For a wound-rotor motor, the conductor connecting the secondary to the controller must have an ampacity not less than 125% of the motor's full-load secondary current. Which option correctly states this requirement?

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

Answer: (C)

When sizing conductors for a wound-rotor motor, you have to plan for the rotor current during starting and during control adjustments. The rotor (the secondary) current can spike well above its running value, so the conductor that connects the rotor to the controller must be able to carry those higher currents without overheating. That’s why the rule specifies an ampacity not less than 125% of the motor’s full-load secondary current—the extra margin protects the conductor during surge and ensures safe, continuous operation.

The other options would either underestimate the needed capacity (100% or 110%), risking overheating during startup, or go beyond the minimum more than necessary (130%), whereas the specified standard requirement is 125%.