In accordance with Article 250, metal boxes must be:

In accordance with Article 250 of the National Electrical Code (NEC), metal boxes must be both grounded and bonded to ensure safety and proper functioning within an electrical system. Grounding provides a path for electrical current to safely flow to the ground in the event of a fault, which helps to prevent electric shock hazards. Bonding ensures that all metal parts of the electrical system are interconnected and at the same electrical potential. This reduces the risk of voltage differences between conductive elements, which could lead to shock or fire hazards.

By requiring both grounding and bonding, the NEC establishes a clear standard for how metal boxes should be handled within electrical installations, thereby enhancing the overall safety of the electrical system. Simply grounding or bonding the boxes alone would not provide the same level of protection and could leave vulnerabilities that might be exploited under certain fault conditions. Isolating the boxes would be contrary to the requirements outlined in the NEC as it could lead to unsafe conditions where fault currents cannot be effectively managed.