For a motor on a 120/240-volt, 3Ø, 4-wire panelboard, what size SO cord is considered minimum for flexible connections?

The minimum size of SO cord for flexible connections in a 120/240-volt, 3-phase, 4-wire system is determined by the ampacity required for the motor. The SO cord must be sized appropriately to handle the current load without risking overheating or failure, while also complying with the National Electrical Code (NEC) requirements.

In this specific case, a size 9 SO cord would typically indicate a wire gauge that meets the necessary criteria for a standard motor application, considering factors such as voltage drop, current carrying capacity, and environmental conditions. It is important that the chosen cord can adequately support the motor's starting and running currents, ensuring safety and reliability in operation.

The other options represent larger or smaller wire sizes that may not be suitable for the required application. A larger size could be unnecessarily cumbersome and more expensive, while a smaller size may not provide sufficient current capacity, risking overheating and potential failure. Therefore, the selection of size 9 SO cord as the minimum appropriate for flexible connections aligns with industry standards and ensures the safe operation of the motor within the specified system parameters.