When connecting a transformer to an electrical power source, you need to calculate the current, it will draw through the primary. You should then hook the transformer up to a circuit breaker of an equal or higher current rating so that the breaker will not trip under normal operation of the transformer. Make use of this simple online transformer current calculator to calculate the flow of current (I) in the transformer. It is the rate of change of power with respect to velocity.
Calculate the flow of current in the transformer having a power rating of 5 VA and voltage of 7 volts.
I = 5 / 7
= 0.7143 A