For a wye-start, delta-run motor, the ampacity of conductors between the controller and the motor must be what percentage of the motor full-load current?

For a wye-start, delta-run motor, the ampacity of conductors between the controller and the motor must be what percentage of the motor full-load current?

• A. 60%
• B. 72%
• C. 80%
• D. 90%

Answer: (B)

In a wye-start, delta-run motor, the wiring between the controller and the motor does not have to carry the motor’s full-load current all the time because the starting arrangement reduces the current drawn from the controller during startup. When starting, the windings are energized in star, which lowers the voltage across each winding and therefore reduces the current in the feeder conductors. This reduced current is why the ampacity of those conductors is specified as a fraction of the motor’s full-load current—approximately 72%. Using about 72% provides enough capacity to handle the startup transient and normal running without oversizing the conductors for the full-load current. The other options would either undersize for the startup surge (60%) or be more conservative than necessary (80% or 90%).

In a wye-start, delta-run motor, the wiring between the controller and the motor does not have to carry the motor’s full-load current all the time because the starting arrangement reduces the current drawn from the controller during startup. When starting, the windings are energized in star, which lowers the voltage across each winding and therefore reduces the current in the feeder conductors. This reduced current is why the ampacity of those conductors is specified as a fraction of the motor’s full-load current—approximately 72%. Using about 72% provides enough capacity to handle the startup transient and normal running without oversizing the conductors for the full-load current. The other options would either undersize for the startup surge (60%) or be more conservative than necessary (80% or 90%).