TRANSFORMER CALCULATIONS

[Josh Clark]

It appears to me that there is a mistake in Mike Holts Electrical Exam Preparation Handbook. Example 1 on page 243 assumes that there is only primary protection on the 3 phase transformer feeding a panel however 408.36 (B) requires that panelboards supplied by a transformer have overcurrent protection on the secondary side of the transformer which in turn would provide secondary protection of the transformer. The exception only applies to single phase transformers. Given this, the maximum rating of the primary overcurrent protection for the transformer should be 250% not 125%. The answer given is misleading to me. Is there something I am missing?

Thanks

 

[petersonra]

I think three phase Delta-delta transformers can also have primary only over current protection.

 

[Josh Clark]

You are correct for a delta-delta transformer however the example is for a 480 to 208/208 transformer which would indicate a delta-wye. I tried to include a picture of the example but I couldn't find a way to include in my post

 

[Josh Clark]

I meant 208/120 not 208/208

 

[electrofelon]

I am unsure of the specifics of the example, but note 450.3 transformer protection is for the transformer ONLY. There are other requirements for an OCPD for a panelboard (as you note) and also the transformer secondary conductors (240.21(C). One OCPD can serve all 3 of these requirements but doesn't have to. Perhaps the example was specifically and only about the transformer protection?

 

[jim dungar]

The answer given is misleading to me. Is there something I am missing?

It would be possible for the panel board main OCPD, required per 408.36, to be too large to count as the secondary side transformer protection per 450.3.

 

[Josh Clark]

Jim Dungar, thanks for understanding my question. I’ll look into that, I think that was they key I was missing, thanks for your response.