Which feeders are allowed to use a common neutral conductor?

The correct choice indicates that up to three sets of 3-wire feeders are allowed to use a common neutral conductor under the National Electrical Code (NEC) guidelines. This rule is applicable because when multiple sets of 3-wire feeders are used in a multi-wire branch circuit, the return current from the phases can balance each other out if they are evenly loaded. Consequently, this allows the neutral conductor to safely carry only the unbalanced load, which can often result in reduced conductor sizes and more efficient systems.

Understanding this concept is essential in electrical design to ensure that the load on the neutral does not exceed the conductor's rating and that the system operates safely. It also emphasizes the importance of proper loading and circuit design to prevent overheating of the neutral conductor or voltage imbalances.