What is the required rating of the service disconnect for a building supplied by a feeder?

The required rating of the service disconnect for a building supplied by a feeder is determined by the total load that the service is intended to carry and the specific requirements outlined in the National Electrical Code (NEC). In many scenarios, the size and capacity of the service disconnect must accommodate the maximum expected load, which can vary depending on the design and use of the building.

In this case, a 15A service disconnect may reflect a low load scenario typically used in smaller installations, such as single-family residential units or small commercial applications where the overall load is not expected to exceed that capacity. This ensures that overcurrent protection is provided appropriately for the connected equipment, and that the circuit can safely manage normal operations without risking tripping or damage.

It's essential to match the service disconnect rating to the needs of the electrical system while also complying with local codes and requirements. Higher ratings like 30A, 40A, and 20A would be appropriate for larger loads and applications, ensuring they provide sufficient capacity for heavier duty systems, but they may not be necessary in a small-scale application, making a 15A rating suitable for certain situations. Choosing the correct rating is critical to maintaining safety and efficiency in electrical systems.