Main breaker size

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KMeni

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I have a 300KVA 12.47kV-208V 3P 4W Transformer with FLA 833A at 208V.
Service Switchgear has an C-H MDL 800A Main CB. Is the of the Main Circuit to High??
 
I must not understand your question as 832A is 300kVA/208*1.7321. Then you downsize to the next breaker since it is over 800A which would be a 800A breaker.
 
Bjenks said:
I must not understand your question as 832A is 300kVA/208*1.7321. Then you downsize to the next breaker since it is over 800A which would be a 800A breaker.
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You "may" upsize to the next higher standard size rating as long as it does not exceed 800 amps.

So yes it looks like you would have to downsize or install a larger service.
 
In terms of transformer protection (not conductor protection) what Code section prohibits you from going to the "next largest" when the OCP is over 800 amps ?
OP did not mention conductor size and if the conductor were protected, would would 450.3(a) not allow you to go to a 1000 amp ?
 
I am making an assumption he is talking about secondary protection only since he mentioned 800A. Table 450.3(B) says it can't be more than 125% of the current rating. Note 1 does say you can go up to the next size, but you still have to honor 240.4(C). So I guess you can put a 1000A breaker in there as long as the conductor are rated for 1000A. I don't see how you could put 800A of conductors in and protect it with a 1000A breaker.
 
augie47 said:
In terms of transformer protection (not conductor protection) what Code section prohibits you from going to the "next largest" when the OCP is over 800 amps ?
OP did not mention conductor size and if the conductor were protected, would would 450.3(a) not allow you to go to a 1000 amp ?
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With out pulling out my book, if 450.3(A) tells you that you can protect it at 125%, etc etc, then you may be correct and I won't say that I didn't read the question wrong. I saw the FLA at 833 and thought that was his load.
 
Bjenks said:
I am making an assumption he is talking about secondary protection only since he mentioned 800A. Table 450.3(B) says it can't be more than 125% of the current rating. Note 1 does say you can go up to the next size, but you still have to honor 240.4(C). So I guess you can put a 1000A breaker in there as long as the conductor are rated for 1000A. I don't see how you could put 800A of conductors in and protect it with a 1000A breaker.
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I'm not saying that you can, but, KMeni did not state his conductor size, he asked about the breaker rating. Also, I would not want to use that size breaker for my transformer secondary protection. I was simply pointing out that 450.3(a) {the applicable table} has allowances for a larger breaker.
We would need a lot more specifics to determine the required size for this application.
 
I will admit that I didn't even notice I was looking at the 600v or less table and not the > than 600V. You are correct we just don't have enough info to give a full answer.
 
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