another code challenge question

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electricrw

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I am taking my supervisers test soon. Hope you can put up with me asking all the questions that I am stuck on. Thanks again

What is the secondary over current protection on a 300kva transormer 480/120/208v hint; table430.3b
I dont know this answer. need help
 
electricrw said:
I am taking my supervisers test soon. Hope you can put up with me asking all the questions that I am stuck on. Thanks again

What is the secondary over current protection on a 300kva transormer 480/120/208v hint; table430.3b
I dont know this answer. need help
Click to expand...


did you mean to type 450?
 
300000/480/1.73 =361.3 amps (primary)

300000/208/1.73 =833.7 amps (secondary - max)

my guess is 800 amps
 
Why don't you post your answers and how you arrived at them, then someone can help you by reassuring you or pointing you in the right direction.

Roger
 
oops, the table says you can go next higher (note 1), plus it says you can use 125%, so I guess that would be

833 x 1.25 =1042, or next higher breaker.


hmmm, that sounds about right actually. next higher is, what, 1200 ?
 
electricrw said:
isnt table 450.3b the max rating. should you go down to the next size ocd?
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Yeah, IIRC, the "next higher breaker rule" is rescinded for secondary OCPDs (but look it up before quoting me :grin: because that may be only when the primary isn't protected for max current)
 
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electricrw said:
ya that was a stupid type-o. I mean 450.3b
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Do you have a copy of the NEC? Look up the table and post what you think the answer should be or post as much as you can up to the point where you get stuck. This will help you learn it better.
 
nakulak said:
833 x 1.25 =1042, or next higher breaker.
Click to expand...
This is where article 450 differs significantly, and unexpectedly, from 240. If you are protecting conductors, and if you are over 800 amps, you cannot use the next higher standard size overcurrent protection. If you are protecting transformers, you can. The 800 amp limit is not included in Table 450.3(A) or (B).
 
Originally Posted by electricrw
What is the secondary over current protection on a 300kva transormer 480/120/208v hint; table430.3b
I dont know this answer. need help

U see thats where I get confused. In my question how would a person know if the transformer had primary only protection or primary and secondary protection when using table 450.3b. Not sure what protection method to use. If my question asked for the max secondary over current protection then I could understand using 125%. U see the primary over current protection is unknown. I have had this question befor and got it wrong. trying to understand why
 
If it is multiple choice then I would say that more information is needed, unless all answers but one are completely out in left field.

If it is an essay question, then I think you should be able to choose your method of solution and make your own assumptions outlining them at the beginning of the solution.
 
wirenut1980 said:
If it is multiple choice then I would say that more information is needed, unless all answers but one are completely out in left field.

If it is an essay question, then I think you should be able to choose your method of solution and make your own assumptions outlining them at the beginning of the solution.
Click to expand...

I gotta agree with you, but I think maybe the intent of the test is to make sure the testee at least knows how to figure the max protection (?) So in the absence of other information, max seems to apply.

(I just reread my sentece and it made me wonder - if one person being tested is a testee, is a bunch of them testees ? lol)
 
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