A secondary distribution box (sub panel) requires a separate grounding bus bonded to the panel enclosure, isolated neutral bus, and proper earth grounding with low-resistance rods or electrodes.

Key Grounding Principles

Separation of Neutral and Ground: In a sub panel, the neutral bus must be isolated from the panel enclosure, while a separate grounding bus is bonded to the metal enclosure. This prevents neutral current from flowing on grounding conductors, which could energize metal parts and delay breaker tripping during a fault . Grounding Bus Installation: The grounding bus bar should be securely attached to the panel enclosure. All equipment grounding conductors from branch circuits are connected to this bus, providing a low-impedance path for fault currents . Earth Ground Connection: The sub panel must be connected to the earth using grounding electrodes, such as copper-bonded rods. The resistance of the grounding system should ideally be 25 ohms or less, and multiple rods may be required if soil conditions are poor . Continuous conductors should interconnect all rods to ensure reliable fault current dissipation. Equipment and Personnel Safety: Proper grounding stabilizes system voltage, protects equipment from surges and lightning, and reduces the risk of electric shock . Grounding also ensures that protective devices like circuit breakers operate correctly during faults. Cable Shielding and Maintenance: Any metallic shielding on cables should be grounded to the panel to prevent high-frequency interference. Regular inspection is necessary to account for corrosion, loose connections, or environmental changes that could increase resistance .

Summary of Steps

1. Install a floating neutral bus isolated from the panel enclosure.
2. Install a grounding bus bar bonded to the panel enclosure.
3. Connect all branch circuit grounds to the grounding bus.
4. Drive grounding rods into the earth and connect them with continuous conductors to the grounding bus.
5. Test the grounding system to ensure resistance is ≤25 ohms.
6. Maintain separation of neutral and ground to prevent parallel current paths. Following these practices ensures compliance with electrical codes, protects personnel, and maintains system stability in a secondary distribution box.