1. The problem statement, all variables and given/known data An 11kV to 415v transformer has a rating of 100kVA. The winding resistance and leakage reactance when referred to the secondary are 0.0142 ohms and 0.0569 ohms respectively. Determine the % regulation of the transformer for a 0.8 lagging power factor. 2. Relevant equations Voltage regulation = (No load secondary voltage - rated secondary voltage) /(rated secondary voltage) * 100 3. The attempt at a solution I have come across a few formulas to solve this (this easiest seems to be the one stated above. I assume that the rated voltage would be 415V but not sure how to work the no load voltage? I also found another formula: I2(full load)/V2(fl) *(R(secondary) * cos∅ +Xs sin∅) * 100 I calculated I2 at 100000/415 = 241A ∴241/415 ((0.0142*0.8)+(0.0569*sin36)) *100 which gives me 0.581(0.01136)+(-0.056)*100 This gives me a % of -2.62%. Im guessing this is incorrect as result is a minus. If someone could point out where im going wrong but keep it in laymans terms as im just learning this stuff it would be much appreciated.