Each distribution box must be grounded individually and connected to a common grounding system using appropriately sized conductors to ensure safety and compliance.

Grounding Principles

1. Individual Grounding: Each distribution box should have its own ground connection. For metal enclosures, attach a ground wire from a threaded stud or dedicated grounding point on the box to a grounding bus or mounting plate . Plastic boxes do not require grounding of the enclosure itself, but all devices inside still need grounding .
2. Connection to Central Ground: Both boxes should connect to a central grounding point or electrode system. This can be a grounding rod, Ufer ground (concrete-encased electrode), or a dedicated grounding bus bonded to the building's main panel . The ground resistance should be kept below 25 ohms for safety .
3. Wire Sizing: Use the correct wire gauge for grounding. In the US, a 10 AWG wire is standard for distribution boxes, while other markets may require 6 mm² . For connections between mounting plates or multiple boxes, larger conductors (e.g., 6 AWG or 13.29 mm²) may be needed depending on cable length and fault current requirements .
4. Pigtail Technique: When grounding multiple devices in one box, use a pigtail: a short wire connecting all device grounding screws to the main ground wire. For metal boxes, include a pigtail to the box itself to ensure the enclosure is part of the fault path .
5. Separation of Neutral and Ground: If the boxes are sub-panels, maintain a floating neutral isolated from the enclosure, and connect all equipment grounds to a dedicated grounding bus . This prevents unintended current flow on metal parts and ensures proper operation of overcurrent protection.
6. Testing: After installation, measure the resistance between all grounded components. The total resistance should be very low (typically <0.1 ohm for industrial systems or <25 ohms for building grounding) to ensure effective fault current paths .

Step-by-Step Summary

1. Mount each distribution box securely.
2. Attach a ground wire from the box's grounding stud to a mounting plate or grounding bus.
3. Connect the mounting plate or bus to the central grounding electrode system.
4. Use pigtails to connect multiple devices within the box.
5. Ensure proper wire sizing according to local standards.
6. Verify resistance and continuity of the grounding system. Following these steps ensures that both distribution boxes are safely grounded, protecting equipment and personnel from electrical faults and surges.