Three phase Wye Transformer KVA calculation/ Homework typo?

[JasonCo]

Three phase loads of 16, 42, and 75 amps are fed from a 208-volt panel. The smallest Group D transformer required to feed the 120/208-volt 3-ph, 4-wire panel has a rating of ___ KVA.


Group D transformer KVA ratings:
3KVA, 6KVA, 9KVA, 15KVA, 25KVA, 30KVA, 37.5KVA, 45KVA, 50KVA, 75KVA, 112.5KVA, 150KVA, 225KVA, 300KVA, 500KVA, 750KVA, 1000KVA


So this is what I thought I would do...
75 + 42 + 16 = 133amps
3phase power = 1.732 x 133 x 208 = 47914 = 47.9KVA


My answer choices are:
A. 150
B. 225
C. 45
D. 300
E. 112.5
F. 75


There is no answer... Because looking at the Group D transformers, the next one up that I need is rating for 50 KVA, which is not one of the answer choices.. Have I gone crazy or is this just a homework typo?

 

[Smart $]

Perhaps a typo. Possibly looking for the most correct of the choices. Can't decide which from here.

:angel:

 

[kwired]

Three phase loads of 16, 42, and 75 amps are fed from a 208-volt panel. The smallest Group D transformer required to feed the 120/208-volt 3-ph, 4-wire panel has a rating of ___ KVA.


Group D transformer KVA ratings:
3KVA, 6KVA, 9KVA, 15KVA, 25KVA, 30KVA, 37.5KVA, 45KVA, 50KVA, 75KVA, 112.5KVA, 150KVA, 225KVA, 300KVA, 500KVA, 750KVA, 1000KVA


So this is what I thought I would do...
75 + 42 + 16 = 133amps
3phase power = 1.732 x 133 x 208 = 47914 = 47.9KVA


My answer choices are:
A. 150
B. 225
C. 45
D. 300
E. 112.5
F. 75


There is no answer... Because looking at the Group D transformers, the next one up that I need is rating for 50 KVA, which is not one of the answer choices.. Have I gone crazy or is this just a homework typo?

I don't know exactly what group D means, but all the sizes you mentioned are standard sizes, however some of them are only common sizes for single phase, some of them for three phase.

On the lower capacity end of what you mentioned the 25, 37.5 and 50 are usually only seen as single phase transformers, which would leave you with 75 being next higher three phase rating over your load.

 

[charlie b]

I concur with your math. In a real world, I would want to know the nature of the three loads. Are they motors, and should I therefore include a 125% factor? Are they pumps that run intermittently, so that I don't have to include all the load at the same time? Are there any other loads, or do I need to account for future load additions? From a homework or test perspective, however, I think they chose numbers that would go above the 45 KVA transformer rating, and make you struggle to infer whether using a 75 KVA would be overkill. My answer would be 75 KVA, because that is the next higher standard rating, and because the question asked for the smallest required size.

 

[JasonCo]

I concur with your math. In a real world, I would want to know the nature of the three loads. Are they motors, and should I therefore include a 125% factor? Are they pumps that run intermittently, so that I don't have to include all the load at the same time? Are there any other loads, or do I need to account for future load additions? From a homework or test perspective, however, I think they chose numbers that would go above the 45 KVA transformer rating, and make you struggle to infer whether using a 75 KVA would be overkill. My answer would be 75 KVA, because that is the next higher standard rating, and because the question asked for the smallest required size.

Great responses from everyone! Yes I decided that 75KVA would be the only answer that makes sense. 45KVA is too low... Just wish they had put 50KVA in the answer choices Because that is what the question leads me to believe the answer should be. No point in using a 75KVA when a 50KVA is all I need. But yes I'm 100% agree with you, that all these questions it would be nicer if they specified if it is a heater motor or something, or a continuous and etc... But they are all pretty straight forward, which in some cases makes it hard for me to answer the questions. Anyways, teacher gave us 100 question homework and each question involves math. Really enjoy this stuff though, almost done with the homework! Thanks for all your help

 

[kwired]

Great responses from everyone! Yes I decided that 75KVA would be the only answer that makes sense. 45KVA is too low... Just wish they had put 50KVA in the answer choices Because that is what the question leads me to believe the answer should be. No point in using a 75KVA when a 50KVA is all I need. But yes I'm 100% agree with you, that all these questions it would be nicer if they specified if it is a heater motor or something, or a continuous and etc... But they are all pretty straight forward, which in some cases makes it hard for me to answer the questions. Anyways, teacher gave us 100 question homework and each question involves math. Really enjoy this stuff though, almost done with the homework! Thanks for all your help

But as I mentioned earlier you are not all that likely to find a 50 kVA three phase transformer unless it is custom made for you. 50 is a common single phase size though.

 

[topgone]

Three phase loads of 16, 42, and 75 amps are fed from a 208-volt panel. The smallest Group D transformer required to feed the 120/208-volt 3-ph, 4-wire panel has a rating of ___ KVA.


Group D transformer KVA ratings:
3KVA, 6KVA, 9KVA, 15KVA, 25KVA, 30KVA, 37.5KVA, 45KVA, 50KVA, 75KVA, 112.5KVA, 150KVA, 225KVA, 300KVA, 500KVA, 750KVA, 1000KVA


So this is what I thought I would do...
75 + 42 + 16 = 133amps
3phase power = 1.732 x 133 x 208 = 47914 = 47.9KVA


My answer choices are:
A. 150
B. 225
C. 45
D. 300
E. 112.5
F. 75


There is no answer... Because looking at the Group D transformers, the next one up that I need is rating for 50 KVA, which is not one of the answer choices.. Have I gone crazy or is this just a homework typo?

Take it like this: You are given per phase amps and you need to size a transformer to serve those loads and not get overloaded! You have a max load of 75A, and you need to find a transformer that when loaded with 75 amperes on one phase runs fine. So:
kVA required = 75 x 1.732 x 208 = 27 kVA!
The nearest standard transformer size should be 30 kVA! But on your list, you are given 45 kVA!

 

[Besoeker]

I concur with your math.

To add the currents arithmeticaly, they would all need to be in phase but that piece of information seems to me missing.

 

[kwired]

Take it like this: You are given per phase amps and you need to size a transformer to serve those loads and not get overloaded! You have a max load of 75A, and you need to find a transformer that when loaded with 75 amperes on one phase runs fine. So:
kVA required = 75 x 1.732 x 208 = 27 kVA!
The nearest standard transformer size should be 30 kVA! But on your list, you are given 45 kVA!

What if the mentioned 16 and/or 42 amp loads are to run at same time as the 75 amp load?

 

[charlie b]

To add the currents arithmetically, they would all need to be in phase but that piece of information seems to me missing.

No, it is there. All three of the loads are 3-phase loads, and therefore they all in phase with each other. The inherent assumption is that each load is balanced within itself. But given that, we can add amps to amps and get a correct answer in amps. Not a good general practice, mind you. But this time it works.

 

[Smart $]

No, it is there. All three of the loads are 3-phase loads, and therefore they all in phase with each other. The inherent assumption is that each load is balanced within itself. But given that, we can add amps to amps and get a correct answer in amps. Not a good general practice, mind you. But this time it works.

What if each motor operates at a different power factor?

What you're saying would only be true if all three motors operate at the same power factor.

 

[Besoeker]

No, it is there. All three of the loads are 3-phase loads, and therefore they all in phase with each other.

The voltages might be. The currents might not.

 

[Sahib]

No power factor is given in the problem. As such, charlie's approach is correct and also applicable in other practical estimates of transformer capacity for loads of known KVA but unknown power factor.

 

[Smart $]

No power factor is given in the problem. As such, charlie's approach is correct and also applicable in other practical estimates of transformer capacity for loads of known KVA but unknown power factor.

I agree with this but not how he stated all three loads will be in phase. This works where no power factor is given because no matter the power factor, the combined current magnitude can never exceed the arithmetic sum of currents.

 

[Sahib]

This works where no power factor is given because no matter the power factor, the combined current magnitude can never exceed the arithmetic sum of currents.

Why don't you just apply it for the thread below and find the solution for the OP?:

http://forums.mikeholt.com/showthread.php?t=175384

 

[Smart $]

Why don't you just apply it for the thread below and find the solution for the OP?:

http://forums.mikeholt.com/showthread.php?t=175384

Because OP'er already tried this approach...

...
Knowing this, I did the calculations
A phase circuits will have 17.5 amps + 114.6 amps + 183.3 amps + 86.8 amps = 402.2 amps on A phase.

Answer choices are...
A. 234.2 amps
B. 266.2 amps
C. 283.2 amps
D. 299.2 amps
E. 345.3 amps
F. 323.8 amps
...

 

[Sahib]

But you have not given your own solution based on it there. Have you?

 

[Smart $]

But you have not given your own solution based on it there. Have you?

My initial response was...

This is another no correct choice Q&A.

You can't arithmetically add Line A amperes when you have a mix of 3Ø and 1Ø L-N with 1Ø L-L.

You should be posting this over on that thread. Quit hijacking this thread.

 

[Sahib]

The two threads are interconnected in that currents, in a phase are to be arithmetically summed up to find the solution.

 

[Smart $]

The two threads are interconnected in that currents, in a phase are to be arithmetically summed up to find the solution.

Well that premise obviously don't work over in the other thread.