OCPD for transformer secondary

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texie

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In vetting some exam questions....

4/0 AL 75* conductors on the secondary of a 208Y transformer. What is the maximum OCPD allowed?
The author says it is 200 amp. I say this should be max 175 per 240.21(C).
What say all?
 
texie said:
In vetting some exam questions....

4/0 AL 75* conductors on the secondary of a 208Y transformer. What is the maximum OCPD allowed?
The author says it is 200 amp. I say this should be max 175 per 240.21(C).
What say all?
Click to expand...
Have you considered Art 450 table 450.3(A)?
Also, why did you omit the KVA rating of the transformer in your post?
 
texie said:
In vetting some exam questions....

4/0 AL 75* conductors on the secondary of a 208Y transformer. What is the maximum OCPD allowed?
The author says it is 200 amp. I say this should be max 175 per 240.21(C).
What say all?
Click to expand...
Depends on how exhaustive you want to make it.

4/0 AL 75* conductors have an allowable ampacity of 180A. They could be installed in an ambient of 10?C or less, so 180A ? 1.20 correction factor = 216A.

:D
 
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What exactly did the question ask for? The rules in 450 for the protection of the secondary winding of the transformer are not the same as the rules in 240 for the protection of the secondary conductors.
 
don_resqcapt19 said:
What exactly did the question ask for? The rules in 450 for the protection of the secondary winding of the transformer are not the same as the rules in 240 for the protection of the secondary conductors.
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I know that and that is why I think the answer of 200 amps is wrong. The question is what is the maximum OCPD for 4/0 AL 75* conductors on the secondary of a Y transformer. In my view 450 rules have nothing to do with the question. 240.21(C) applies. It would not matter what size the transformer is (as long as the transformer is protected per 450 rules). If you only have 180 amps of conductor ampacity on the secondary, the max breaker would be 175 amps (largest standard size).
 
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I would agree, for a transformer secondary with 4/0 AL, 175 would be the correct size
 
texie said:
I know that and that is why I think the answer of 200 amps is wrong. The question is what is the maximum OCPD for 4/0 AL 75* conductors on the secondary of a Y transformer. In my view 450 rules have nothing to do with the question. 240.21(C) applies. It would not matter what size the transformer is (as long as the transformer is protected per 450 rules). If you only have 180 amps of conductor ampacity on the secondary, the max breaker would be 175 amps (largest standard size).
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Correct, but when I see "transformer secondary" I only think of 450. If you want me to think of Article 240, you need to use the term "transformer secondary conductors".

I know that what was posted did not give enough information to get the correct size of the transformer secondary OCPD, but we have not seen the complete question. Often questions are written with information that had nothing to do with the correct answer to the question.
 
don_resqcapt19 said:
Correct, but when I see "transformer secondary" I only think of 450. If you want me to think of Article 240, you need to use the term "transformer secondary conductors".

I know that what was posted did not give enough information to get the correct size of the transformer secondary OCPD, but we have not seen the complete question. Often questions are written with information that had nothing to do with the correct answer to the question.
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Exactly. To my point of my previous post.
 
texie said:
In vetting some exam questions....

4/0 AL 75* conductors on the secondary of a 208Y transformer. What is the maximum OCPD allowed?
The author says it is 200 amp. I say this should be max 175 per 240.21(C).
What say all?
Click to expand...

I agree assuming what the actual question was asking but as Smart pointed out vague questions can have many answers.
 
infinity said:
I agree assuming what the actual question was asking but as Smart pointed out vague questions can have many answers.
Click to expand...

Yes, should the OP include all the necessary information most question could be addressed in a reply or two.
But, it's a lot more fun the guess and to make assumptions on what the OP is trying to ask which is all to familiar.
 
It is a mystery to me why you all are making such a big deal out of this. It is a simple question and it has enough info to answer the question. Why not just look at the question and answer based on the info given. This has nothing to do with the size of the transformer and article 450. That I thought that would be obvious, but evidently you all have such egos that you can't see the forest for the trees.
 
templdl said:
Yes, should the OP include all the necessary information most question could be addressed in a reply or two.
But, it's a lot more fun the guess and to make assumptions on what the OP is trying to ask which is all to familiar.
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Gee, forgive for not including all the info you deem necessary. I didn't think I need to state the obvious.
 
don_resqcapt19 said:
Correct, but when I see "transformer secondary" I only think of 450. If you want me to think of Article 240, you need to use the term "transformer secondary conductors".

I know that what was posted did not give enough information to get the correct size of the transformer secondary OCPD, but we have not seen the complete question. Often questions are written with information that had nothing to do with the correct answer to the question.
Click to expand...

Condescending nonsense. I'll think long and hard before I continue around here.
 
texie said:
Condescending nonsense. I'll think long and hard before I continue around here.
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Ahh... come on now. Lighten up... please. I consider you a valuable member of this forum. :happyyes:

I've been avoiding additional posts to this thread simply to avert getting anyone to this point. I apologize for those that haven't the foresight or the respect.
 
texie said:
Condescending nonsense. I'll think long and hard before I continue around here.
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The title of the thread is "OCPD for transformer secondary". The body of the OP does not exactly match the title...I have no idea what the actual question was.
 
texie said:
It is a mystery to me why you all are making such a big deal out of this. It is a simple question and it has enough info to answer the question. Why not just look at the question and answer based on the info given. This has nothing to do with the size of the transformer and article 450. ....
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It's a good question - a bit trickey, but still good. As noted, "transformer secondary" pulls one toward art. 450.

Transformary secondary conductors should lead one back to 240.21.C - as you pointed out in post 1.

And, of course as noted by jumper, 240.21.C does not allow upsizing per 240.4.B.

texie said:
....That I thought that would be obvious, but evidently you all have such egos that you can't see the forest for the trees.
Click to expand...
Oh No. Tell me it's not so.:eek: Where could you possibly have gotten that from? :rotflmao:

ice
 
texie said:
It is a mystery to me why you all are making such a big deal out of this. It is a simple question and it has enough info to answer the question. Why not just look at the question and answer based on the info given. This has nothing to do with the size of the transformer and article 450. That I thought that would be obvious, but evidently you all have such egos that you can't see the forest for the trees.
Click to expand...


The question was obvious to me and I said so in post #11, but the answer that you wanted, as Smart pointed out 175 amps is not the only answer based on the information provided. Now in the real world would anyone ever have a test question that uses an ambient of 10? C, I doubt it.
 
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