Smaller Size Sub-Panels Connected to Larger Transformer

[Samardas]

My client has a 45KVA, 480-208/120V, 3 Phase transformer, a 480V, 3 Ph. Distribution Panel and a Sub-panel. He also has a 125A CB on the Distribution Panel. His Sub-panel is rated for 125A. Although the calculation on the primary shows ~70A and the secondary ~156A (including 1.25 multiplying factor), can the above setup be used? In other words, can I use 125A CB on the primary and 125A subpanel on the secondary? There is no distribution panel on the secondary side. I understand the cables also have to be sized accordingly. Can someone advice please? I would truely appreciate a quick response. Thanks a LOT!!!

 

[charlie b]

I don't quite understand the question, mostly because of the way you are using the term "sub panel." That term does not really exist in any official sense.

If you have a 480 volt panel that has an available spare 125 amp breaker, you can use it to provide power to the primary side of a 45 KVA transformer. The limit is 250% of rated primary current. A 125 amp breaker is 231% of the rated primary current of 54 amps.

If you have a 125 amp rated panel with a main breaker of 125 amps, you can feed it directly from the secondary of a 45 KVA transformer. The limit is 125% of rated secondary current. A 125 amp breaker is exactly 100% of the rated secondary current of 125 amps. I would prefer to use a 150 amp panel, so as not to waste the available capacity.

 

[texie]

My client has a 45KVA, 480-208/120V, 3 Phase transformer, a 480V, 3 Ph. Distribution Panel and a Sub-panel. He also has a 125A CB on the Distribution Panel. His Sub-panel is rated for 125A. Although the calculation on the primary shows ~70A and the secondary ~156A (including 1.25 multiplying factor), can the above setup be used? In other words, can I use 125A CB on the primary and 125A subpanel on the secondary? There is no distribution panel on the secondary side. I understand the cables also have to be sized accordingly. Can someone advice please? I would truely appreciate a quick response. Thanks a LOT!!!

Based on your description the primary OCPD can be up to 250% as long as the the secondary is not more than 125% of the transformer capacity. So it would seem that this is fine.

 

[Samardas]

Smaller Size Sub-Panels Connected to Larger Transformer

Thanks to both Texie and Charlie. I also think he is wasting the capacity of the transformer. However, since he wants to use what he has, I will go ahead and design it based on your advice. Again, "Thanks a LOT" for your quick responses.