How to connect the grounding wire in a secondary distribution box

In a secondary distribution box (subpanel), grounding wires are separate from neutral and typically use a dedicated grounding bus connected to the panel enclosure and, if required, to a grounding electrode.

Grounding Wire Types and Sizing

For subpanels, the grounding system consists of equipment grounding conductors (EGC) that connect all metal parts of the panel and downstream circuits to a safe ground. The wire size depends on the market and application: United States: A 10 AWG (5. 26 mm²) ground wire is commonly used for standard subpanel installations, with larger sizes (e.g., 6 AWG or 13.

29 mm²) for higher current or longer runs to maintain low resistance. Other markets: A 6 mm² grounding wire is standard. For specialized equipment, such as tool spindles or long cable runs, grounding wires may be sized up to 16 mm² to ensure total resistance remains below 0.

1 Ohm.

Installation and Bonding

The grounding bus bar in the subpanel is bonded to the panel enclosure but must remain isolated from the neutral bus to prevent parallel neutral paths. A grounding electrode conductor (GEC) may connect the subpanel to a ground rod, especially if the subpanel is in a separate building. All metallic parts, including mounting plates and equipment, should be connected to the grounding bus to ensure a low-impedance path for fault currents.

Separation of Neutral and Ground

In a subpanel, the neutral bus is floating and not bonded to the panel enclosure. This prevents neutral current from flowing on grounding conductors, which could create shock hazards or interfere with protective devices. The only point where neutral and ground are bonded is at the main service panel or first service disconnect.

Summary

A properly grounded secondary distribution box uses: Dedicated grounding wires (EGC) sized according to NEC or local standards (10 AWG in the US, 6 mm² elsewhere). Grounding bus bar bonded to the panel enclosure. Optional grounding electrode conductor if required by location or code.

Strict separation of neutral and ground to ensure safety and proper operation of overcurrent protection devices. This setup ensures that fault currents are safely directed to the source, stabilizes system voltage, and protects both equipment and personnel.