Primary and secondary OCPD for transformers

[JdoubleU]

We are upgrading our electrical system at one of our building. 12,470 goes to a 480 volt transformer, to a 480 volt MDP, then to a 208 volt transformer, and then to a 208 volt MDP. I get confused when in come to OCPD for conductor and OCPD when it comes to the transformer. This question applies to the 208v transformer. How many OCPD do I need if the transform is not in the same room as the 208 volt MDP or the 480 volt MDP. Then, how many OCPD would I need if the 208 volt transformer is in let's say the same room as the 480 volt MDP?


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[don_resqcapt19]

The primary OCPD can be anywhere. If it is not within sight from the transformer, then 450.14 requires that the location of the disconnect be marked on the transformer. If the primary OCPD does not exceed 125% of the primary full load current there is no requirement for the transformer to have secondary OCPD. Most of the time the controlling rules are found in 240.21(C) that apply to the protection of the secondary conductors.

 

[JdoubleU]

Awesome, thank you very much


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[JdoubleU]

When do you size OCPD for primary conductors and OCPD for the transformer and need to install both.


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[waedwards87]

When do you size OCPD for primary conductors and OCPD for the transformer and need to install both.


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In my experience, we protect the weakest link. In a properly designed system consider that you should have an oversized ampacity on the conductors so they do not overheat. The transformer current SHOULD be your limiting factor. If you protect the transformer you should never have a problem with your conductors. They should be protected by OCPD that are upstream.

 

[don_resqcapt19]

When do you size OCPD for primary conductors and OCPD for the transformer and need to install both.


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The rules in Article 450 permit the transformer primary protection to be up to 250% of the rated primary current. The conductors between the primary OCPD and the transformer must have an ampacity equal to or greater than the rating of the primary OCPD.