Originally Posted by Wingnut
So have we gotten to the size of the transformer yet?
We cannot size the transformer until we know the load it is intended to supply. Simply saying 200 amps is just a guess.
I've seen a large number of installations like the OP described that have a 75 KVA transformer, 3 phase, 480 to 120/208 wye. The 480 breaker is usually a 100, and the secondary feeds a panel with a 225 amp main.
The vast majority of these have about 30 amps on the secondary. Way overkill.
If the expected load allows for it, a 45 KVA can be used. It takes a 70 on the 480 side, and a 150 on the secondary.
There's a very good possibility that a 30 KVA will do. This takes a 45 on the primary, and a 100 on the secondary. You'll find that a panel with a 100 amp main is a lot less money than one with a 150 or 225.
The compressor, if it's 3 phase, almost certainly has a dual voltage motor. Usually, a 460-480 volt kit can be ordered. It has a 480 coil for the starter, and new heaters for the now-lower amperage. Easy to install, and it'll run much better on 480. Plus, the transformer can be smaller, and no problems with office equipment.
P.S. Transformers come with 120/208 as well as 120/240 secondaries. The 120/208 is by far the most common. If you get one that's 120/240, it'll have a high leg. Two of the phases will be 120 to neutral, the other will be 208 to neutral. Not suitable for an office.