The main function of a voltage transformer is to change alternating current (ac) voltage from one voltage level to another. The relationship between the primary coil and secondary coil of a transformer is a function of ratio of the voltage or current and number of turns or loop. The ratio of the voltages between the two coils is equal to the ratio of number of loops in the two coils.
Varying the number of winding in the primary and secondary coils allows the transformer to step-up or step-down voltages while power is neither increased nor decreased in the primary or secondary except for some minor losses such as iron losses, copper losses, hysteresis losses, eddy current losses, stray loss, and dielectric losses. Therefore, the power in the primary and secondary are the same. The power supplied to the 20-ohm resistor terminal in the secondary coil will be 400 Watts.