What grounding wires are used in a secondary distribution box

Yes, a secondary distribution box (subpanel) requires a grounding wire, but the neutral and ground must remain separate.

Grounding Requirements

A secondary distribution box, commonly referred to as a subpanel, must have a dedicated grounding conductor that connects the panel's ground bus to a grounding electrode, such as a ground rod, to ensure safety and proper fault current path (NEC 250.24(A), 408.40) . This grounding wire does not carry normal load current; it only carries current during fault conditions to allow protective devices to trip quickly .

Separation of Neutral and Ground

In a subpanel, the neutral bus and ground bus must remain separate. Unlike the main service panel, where the neutral is bonded to the panel and grounding electrode, bonding in a subpanel is prohibited. This separation prevents neutral current from flowing on grounding conductors and metal enclosures, which could create shock hazards or interfere with protective devices . The neutral bus in the subpanel is connected only to the incoming neutral from the main panel, while the ground bus is connected to the grounding electrode system.

Practical Implementation

• Grounding conductor size: Typically, a 10 AWG (5.26 mm²) wire is used in the US, or 6 mm² in other markets, connecting the subpanel to the grounding electrode .
• Connection points: The grounding wire is attached to the subpanel's ground bus and then to a ground rod or central grounding point. In some industrial setups, additional grounding wires may connect mounting plates or equipment to ensure low-resistance paths .
• Fault protection: Proper grounding ensures that in the event of a short circuit or fault, the current has a low-impedance path back to the source, allowing breakers to trip and preventing equipment damage or electric shock .

Summary

A secondary distribution box does have a grounding wire, but it is strictly separated from the neutral. This configuration ensures safety, compliance with the NEC, and proper operation of protective devices. The grounding wire provides a dedicated path for fault currents, while the neutral handles normal return currents, maintaining the integrity of the electrical system .