NEC Requirements for Office Load

[Carter216]

I am working on adding office space to a manufacturing plant. I need a transformer to supply this space. 480V-208/120V. My secondary load is a 125A main lug 24 space panel feeding (18) duplex receptacles and (16) 110V LED fixtures. There will also be (4) 20A ptac units.

Choosing a transformer, I assumed 200A secondary.
sqrt(3) * 200A * 208V = 72 KVA --> 75KVA transformer

Should I need this big of a transformer? What does the NEC state about load currents in this environment? Also, calculations in 120V vs 208V?

 

[jaggedben]

Familiarize yourself with article 220.

The transformer calculation is correct based on 200A but why are you assuming 200A for that when you stated you have a 125A panel? A 125A panel would only need 45KVA.

I'm also guessing your actual load is substantially less than either 75 or 45kVa. The NEC requires 180VA per receptacle so that's 3240VA. Then we need the nameplate on the A/C units and LEDs although the latter is probably a rounding error compared to the former.

 

[charlie b]

Also, you can't feed a 24 circuit MLO panel directly from a transformer. The panel must have a main breaker or there must be a separate overcurrent device between the transformer and the panel.