For a transformer rated below 600 volts with coordinated thermal overload protection, the primary overcurrent protection may be set at up to what percent of the transformer's rated current for a transformer with 8% impedance?

For a transformer rated below 600 volts with coordinated thermal overload protection, the primary overcurrent protection may be set at up to what percent of the transformer's rated current for a transformer with 8% impedance?

• A. 125
• B. 225
• C. 400
• D. 300

Answer: (C)

Understanding how impedance and coordination affect protection helps here. A transformer's impedance determines how big the fault current on the primary would be if the secondary were shorted. With an 8% impedance, the primary short-circuit current is about 100/8 = 12.5 times the transformer’s rated primary current. That means faults can drive currents many times what the transformer normally carries. When you have coordinated thermal overload protection on the transformer, the primary overcurrent device can be set higher than the transformer's normal full-load current. The idea is to avoid nuisance trips from temporary overloads or inrush, while still providing fast clearance for actual faults. The NEC setting guidance for this scenario allows the primary overcurrent protection to be as high as four times the transformer's rated primary current (400%). So, even though a fault could pull well above that (since it would try to reach the 12.5× rated fault current), the protection strategy relies on the transformer’s own thermal overload protection to handle sustained overloads, and the primary device trips for short-circuit conditions. That balance is why 400% is the best answer here.

Understanding how impedance and coordination affect protection helps here. A transformer's impedance determines how big the fault current on the primary would be if the secondary were shorted. With an 8% impedance, the primary short-circuit current is about 100/8 = 12.5 times the transformer’s rated primary current. That means faults can drive currents many times what the transformer normally carries.

When you have coordinated thermal overload protection on the transformer, the primary overcurrent device can be set higher than the transformer's normal full-load current. The idea is to avoid nuisance trips from temporary overloads or inrush, while still providing fast clearance for actual faults. The NEC setting guidance for this scenario allows the primary overcurrent protection to be as high as four times the transformer's rated primary current (400%).

So, even though a fault could pull well above that (since it would try to reach the 12.5× rated fault current), the protection strategy relies on the transformer’s own thermal overload protection to handle sustained overloads, and the primary device trips for short-circuit conditions. That balance is why 400% is the best answer here.