What is the minimum ampacity percentage applied to conductors supplying the secondary of a wound-rotor motor for continuous duty?

What is the minimum ampacity percentage applied to conductors supplying the secondary of a wound-rotor motor for continuous duty?

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

Answer: (C)

Continuous-duty operation pushes a steady amount of heat into motor conductors, so they must be sized to carry more than the motor’s running current. For a wound-rotor motor, the rotor circuit (the secondary) is energized whenever the motor is running, and its windings and external rotor resistors must not overheat. To provide a safe margin for temperature rise and long-term reliability, the conductors feeding the rotor are required to have an ampacity at least 125 percent of the rotor’s full-load current. That extra margin helps prevent insulation damage and voltage drop issues during continuous operation. The standard minimum is 125 percent.

Continuous-duty operation pushes a steady amount of heat into motor conductors, so they must be sized to carry more than the motor’s running current. For a wound-rotor motor, the rotor circuit (the secondary) is energized whenever the motor is running, and its windings and external rotor resistors must not overheat. To provide a safe margin for temperature rise and long-term reliability, the conductors feeding the rotor are required to have an ampacity at least 125 percent of the rotor’s full-load current. That extra margin helps prevent insulation damage and voltage drop issues during continuous operation. The standard minimum is 125 percent.