Transformer Protection

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jap

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Electrician
I've noticed on most of the Prints I've seen that the Primary and Secondary protection are at sized at 125%.
Take for instance 480v Delta to 120/208v.
30kva 45 amp p, 104 amp s,
75kva 113 amp p, 260 amp s,
112.5 169 amp p, 390 amp s,

But 450.3b indicates for primary and secondary protection you could use
250 % for the primary and 125% for the secondary.

Why are they sizing the primary OCP at 125%? Doesnt this apply if there is primary protection only on transformers that have currents of 9 amps or more?
 
250% is a maximum. I generally don't encounter a need to use the maximum. I use 125%, rounded up to the nearest standard OCPD size, because that is usually enough to prevent a trip during inrush, and because it provides a better degree of protection.
 
If the Inrush does Neusance trip the Primary and you increase the size of the Primary OCP.Then are the Primary conductors that were originally installed at 125% still considered protected ? Or does the size of the Primary conductors need to be increased also.
 
jap said:
If the Inrush does Neusance trip the Primary and you increase the size of the Primary OCP.Then are the Primary conductors that were originally installed at 125% still considered protected ? Or does the size of the Primary conductors need to be increased also.
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There is no provision of the NEC that lets you use an over sized OCPD on the primary conductors. They must be protected at or below their ampacity.
 
don_resqcapt19 said:
There is no provision of the NEC that lets you use an over sized OCPD on the primary conductors. They must be protected at or below their ampacity.
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Thats my point.Seems like all the more reason not to be too low on sizing the Primary OCP to begin with. Thanks,
 
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