1. The problem statement, all variables and given/known data A single-phase, 2 kVA, 100/200 V transformer is reconnected as an auto-transformer such that its kVA rating is maximum. What is the new rating in kVA? 2. Relevant equations Rating is KV times the current. 3. The attempt at a solution For single winding transformer: O/P voltage is Vab = Vac - Vbc. Vab is 200 V as this is given in question. O/P current is 2KVA/200 is 10A. For auto transformer: O/P voltage is Vac Vac is Vab + Vbc is 200 + 100 is 300V. O/P current is same in both cases that is flowing through the load and it is 10A. So KVA of auto transformer is 300 * 10 equals 3KVA. But books goes through other method where Vac is 200V So Vab is 200 - 100 = 100 V. KVA auto / KVA single phase = Vac/(Vac - Vbc) = 1/(1-Vbc/Vac) = 1/(1-0.5) = 2. So KVA auto is 2 * KVA single phase = 2*2 = 4KVA. Which method to use and why? Why is o/p voltage 200V in case of auto? Shouldn't it be 200 + 100 as i've drawn in picture?