Secondary Side of Transformer Disconnect???

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jwatts

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Doing a job where the engineer has a 150kva 480/120-208v transformer being fed with a 250amp breaker in the distribution panel adjacent to the transformer. The secondary side then shows a 400amp disconnect fused at 300amps feeding a 400amp MLO panel. First, the 150kva transformer is only rated at 180amps @480v so I am not sure why there is a 250 amp breaker. My question is whether or not a fused disconnect is required on the load side of the transformer. Would a nonfused 400amp disconnect suffice since I am feeding a 400a MLO panel? Or according to the table in NEC2011 450.3(B), if I were to install a 225amp breaker, I would not need a load side disconnect because the OCPD would be equal to 125% of the rated input current. I am not certain I am interpretting the code correctly.
 
No, a 400A non-fused disconnect switch no the secondary side would not suffice. You need to protect the transformer secondary conductors per 240.21(C), and you need to protect the panelboard per 408.36(B).
 
would exception #1 in 408.36 apply if I only have 5 switches in the panelboard... which would allow for not protecting the panelboard with a OCPD?
 
jwatts said:
... Or according to the table in NEC2011 450.3(B), if I were to install a 225amp breaker, I would not need a load side disconnect because the OCPD would be equal to 125% of the rated input current. I am not certain I am interpretting the code correctly.
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You would not need secondary overcurrent protection for the transformer in that case, but the secondary conductors must be protected per one of the rules in 240.21(C). You have to be sure that you satisfy the rules in both Article 450 and 240.
In general the rules in 450 apply to the protection of the transformer itself and the rules in 240 apply to the protection of the primary and secondary conductors.
 
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