What does the size of the grounded conductor in a separately derived system need to be based on?

The size of the grounded conductor in a separately derived system should be determined based on the size of the ungrounded conductor. This is because the grounded conductor's size must provide adequate capacity to safely carry potential fault current, ensuring that the system operates safely under normal and fault conditions. The grounding conductor serves to ensure that any fault current can return to earth, thus helping to clear faults by tripping overcurrent devices.

Choosing the size of the grounded conductor to match the ungrounded conductor is essential for ensuring that both conductors operate effectively within their intended current-carrying capacity. This practice aligns with the principles of electrical system design which aim to provide an adequate balance between system protection and functionality.

In contrast, while current ratings of circuit breakers and specific code references are essential when designing electrical systems, they often guide other aspects of installation rather than the specific sizing of the grounded conductor. An arbitrary approach to sizing would not comply with safety and regulatory standards.