Transformer Loading & OCPD Sizing.

[Provision&installation]

Hello,

From what I have gleaned from extensive google-fu, i have found the following to be a possible code compliant transformer installation, my questions will be posed after:

75KVA 480D - 120/208Y

Primary OCPD:

75000 / (480x1.732) = 90.21A ----------> 90.21A x 1.25 = 112.76A --------> Next size up = 125A OCPD

Secondary OCPD (Transformer will feed panelboard with branch circuits):

75000 / (208x1.732) = 208.19A -----------> (assuming continuous load is 100% of transformer rating) 208.19 x 1.25 = 260.23A ---------> Next size up = 300A OCPD

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If what i've suggested above is correct, and one can provide a 300 amp panelboard from a 75KVA transformer, is it then kosher to load this panel up to 80%, which would be 240A/phase and approximately 15% overload of the 75KVA transformer rating? I want to say no, because the load was only calculated for a 75KVA transformer, but i could see people in the field seeing a 300 amp panel with room to spare and not considering the load on the transformer.

I guess i'm also having trouble understanding the usable KVA that can be loaded on a transformer vs. acceptable OCPD.

 

[Smart $]

You've discovered an area of Code that is not [overly] conservative. Let me skip all the minutia and sum it up with "Just because you can don't mean you should".

 

[ron]

As mentioned, the transformer should only be loaded to 75kVA.

If the breakers are regular molded case (not 100% rated), then you can load it to 80% of continuous loading, but no more than 75kVA since that is part of the power path. So the 75kVA is the bottleneck.

 

[charlie b]

Our company standard is to have a 250 amp breaker protecting the 120/208V secondary of a 75 KVA transformer, pretty much for the reasons that were stated already.

 

[Provision&installation]

Our company standard is to have a 250 amp breaker protecting the 120/208V secondary of a 75 KVA transformer, pretty much for the reasons that were stated already.

My thoughts were to provide a 225a breaker so we can stick with a cheaper panelboard and we just won't load it beyond 180a/phase. I don't see the 400 amp panelboard being worth the added cost to get 20 more amps of useful load.