480VAC (from 3 phase 225 amp breaker delta panel serviced directly from City Light) transformer to 240/120 1 phase 200 amp panel

[CAEmerson]

Need to size a transformer and breaker. Source is 480VAC 3 phase into a main panel from city drop. 225 amp breaker in that panel will feed 225 amp 3 phase 480VAC sub-panel. 2 pole breaker in sub-panel (need size) will feed a new 1 phase transformer (480 to 240/120 VAC). I thought I had this figured out, but have become unsure. Originally I calculated: 200 amp at 240 VAC 1 phase => 200 amp * 240 VAC = 48,000 KVA (assuming resistive load). So, I would need a 50 KVA single phase transformer. Breaker in the 3 phase panel would be 2 pole 100 amp (48,000 KVA / 480 VAC => 100 amps).

Is this correct, and if not what should the 1) transformer size be, 2) breaker size and type be, 3) how is this calculated?

Thanks, and stay healthy!
Chris

 

[tom baker]

CA, need to lnow how you are related to the electrical industry, retired from what? PM me.