Transformer for 2 - 200 amp 120/208 panels

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Matthew132

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Detroit
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Electrician
the job I’m currently running has a 120/208 3ph panel with a 200amp main, and it feeds through to another 200amp 120/208 3ph panel. The total load on the two panels is ~110kva. I'm having trouble understanding why I can feed this with a 75kva transformer. The engineer assured me it works like this, can someone explain what I'm not understanding?
 
How did you derive the 110 kva load ?
If the actual load is less than the transformer output and the transformer has Art 450 proper OCP on the primary (125%) then there is no violation.
Similar situation to adding up all the breakers in a 200 amp panel.
 
Connected load vs calculated load.

Calculated considers non-simultaneous loads like AC and heat.
 
Matthew132 said:
the job I’m currently running has a 120/208 3ph panel with a 200amp main, and it feeds through to another 200amp 120/208 3ph panel. The total load on the two panels is ~110kva. I'm having trouble understanding why I can feed this with a 75kva transformer. The engineer assured me it works like this, can someone explain what I'm not understanding?
Click to expand...

How is the 110kva figure arrived at? Calculated per NEC or measured? This design could be based on the conservative nature of NEC load calcs I suppose, where the actual load will be under 75kva. There isnt really a specific article that says a transformers nameplate shall not be exceeded, but the 450.3 protection has to comply.
 
augie47 said:
How did you derive the 110 kva load ?
If the actual load is less than the transformer output and the transformer has Art 450 proper OCP on the primary (125%) then there is no violation.
Similar situation to adding up all the breakers in a 200 amp panel.
Click to expand...
The 110kva was calculated by the engineer who designed the system. Not sure how he came up with that, but I know it's the total demand, not total connected load.
 
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