Transformer secondary conductor protection 240.21(C)(6)

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mull982

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I am trying to determine whether or not a set of transformer secondary conductors are adequately protected by a transofmer secondary breaker. A 225KVA transformer has (2) sets of 500MCM cables feeding a 208V panel which is 16ft away having a main breaker rated at 800A. Now I know that 240.21(C) does not allow the provisions of 240.4(B) to be used to allow the (2) sets of 500MCM with a combined ampacity of 760A to be protected by an 800A breaker.

I'm trying to determine however if there are any other provisions in 240.21(C)(1-6) that would allow these secondary cables to be protected byt the 800A secondary breaker. It would appear that all the provisions require that the secodnary breaker be no larger than the rating of the secondary conductors.

240.21(C)(6) appears to have a provision for the secondary conductors being rated at a fraction of the primary protective device, but also requires that that secondary conductors terminate into a single breaker that is no larger than the conductor ampacity.

Is it correct to say that the 800A breker does not adequately protect these cables and must be reduced to a 750A breaker or less?
 
Several questions, not enough information to answer your question

Several questions, not enough information to answer your question

What is the insulation on the conductors? THHN, TW, ...
Are the paralleled conductors in one raceway or two?
What is the temperature rating of the raceway?
What is the temperature rating of the terminations for the conductors? 60?, 75?, or 90?
What is the actual or calculated load?
What is the nature of the load? Lighting, outlets, motors, ...
What is the circuit? 120/208

Answering those questions and looking at articles 215, 240, 310, 430, & 450 will lead us to the answer.
 
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Mull I think you pretty well reasoned this out yourself. 240.4(B) cannot apply and none of the C1-C6 situations allow for upsizing. The full load rating of the transformer is 625A. You also want to look at the requirments in Article 450. There is no 750A breaker that I am aware of? Per 450.3(A) assuming it is less than 6%Z then you could use an 800A breaker on the secondary. If you had 90C rated termination on both the breaker and the transformer (doubt it but you could check) the 500's would be OK. Would need 3x350 or 4x250 in this situation.
 
BPoindexter said:
Per 450.3(A) assuming it is less than 6%Z then you could use an 800A breaker on the secondary
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450.3(A) applys to transformers over 600A and deals with primary protection at over 600V.

Also these tables in 450.3(A) deal with required protection of the transformer and not necessarily the cables which we are talking about here and are addressed in 240.21?
 
Sorry should have been table 430.3(B) my bad. Still 125% protection for transformer secondary though which still comes out to 800A. 625 x 1.25 = 781 per note 1 next size up is 800A.

I was only pointing out the protection required for the transformer which is most likely where they got the 800A breaker. Did not mean to distract you with that. Which brings us back to my original statement based on 240.21 and 310.16:

BPoindexter said:
Mull I think you pretty well reasoned this out yourself. 240.4(B) cannot apply and none of the C1-C6 situations allow for upsizing............ If you had 90C rated termination on both the breaker and the transformer (doubt it but you could check) the 500's would be OK. Would need 3x350 or 4x250 in this situation.
Click to expand...
 
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