According to regulations, how must the system bonding jumper for a separately derived system be sized?

The correct answer focuses on the specific guideline set forth in Section 250.28(D) of the National Electrical Code (NEC), which outlines the requirements for sizing the system bonding jumper in a separately derived system. This section details that the bonding jumper must be sized based on the size of the service-entrance conductors feeding the derived system and considers the overall system's capacity. Proper sizing is crucial for ensuring safety and functionality, particularly in preventing overcurrent and ensuring a reliable path for fault current.

In contrast, regional standards may vary and would not necessarily provide the specific guidance necessary for determining the size of a bonding jumper within the context of a separately derived system. Sizing based on the length of the conductor is not a standard practice for bonding jumpers and does not take into account the essential factors that Section 250.28(D) emphasizes. The option suggesting that it can be any practical size compromises safety by indicating a lack of adherence to precise sizing requirements outlined in the NEC, which are in place to protect both the electrical system and its users.