Transformer secondary protection

[ryan_618]

Regarding a 480 to 208Y/120 transformer, does 240.4(F) imply that secondary protection is required from Table 450.3(B)?

How does a person come up with the "voltage ratio" being refered to?

 

[ryan_618]

Re: Transformer secondary protection

I'm replying to my own post here

I didn't read carefully enough to see that the voltage ratio concept doesn't apply. With that in mind, I guess you would have to provide primary and secondary protection, unless you comply with 240.4(C)???

 

[jim dungar]

Re: Transformer secondary protection

Transformer secondary protection is never required.

Lighting and Appliance branch circuit panels located on the secondary need to be properly protected.

Secondary conductors need to be properly protected (tap rules can be used).

If one, or both, of the above required protective devices also is not more than 125% of the transformer secondary current, then the primary OCPD can be sized larger.

 

[ryan_618]

Re: Transformer secondary protection

Originally posted by jim dungar:
Transformer secondary protection is never required.

Thanks for your help Jim, but if that is the case, why is footnote 2 to Table 450.3(B) written?

 

[charlie b]

Re: Transformer secondary protection

Let me expand on Jim?s statement: Secondary protection is never required, but that is because you have the option of keeping the primary protection below the 125% value. On the other hand, if you choose to protect the primary at 200%, then that choice forces you to add secondary protection.

To answer your first question, you look at the line-to-line voltages on both sides of the transformer. On a 480 ? 120/208, the line-to-line voltages are 480 and 208. Therefore, the ?secondary to primary transformer voltage ratio? is 208/480, or about 0.433. Now suppose that, based on the transformer?s KVA rating, you used #4 THHN for the secondary conductors. That wire is good for 85 amps. You multiply 85 times 0.433, and you get 36.8. The next higher standard OCPD rating is 40 amps. Conclusion, the primary OCPD must be rated no higher than 40 amps, else the secondary conductors would not be protected by that primary OCPD. The reason that this is necessary is that a transformer that reduces voltage from primary to secondary will increase current from primary to secondary by the same ratio.