NEC Commercial Electrical Practice Test

For a wye-start, delta-run connected motor, what percentage of the motor full-load current is used for selecting conductors?

1. 50 percent

2. 58 percent

3. 65 percent

4. 75 percent

The correct answer is: 58 percent

In a wye-start, delta-run configuration, the starting current of the motor is significantly higher than the full-load current. When the motor is started in a wye configuration, the voltage across each winding is reduced to 1/√3 or approximately 58% of the line voltage. This lower voltage results in the motor drawing about 50% of its full-load current during the wye start phase. However, when selecting conductors, it is crucial to consider the maximum current draw, which occurs when the motor transitions to the delta configuration. At this point, the motor will run at full-load currents once it is up to speed. For conductor sizing, the NEC (National Electrical Code) often references the starting current as a guideline. The appropriate size for conductors should thus be based on the maximum full-load current, which, in the case of a wye-start, delta-run setup, translates to considering approximately 58% of the full-load current for continuous application after start-up. This aligns with common practices in the industry, emphasizing the importance of ensuring that conductors can handle the expected load with a suitable safety margin. Therefore, using 58% of the motor full-load current for selecting conductors in this