Copper grounding conductors in distribution boxes must be properly sized, bonded, and connected to ensure a continuous low-impedance path for fault currents, in compliance with NEC standards.

Grounding Conductor Sizing

The equipment grounding conductor (EGC) must be sized according to the rating of the overcurrent protection device (OCPD) protecting the circuit, as specified in NEC Table 250.122. For most circuits, the minimum size is #12 AWG copper, except for control circuits where #14 AWG may be used. Larger conductors are required for higher-rated circuits to ensure safe fault current dissipation .

Connection and Bonding Requirements

• Panelboards: Connect the ground conductor from the feeder to the ground bus in the panel. All metal enclosures must be bonded to this bus to maintain continuity .
• Pull Boxes and Junction Boxes: Install a bolted ground connector and connect all incoming and outgoing conduits using a copper conductor. Ensure continuity across all metal parts and conduit systems .
• Conduits: Use the metal conduit system as part of the grounding path. Bond all metallic conduits to equipment enclosures with locknuts that cut through paint, ensuring a continuous electrical path .
• Motors and Equipment: Bond motor frames and other equipment to the grounding system using an independent green insulated copper wire sized according to NEC requirements. For VFDs, flat braided tinned copper straps may be used instead of wire .
• Expansion Fittings: Provide bonding jumpers across all expansion or deflection fittings to maintain continuity .

Installation Practices

• Insulation and Identification: Ground conductors should be insulated and identified with green insulation or green tape at all accessible locations .
• Continuity Testing: All ground cables must be tested for continuity, and grounding electrode system resistance should be verified to meet safety standards, typically less than 25 ohms .
• Separate Grounding Conductors: Each feeder and branch circuit should have a separate insulated green ground conductor connected from the panel ground bus to all connected equipment .

Additional Considerations

• Bonding Metal Piping: All metal piping systems in the building must be bonded to the service equipment ground bus .
• Raised Floors and Structural Steel: Bond raised floor systems to building steel using a #6 AWG bare copper conductor at each corner, with additional connections for non-bolted stringers .
• Grounding Electrodes: Grounding electrode conductors should connect to approved electrodes such as water service piping or structural steel, sized per NEC Table 250.66 . By following these requirements, grounding copper wire in distribution boxes ensures electrical safety, proper fault current dissipation, and compliance with NEC standards, reducing the risk of electrical shock, equipment damage, and fire hazards.