Which statement best describes the ampacity requirement for conductors supplying the secondary of a wound-rotor motor under continuous duty?

• A. 100%
• B. 110%
• C. 125%
• D. 130%

Answer: (C)

For continuous-duty motor circuits, you size the supply conductors at 125% of the motor’s full-load current. In a wound-rotor motor, the rotor winding (the secondary) carries current through the external resistor bank, and this current can cause significant heating over the motor’s extended run time. To keep temperature rise within limits during continuous operation, the rotor-side conductors and resistor connections must be rated for at least 125% of the motor’s full-load current. That’s why 125% is the correct requirement. The other percentages don’t meet the standard protection for continuous-duty motor circuits.