transformer sizing

[rickl]

first thing, i'm not a expert on transformers, i took this job thinking the power company would supply the transformer ( 120/240 3phase) & do all the primary feed work to the property ( theres is a transformer 10 feet from property line which i figured the pc would come off the primary side and feed a new 120/240 3p transformer). after this morning meeting with the pc engineer at the job site, he said it would be easier if i did all the primary work ( come off the transformer secondary 480 volt & install my on 120/240 3 phase transformer on the property) this is what i'm going to install 200 amp 120/240 3 phase main breaker panel, from this panel i'll feed a 20 x 60 office trailor( thats all i know about it now) and a 240 volt 3phase 7.5 bandsaw. if i use the 200 amp to calculate my tran i'll need a 112500 va does that sound right,
after the meter ( 480 volts) i would have to hit a disconnect before the trans right? i was told i didn't have to. also if the 480 primary is a delta config i cannot get 120/240 only 120/208.
thank

don't have time to proof read my mesage hope every body understands

 

[augie47]

transformer

transformer

rick, I hate to even start. You need to spend a lot of time in Art 450 and 240 as there are so many things to consider. If you are dealing with 3 phase you will probably end up with a 208Y/120 secondary rather than a 240/120 (these transformers won't have the neutral capacity you need). Of course you would need to verify that voltage is compatable with your needs. If its a single phase trailer you might consider two transformers, a single phase 240/120 (probaly at 37.5 kva) and a seperate three phase 480/240 for your saw.
In any event you need to study Art 450 for your transformer protection (and yes primary protection is always required) and Art 240 for overcurrent protection of your conductors.