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At Max eff. the core losses are equal to copper losses. Start from assuming core losses to be equal to x. You must each the answer. Okay I will do it.  Let I be the full load current At max eff (.45I)**2 X R=x This gives me Full load losses= I**2 X R=4.94x Now total losses at full; load=...

Lol. Becoming interesting. That is the reason I mentioned initially that we are discussing one dimension of the issue- Only flux. There are other dimensions to the issue- economy, Operating Temperatures, cooling, cooling medium, life, eddy currents, Insulation and many more.   

I think you are thinking about electrical resistance. Yes water (impure) is better conductor of electricity, so wet wood becomes a better conductor of electricity but as far as magnetic ckt is concerned, both wood and water behave same. I will now give one spin to tickle your brain- Good heat...

It does not matter, reluctance to flux of both water and wood are same. Even copper core will have same size as wood. This is Reluctance to flux . Only magnetic materials like iron offers less reluctance; all others offer high reluctance. 

In this question the Voltage Ratio and turns ratio do not match. So take turns Ratio (5:2) and MVA to get the answer of the solution. Do not mix Voltage with turns ratio. 

Yes @Pixie the size of wooden core  Xmer will be bigger than Iron cored for same ratings. Why?  and how big? Let me try to get it like this. For a particular rating of Xmer there will be a particular amount of flux flowing in the core of that transformer. Say the amount of flux be x Wb. As the...

Q- If Xmer core were made from wood having same ratings as an iron cored. 1. Not possible technically 2. The  Xmer size will be very large compared to that of Iron cored 3. The size will be same as that of Iron core 4. Size will be smaller than Iron cored. Give your answers with reasoning

@Omer you may be right. It is actually not Vt@o but some reference other than Vt. All angles are from that reference. Yea something wrong. However I propose to leave it here; I feel this question is eating away more time than benefits. We will take it up again after exam.

Thanks Omer. The solution starts with changing reference of current , that too is done wrong I think and gets answer 455<9.7. 

I think the answer to this question is wrong by CI. What do you say?

I do not know if there is a code legislation for this, @FPar may let us know. But there is a convention; while deciding a protection setting, it is prudent  and safer to go for lesser setting provided it does not start giving nuisance tripping while energizing a transformer or starting a motor...

Your logic makes absolute sense and I would go with this- round off towards safer side should always be a choice but the question here is interpretation of NEC and selection as per that. Would you please read the paras of NEC and prove concurrency  between the logic and  NEC. 

IMHO if the sequence is A, B, C and Iab<0, Ibc<-120, Ica<120 are phase currents in Delta then IA=Iab-Ica will lag 30 degrees to Iab. However in case Voltages in Star the line Voltages lead the phase Voltages by 30 degrees. 

That makes sense. Then CI guys have done it wrong in the solution. They are changing both. Thanks Omer.

Can some one attempt this. I feel it is only MVA which changes  and not the Voltage to find new pu value; in such questions. WE discussed such question in another thread. 

I also got 195A and so I too feel it should be 200A. Any other expert advise? I do not want to miss on NEC!!!

Omer. Very nice doubt!!!. I will tell you where you are wrong when dealing with pu and 3/1 phases. In pu if you have taken three phase voltage as 1pu, then single phase voltage is also 1pu and not to be divided by sqrt3 while dealing with pu. Now I solve a simple question for you. Say the...