The maximum circuit breaker rating as primary protection for a transformer of over 1,000 volts with an impedance of 3% in an unsupervised location would be calculated using what percent of the primary current?

The maximum circuit breaker rating as primary protection for a transformer of over 1,000 volts with an impedance of 3% in an unsupervised location would be calculated using what percent of the primary current?

• A. 300%
• B. 600%
• C. 400%
• D. 200%

Answer: (B)

In this case the key idea is how to size the primary protection for a transformer installed in an unsupervised location. For such installations, the guideline is to set the primary overcurrent protective device at six times the transformer’s rated primary current. Expressed as a percent of the primary current, that’s 600%. The impedance value of 3% tells you the transformer’s short-circuit current capability (it would be very high in a fault), but it doesn’t change the rule used to size the primary protection in unsupervised locations—the standard practice is to use six times the rated primary current. That’s why the correct choice corresponds to 600%.

In this case the key idea is how to size the primary protection for a transformer installed in an unsupervised location. For such installations, the guideline is to set the primary overcurrent protective device at six times the transformer’s rated primary current. Expressed as a percent of the primary current, that’s 600%. The impedance value of 3% tells you the transformer’s short-circuit current capability (it would be very high in a fault), but it doesn’t change the rule used to size the primary protection in unsupervised locations—the standard practice is to use six times the rated primary current. That’s why the correct choice corresponds to 600%.