The operating duty cycle of a transformer is normally specified as the percentage of time for which the load is drawing its full rated current . It is usual to specify the duty cycle based on a time period of 30 minutes.

Providing a transformer with a low duty cycle enables a physically smaller transformer to be used in situations where it is known that the load will only be used for a certain time during every 30 minute period. This is normally the case with 110 V tool transformers, portable arc welders and car starter-chargers. For most applications a continuous rating is applied.