Just saw this thread, and have a quick comment;
Per the 75 KVA rating...
On the Primary (Delta) Side:
At Full Load, each of the (3) Windings will draw 52.08333- Amps at 480V, with a resulting 25 KVA per Winding.
Input Phase Terminations at Full Load will carry 90.208333- Amps, which is 52.08333- Amps x 1.732.
With a Balanced, Full Load output on the Secondary side, all (3) Input Phases (Phase "A", Phase "B" and Phase "C") will carry 90.208333- Amps.
Multiplying 90.2 Amps by the Three-Phase Voltage of 831 (480v x 1.732 = 831.36) equates in a total Apparent Power flowing in the (3) Primary Windings of:
74,956.2 VA
or simply 75.0 KVA
On the Secondary (Wye) Side:
At Full Load, each of the (3) Windings will carry 208 Amps.
The Voltage Ratio is 4:1, so each Secondary Winding is 120V.
With 208 Amps flowing per 120 V Winding, each Secondary Winding has an Apparent Power rating of 24,960 VA, or simply 25 KVA.
Connecting the (3) Windings together in a Star formation results in a Winding to Winding Voltage of 208 V (120 V x 1.732 = 207.84 V).
Multiplying 208 Amps by the Three-Phase Voltage of 360 (208 V x 1.732 = 360.256 V) equates in a total Apparent Power flowing in the (3) Secondary Windings of:
74,933.248 VA
or simply 75.0 KVA.
--Scott (EE)
