Rule #1: KVA = KVA. By that I mean that (neglecting losses) the KVA supplied by the primary is the same as the KVA taken from the secondary by the loads. That is also to say that amps do not equal amps, and that volts do not equal volts.
Rule #2: When you deal with three phase systems, the factor of “the square root of three” (about 1.732) is going to sneak its way into the calculation somehow. The trick is in knowing where it belongs.
You have 400 amps of 115 volt, single phase loads. Multiply 400 times 115 and you get 46,000 VA, or if you prefer, 46KVA. That is what the loads are going to draw from secondary side. That is also what the primary side is going to have to supply.
That is the minimum size transformer you will need. But this calculation did not take any internal losses into account. So you should get at least the next higher standard size transformer. I usually deal with 480 – 120/208 volt systems, and if that were your operating voltages I would pick a 75 KVA for your application. But I don’t know what the standard sizes are for a 450-115 delta-delta transformer.
Did you also need to determine the primary side current? The primary side is a three phase connection. So the 1.732 factor is going to be involved. Note that the secondary is also a three phase connection. But you gave us the data in terms of single phase loads. So you take the 46,000 VA, divide it by the 450 volts of the primary side, and divide it again by the 1.732 factor, and you will find the primary current will be about 59 amps.
