Secondary distribution box grounding leaf plate

A secondary distribution box must be grounded using a dedicated grounding bus or leaf plate connected to the enclosure and the main grounding system, with neutral and ground kept separate.

Purpose of Grounding

Grounding a secondary distribution box ensures personnel safety, equipment protection, and proper operation under normal and fault conditions. It provides a low-impedance path for fault currents, stabilizes system voltage, and helps dissipate energy from lightning or transient surges .

Grounding Leaf Plate

A grounding leaf plate is a metal plate inside the distribution box used to bond all equipment grounding conductors (EGCs) and the panel enclosure. It serves as a central point for connecting branch circuit grounds and the feeder grounding conductor from the main panel . The leaf plate is typically bolted to the metal enclosure, ensuring a solid electrical connection.

Installation Guidelines

1. Separate Neutral and Ground: In a sub-panel or secondary distribution box, the neutral bus must float, isolated from the enclosure, while the ground bus or leaf plate is bonded to the enclosure .
2. Feeder Grounding Conductor: The feeder supplying the sub-panel must include a dedicated equipment grounding conductor that connects directly to the grounding leaf plate .
3. Ground Electrode Connection: The grounding leaf plate can be connected to a grounding electrode, such as a Ufer ground (concrete-encased rebar) or ground rods, to ensure a low-resistance path to earth .
4. Wire Sizing: Grounding conductors should comply with local codes. For example, in the US, a 10 AWG wire is commonly used for panel grounding, while larger conductors may be required depending on the panel rating .
5. Resistance Requirements: The total ground resistance between the panel, leaf plate, and grounding electrode should be less than 0.1 ohm to ensure effective fault clearing .

Safety Considerations

• Avoid bonding the neutral to the enclosure in a secondary panel to prevent parallel neutral paths, which can energize metal parts and delay breaker operation .
• Ensure all metallic parts, including mounting plates and enclosures, are properly bonded to the grounding leaf plate.
• Verify connections periodically for corrosion or loosened bolts to maintain low-resistance grounding.

Summary

The grounding leaf plate in a secondary distribution box acts as the central bonding point for all equipment grounds and the enclosure. It must be connected to the main grounding system via a dedicated conductor, with neutral kept isolated, and sized according to code requirements. Proper installation ensures safety, system stability, and compliance with electrical standards .