Each distribution box must be grounded separately

Yes, each distribution box must be grounded, and in many cases, components like the box door require separate grounding connections.

General Grounding Principles

A distribution box serves as a central point for electrical circuits, and proper grounding ensures safety by providing a low-impedance path for fault currents, preventing electrical shock, and protecting equipment from surges or lightning strikes . Grounding is not just a single connection; it often involves multiple points:

• Box enclosure: Metallic distribution boxes must be bonded to ground using a dedicated grounding wire or bus bar .
• Box door: If the door is metallic, it should have a separate grounding connection to the cabinet's grounding bar using a dedicated screw or clip, typically with a 12 AWG copper wire .
• Internal components: Controllers, mounting plates, and spindles may require additional grounding connections to maintain low resistance (<0.1 Ohm) across the system .

Wire Sizing and Resistance

• In the US, grounding wires for distribution boxes are typically 10 AWG for the main connection, while other markets may use 6 mm² .
• For connections between internal components, larger wires may be required depending on cable length and number of spindles (e.g., 6 AWG for long tool cables) to ensure total resistance remains below safety thresholds .
• Grounding resistance should generally be less than 25 Ohms for sub-panels or separate structures, with multiple rods used if necessary .

Code Compliance

• NEC 250.148 requires metallic junction boxes and cabinet doors to be bonded to ground.
• NEC 314.28 mandates that metal boxes must be reliably grounded and bonded.
• NEC 110.26 ensures safe access to equipment without obstruction from grounding wires .
• For sub-panels in separate structures, a Grounding Electrode System (GES) is required, with a floating neutral bus and a separate grounding bus bonded to the enclosure .

Practical Recommendations

1. Always use dedicated grounding conductors; do not rely on mounting bolts or hinges alone.
2. Verify grounding resistance with proper testing to ensure compliance.
3. Use high-quality materials to prevent corrosion and maintain long-term safety.
4. For DC systems, follow NEC Part VIII rules for grounding and bonding, ensuring proper conductor sizing and connection to the common point of the system . In summary, distribution boxes require grounding for the enclosure, internal components, and metallic doors, often with separate connections to ensure low resistance and compliance with electrical codes. Proper installation is critical for safety and system reliability.