Each phase of a three phase transformer must be considered as a single phase transformer when determining loading.
For example: A 45 kVA three phase transformer with a 208Y/120 volt secondary is to service 4 loads at 120 volts single phase each. These loads are 10 kVA, 5 kVA, 8 kVA,and 4 kVA.
NOTE: that maximum loading on any phase does not exceed 10 kVA. Each phase has a 15 kVA capacity.
If an incorrect method is used, phase B will have an 18 kVA load which is 3 kVA above its normal capacity of 15 kVA and failure will result even though we only have a total load of 27 kVA on a 45 kVA transformer.