How is a system bonding jumper for a separately derived system sized?

A system bonding jumper for a separately derived system is sized using the 12.5% rule or based on the largest ungrounded phase conductor. This method ensures that the bonding jumper can adequately carry the potential fault current that may occur in the system, maintaining the safety and reliability of electrical equipment and grounding systems.

The 12.5% rule, as specified in the National Electrical Code (NEC), states that the bonding jumper must be sized at a minimum of 125% of the largest ungrounded phase conductor if it is used for bonding a separately derived system. This sizing ensures that, in the event of a fault condition, the jumper can handle the increased current flow without overheating or failing, which is crucial for the effectiveness of the grounding system and the protection of personnel and equipment.

In contrast, sizing by the length of the run does not account for the required capacity to handle fault currents, and simply relating sizing to operational voltage does not provide a direct method to ensure the jumper can carry fault currents safely. Furthermore, while overall amperage load is an important factor in system design, it does not specifically target the bonding requirements necessary for separately derived systems, which are distinctly focused on managing fault currents safely.