Grounding at the end of the cable tray and the distribution box

The cable tray must be properly bonded and grounded to the distribution box using either the tray as an equipment grounding conductor or a dedicated grounding conductor, ensuring compliance with NEC standards and electrical safety.

Grounding Principles

All metallic cable trays must be grounded regardless of whether they are used as an equipment grounding conductor (EGC) or not, as required by NEC Article 250. 96. The grounding connection ensures that any fault current is safely conducted to the earth, protecting personnel and equipment from electrical hazards.

Methods of Grounding

Using the Cable Tray as the EGCThe metal tray itself can serve as the grounding path if it meets the minimum cross-sectional area requirements specified in NEC Table 392. 60(A),. Each tray section should be electrically bonded to adjacent sections using bolted connections or bonding jumpers to maintain continuity.

For galvanized steel trays, connecting plates can act as bridging plates, secured with at least two bolts with locknuts or lock washers. Using a Dedicated Grounding ConductorA separate insulated or bare copper conductor can be installed in or on the tray and bonded to each tray section via grounding clamps. In aluminum trays, use insulated conductors to prevent galvanic corrosion, removing insulation only at bonding points with tin- or zinc-plated connectors.

The conductor should be sized according to NEC Table 250. 122 based on the largest overcurrent protection device in the circuit. Connection to the Distribution BoxThe cable tray grounding system must be connected to the distribution box's main grounding bus or grounding electrode system.

Use tinned copper lugs or connectors to bond the tray or grounding conductor to the distribution box. Ensure mechanical stability so that under fault conditions, the grounding path remains intact and secure.

Additional Considerations

Bonding Jumpers: Required at adjustable splice plates, expansion joints, or discontinuous tray sections to maintain electrical continuity. Inspection: Grounding connections should be inspected before energizing the system to verify compliance with NEC requirements. Corrosion Prevention: Avoid bare copper in aluminum trays in moist environments; use compatible materials to prevent electrolytic corrosion.

By following these practices, the cable tray system will provide a reliable grounding path to the distribution box, ensuring safety, code compliance, and protection against electrical faults.