Breaker size

240.21(C) {1st paragraph} states 240.4(B) shall not be applied to transformer secondary conduction. If you secondary breaker is 800 amps your conductor ampacity must be 800. Parallel 500s won't work.

(Your numbers seem incorrect. 800 amp would be a "normal" breaker for the primary (480v) on a 500 kva)
 
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augie47 said:
240.21(C) {1st paragraph} states 240.4(B) shall not be applied to transformer secondary conduction. If you secondary breaker is 800 amps your conductor ampacity must be 800. Parallel 500s won't work.
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Agree with 240.4(b) wont apply. According to NEC 2017 Article 240.21(C)(6)(1) amapcity shall not be less than 1153A. It doesnt say shall not be less than 800A. How?
 
NEC 2017 Article 240.21(C)(6)(1) says ampacity of the secondary conductors, which is not less than the value of the primary-to-secondary voltage ratio multiplied by one-third of the rating of the overcurrent device protecting the transformer’s primary.

When I do all that minimum secondary conductor ampacity comes to 1153A for those secondary conductors. Am I doing something wrong?
 
hhsting said:
So then how can minimum amapcity fir conductors be 800A as mentioned here before?
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If the secondary conductors have to use part (6) of 240.21(C), the minimum ampacity is higher as you calculated. But if they are outdoors, then part (4) does not have the 1/3 requirement, so 800A is sufficient for an 800A secondary OCPD. Or if they are under 10' in length, then part (2) only requires 1/10 instead of 1/3, and so again 800A is sufficient.

Cheers, Wayne
 
wwhitney said:
If the secondary conductors have to use part (6) of 240.21(C), the minimum ampacity is higher as you calculated. But if they are outdoors, then part (4) does not have the 1/3 requirement, so 800A is sufficient for an 800A secondary OCPD. Or if they are under 10' in length, then part (2) only requires 1/10 instead of 1/3, and so again 800A is sufficient.

Cheers, Wayne
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Transformer is indoor and secondary conductor length is 20 feet. Would 800A breaker have lugs size for 1153A typically?
 
hhsting said:
See NEC 2017 Article 240.4 first paragraph that says exceptions to 310.15 including 240.4(b)
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You have to understand that Article 240 only deals with overcurrent protection of the conductors and does not have anything to do with the ampacity of those conductors, other than the OCPD is based on the conductor ampacity.
As I said before, the rule in 240.4(B) just permits the OCPD to be rated higher than the conductor ampacity but does not change the conductor ampacity.
 
augie47 said:
240.21(C) {1st paragraph} states 240.4(B) shall not be applied to transformer secondary conduction.
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That was added in the 2005 code for the very reasons being talked about in this thread. The rules in 240.21 require the conductors have a specific ampacity and the rule in 240.4(B) does not change that ampacity. The panel rejected the proposal, but a public comment by one of the panel members brought it back and it became part of the code.
 
don_resqcapt19 said:
That was added in the 2005 code for the very reasons being talked about in this thread. The rules in 240.21 require the conductors have a specific ampacity and the rule in 240.4(B) does not change that ampacity. The panel rejected the proposal, but a public comment by one of the panel members brought it back and it became part of the code.
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The Secondary conductor in question is an existing condition they are just adding new feeder breaker in the existing 800 A secondary panelboard. Not sure when it was installed.

Has any AHJ made you change it out to correct size under that situation?
 
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