For continuous duty, the conductors connecting the secondary of a wound rotor AC motor to its controller must have an Ampacity not less than what the percent of the full load secondary current of the motor?

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

Answer: (C)

Continuous duty means the motor runs for extended periods, so the conductors feeding the rotor circuit must have extra heating margin. The standard rule is to size that conductor ampacity at not less than 125% of the motor’s full-load secondary current. That 25% cushion accounts for heat buildup from long operation and any normal surges, keeping insulation and connections from overheating. In a wound-rotor motor, the rotor circuit carries current to the controller, so using 125% ensures safe operation at rated load. The other percentages don’t meet this required margin: 100% is too small for continuous duty, 110% underestimates the needed heat allowance, and 130% is more conservative than the minimum.