calculation question for supervisor test

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leksa1

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Hello Mike I getting ready to supervisor test in Oregon. Can you help me to figure out in this practice question.
I have office space lighting is separate 277 volt.
208 v load include
General use receptacle, Computer receptacle, Copy machine receptacle.Total load is 115000 VA
I have to size transformer, panel and secondary wire size to feed that panel.
To size secondary overcurrent device* (panel) for transformer you have to multiple by 1.25, but is it work in this situation?*
For 115000 VA I need to use 150 kVa transformer
if you size panel by load not by transformer
115000/(208?1.732)= 319 amp So breaker size will be 350 amp or I have to 319 ? 1.25=399 amp and breaker will be 400 amp. Can you explain me what the right way thanks.
 
leksa1 said:
....
To size secondary overcurrent device* (panel) for transformer you have to multiple by 1.25, but is it work in this situation?*
...
115000/(208?1.732)= 319 amp So breaker size will be 350 amp or I have to 319 ? 1.25=399 amp and breaker will be 400 amp. Can you explain me what the right way thanks.
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There's no mention of continuous load, size MCB panel for 319A, i.e. 350A main breaker... which will also serve as transformer secondary and secondary conductor protection.

Technically, you could get away with a 112.5kVA transformer, but probably best to go with 150kVA on test... :angel:
 
Technically, you could get away with a 112.5kVA transformer, but probably best to go with 150kVA on test... :angel:[/QUOTE]

If you can use the 112.5, that is the correct answer when it comes to a test. they are always looking to see if you can correctly calculate the specified load or size cable. With that said, I didn't look over any of the calculations previously mentioned, I am assuming these values are correct.
 
rlundsrud said:
If you can use the 112.5, that is the correct answer when it comes to a test. they are always looking to see if you can correctly calculate the specified load or size cable. With that said, I didn't look over any of the calculations previously mentioned, I am assuming these values are correct.
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I'd agree with your normally, but being able to use a 112.5kVA transformer for a calculated 115kVA load would be taking advantage of a Code "loop hole". Most believe a transformer must be sized not less than the calculated load. Given the majority of electrical professionals out there fall under this premise, a 150kVA transformer would be a lot safer answer. In short, Code does not make that stipulation.

I've taken enough tests to know that it's the desired answer that gets points... and the desired answer isn't always the [most] correct answer.

Now refer to the question as posed in the OP. I could answer using a 300kVA transformer as long as all the other equipment is rated appropriately. The question does not ask one to use the smallest compliant common size. So would my answer be wrong???
 
To use a parallel, if you are asked to calculate the size of a conductor for a specific application you answer with the one that is adequate to handle the load but no larger. If the smallest allowable size is #12 you wouldn't answer with 500mcm. The 500 would certainly be legal to use, but wouldn't answer the question correctly. If you could use a 112.5kva transformer given the requirements postulated in the question, that to me would be the correct answer.
 
rlundsrud said:
To use a parallel, if you are asked to calculate the size of a conductor for a specific application you answer with the one that is adequate to handle the load but no larger. If the smallest allowable size is #12 you wouldn't answer with 500mcm. The 500 would certainly be legal to use, but wouldn't answer the question correctly. If you could use a 112.5kva transformer given the requirements postulated in the question, that to me would be the correct answer.
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What you are discussing is the unwritten assumptions made by the test author. Lacking properly stated criteria, a correct answer may be graded as an incorrect answer... but that doesn't make provided answer any less correct. It just diminishes one's score. :(

This goes back to whether the provided answer is the author's desired answer. Heck, I've got answers scored wrong because I chose what I knew to be in fact a more correct answer than the author's. I've also seen author's answers that were totally wrong. The best you can hope for in these cases is you align with the author's frame of mind when the test questions were written.
 
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