way as to require any one of these answers as the "correct solution."

I don't understand what you meant by talking about neutral being balanced or not balanced. This is a delta system, so it has no neutral.

Question states 120 volt loads????

 

[wirebender]

In ex. #3 you have a 120 volt load on the high leg, don't you.
Or am I confused as usual?

 

[SiddMartin]

As long as you know how to break down the kva and balance them the way you did in all three examples, you'll be okay. If you would get a question like this, it would probally ask you for the highest unbalanced neutral load or something of that sorts. The key in the question is that it says to

balance in kva

if it says

balance in amperes

then it is totally different. You should know how to do both.

 

[SiddMartin]

In ex. #3 you have a 120 volt load on the high leg, don't you.
Or am I confused as usual?

The question doesn't state that there is a high leg so you have to assume for exam purposes that it does not have a high leg.

 

[bamit]

I would assume there is a high leg & assume you would have 120v to ground on the other two legs. my question is my would example 1 be more correct than the others?????

 

[SiddMartin]

I would assume there is a high leg & assume you would have 120v to ground on the other two legs. my question is my would example 1 be more correct than the others?????

Why would you assume there's a high leg? For test purposes, you can't assume that. They would have to tell you that. Either way, doesn't matter to your question. I don't know why one is better then the others, but it won't effect the test question is my point. You're looking too far into it, it looks like you understand how to break it down, thats all they would be looking for.

 

[bamit]

Why would you assume there's a high leg? For test purposes, you can't assume that. They would have to tell you that. Either way, doesn't matter to your question. I don't know why one is better then the others, but it won't effect the test question is my point. You're looking too far into it, it looks like you understand how to break it down, thats all they would be looking for.

Thanks Sid I must have posted at the same time, replying to the post befor. thanks I kidda feel better but I still got the ? wrong.

 

[SiddMartin]

Thanks Sid I must have posted at the same time, replying to the post befor. thanks I kidda feel better but I still got the ? wrong.

so did I :smile:

 

[Lcdrwalker]

I dont think that there is much to debate. The question specified a delta config. Therefore one leg could not have a one phase load connected to it. so no. 3 is eliminated. In no. 2, phases A and B have only two and three phase loads leaving the neutral load entirely on phase C. Only answer no. 1 shares the 'hot' and neutral loads.

 

[bamit]

I dont think that there is much to debate. The question specified a delta config. Therefore one leg could not have a one phase load connected to it. so no. 3 is eliminated. In no. 2, phases A and B have only two and three phase loads leaving the neutral load entirely on phase C. Only answer no. 1 shares the 'hot' and neutral loads.

thanks lcdrwalker, Until this morning it didn't dawn on me that a 240v system only had 2 120v legs to ground ,now it all makes sense, guess I need to study distribution systems more.