Step down autotransformer Question

[osh1397]

Per NEC Handbook
Nse + Nc = 1440, Nc = 1200, SW = 60 kVA.

Since it's a step up autotransformer:

Sio = 60 kVA * 1200 / 1440 = 50 kVA.

However, the answer is


Where is my mistake?

 

[no_concentrate]

Nse + Nc = 1440, Nse = 240 ( 1440-1200)

Sw = (1440-1200)*250 =60 kVA.

(Sio/Sw) = (Nse + Nc)/Nse

Sio = ((Nse + Nc)/Nse )*Sw

Sio = 60* (1440 /240) = 360kVA

(Nse + Nc)/Nse formula is in handbook at page no 61.

Otherwise you can also calculate

IL*VL = IH* VH

(300A)*1200 = (250*1440) = 360 kVA