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The minimum percentage for sizing a bonding jumper of the largest phase conductor is based on the requirement to ensure a safe and functional electrical system. Specifically, a bonding jumper is a conductor that connects parts of an electrical system to provide a continuous path for fault current and to minimize potential differences that could lead to shock or equipment damage.

In most cases, the National Electrical Code (NEC) stipulates that the minimum size of a bonding jumper should be at least 12.5% of the largest phase conductor's size. This requirement ensures that the bonding jumper is sufficiently robust to carry any fault current that may occur in the system, therefore maintaining an effective grounding and bonding scheme.

This percentage takes into consideration the effective current-carrying capacity and the necessary safety margin needed for reliable performance in the event of a fault. As a result, understanding this minimum sizing requirement is critical for electricians to ensure compliance with safety standards and effective system operation.