Secondary distribution box external enclosure grounded

The external enclosure of a secondary distribution box must be properly grounded to ensure safety, with a dedicated grounding bus bonded to the enclosure and neutral isolated from the enclosure.

Key Principles

Separation of Neutral and Ground: In a secondary distribution box or subpanel, the neutral bus must remain isolated from the metal enclosure. Unlike the main service panel, where neutral and ground are bonded, bonding them in a subpanel can create parallel paths for neutral current, potentially energizing metal parts and delaying breaker tripping during a fault . Grounding the Enclosure: The metal enclosure itself must be connected to a grounding system. This is typically done using a grounding bus bar attached directly to the enclosure. All equipment grounding conductors from branch circuits are connected to this bus, which is then bonded to the building's main grounding system or a dedicated grounding rod .

Practical Steps

1. Install a Grounding Bus Bar: Mount a grounding bar inside the enclosure and bond it to the metal box using screws or bolts designed for grounding.
2. Connect Ground Wires: Attach all incoming equipment grounding conductors to the grounding bus. For industrial or tool distribution boxes, additional ground wires may connect the mounting plate and other metallic components to ensure low-resistance paths .
3. Verify Ground Resistance: Ensure the total resistance from the enclosure to the main grounding point is low, typically below 0.1 ohm for industrial applications .
4. Use Proper Hardware: Grounding lugs, bars, or terminals should be UL-listed and installed according to manufacturer instructions. Non-metallic enclosures often have a metal backpanel that should also be grounded .

Safety Considerations

• Never bond the neutral to the enclosure in a subpanel.
• Ensure all metallic parts, including mounting plates and internal panels, are connected to the grounding bus.
• Use appropriately sized ground conductors according to local codes (e.g., 10 AWG in the US for standard distribution boxes, larger for industrial setups), .
• Regularly inspect grounding connections for corrosion or looseness to maintain safety and system reliability . Proper grounding of a secondary distribution box enclosure ensures that fault currents are safely directed to the source, protecting both personnel and equipment from electric shock or fire hazards.