Sizing Transformer

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Butler

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What size transformer do I need if the primary voltage is 480 volts 3phase and the secondary will feed a 120/208 volt 3 phase 100 amp panel.
 
Butler said:
What size transformer do I need if the primary voltage is 480 volts 3phase and the secondary will feed a 120/208 volt 3 phase 100 amp panel.
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Assuming the panel has a 100 AMP MCB:
VA = [(100 A)x(208 V)x3^.5]/1.25 = 28,822 VA

I would size a 480/277 - 208/120 30 KVA transformer.:cool:
 
Assuming the panel has a 100 A MCB:

KVA = [(100 A)*(208 V)*3^.5]/(1000*1.25) = 28.83 KVA

Transformer: 480/277 - 208/120 V, 30 KVA
 
I would size the transformer for the load fed, not the size of the panel, depending on the situation.
 
what is 1.25

what is 1.25

U R absolutely correct Mr. Bcorbin....

KVA = [(100 A)*(208 V)*3^.5]/(1000*1.25) = 28.83 KVA

In above formula What is 1.25?
 
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1.25 is 125%. I calculated the transformer KVA size based on the 100A main circuit breaker. The 100 A main circuit breaker rating is 125% of the panel load, based on amps.

To size the 100 main circuit breaker
I*1.25=100 AMP, I = 80 AMP.
 
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