Sizing transformers

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n240sxy

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How do you size a 277/480V to 120/208V transformer that has a 200A circuit breaker on the primary size? Once the transformer has been sized, how would you size the secondary 120V Panel and it's main circuit breaker?
 
In general you calculate the load you need to supply, that determines the size of the transformer and to some extent the over current protection and conductor sizes you will use.
 
I think you generally size a transformer based on how much power you need. what are the requirements on the secondary ? x safety factor = size of transformer
 
it's not wired. it is in design stage. For a 225A 120/208V, 3ph, 4W panel, what should the transformer and primary side protection size be designed for?
 
appears to be 112kva, but you are assuming that you have a fully loaded 225A panel which may or may not be the case., knock off a few amps and you could easily get by with 75.
 
nakulak said:
appears to be 112kva, but you are assuming that you have a fully loaded 225A panel which may or may not be the case., knock off a few amps and you could easily get by with 75.
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I believe you can use a 75kVA transormer and use a 225A panel main breaker as the transformer secondary protection device (within 125% of transformer secondary) and still be o.k.
 
In this area, based on 480-208Y/120, many people use:
50A breaker -> 30kVA -> 100A panel
80A breaker -> 45kVA -> 150A panel
100/125A breaker -> 75kVA -> 200/225A panel
150A breaker -> 112.5kVA -> 400A panel
200/225A fuse/breaker -> 150kVA -> 600A panel
 
n240sxy said:
it is in design stage. For a 225A 120/208V, 3ph, 4W panel, what should the transformer and primary side protection size be designed for?
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How did you select a 225A panel in the first place? That should not be step 1, as others have already said. First, you calculate the load. Then you pick the equipment ratings (transformer and panel). The last step is picking the overcurrent settings.

There is nothing in the code that would prevent you from feeding a 225 amp panel with a 5 KVA transformer, so long as the calculated load is less than 5 KVA. You have to start by knowing the load.
 
mull982 said:
I believe you can use a 75kVA transformer and use a 225A panel main breaker as the transformer secondary protection device (within 125% of transformer secondary) and still be o.k.
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Yes you can and it is a popular set up but the load must remain under 208 amps.

If your design for 225 amp panel is so close that the load is greater then 208 amps you would have to use the 112.5 transformer.
 
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