Cable tray connected to distribution box grounding

Metallic cable trays must be properly grounded to the distribution box using either the tray itself as an equipment grounding conductor or a dedicated grounding wire, ensuring a low-resistance path for fault currents.

Grounding Principles

All metallic cable trays must be grounded according to NEC Article 250.96 regardless of whether they are used as an equipment grounding conductor (EGC) or not . Grounding ensures that any stray electrical currents are safely diverted to earth, reducing the risk of electric shock, fire, or equipment damage .

Methods of Connection

1. Using the Cable Tray as the EGC
   • The tray itself can serve as the grounding path if it meets the minimum cross-sectional area requirements for the expected fault current .
   • Steel trays should not be used as EGCs for circuits with ground-fault protection above 600 A, and aluminum trays should not be used above 2000 A .
   • Sections of the tray must be bonded together using grounding clamps or bridging plates to maintain electrical continuity .
2. Dedicated Grounding Conductor
   • A bare or insulated copper conductor can be run along the tray and connected to each tray section using tinned or zinc-plated connectors .
   • Insulated conductors are preferred in aluminum trays to prevent electrolytic corrosion .
   • The grounding conductor should also be connected to the distribution box or main grounding bus using secure bolts or lugs .
3. Connection Hardware
   • Use locknuts, bolts, and washers to secure connections. For galvanized trays, the connecting plates can act as bridging plates; for painted or coated trays, use tinned copper wire with crimped lugs .
   • In vibrating environments, spring washers are recommended to maintain tight connections .

Resistance and Safety Requirements

• The grounding system should maintain a low resistance, ideally 4 ohms or less, to ensure effective fault current diversion .
• Keep grounding conductors clear of flammable materials and corrosive environments to maintain long-term reliability .

Best Practices

• Bond each tray section to the grounding system to prevent discontinuities.
• Avoid placing bare copper in aluminum trays; use insulated conductors with exposed ends at bonding points.
• Verify that the tray and grounding conductor meet NEC requirements for the specific circuit rating.
• Ensure mechanical stability so that fault currents do not dislodge the grounding conductor. By following these practices, the cable tray system will be safely and effectively grounded to the distribution box, ensuring compliance with NEC standards and protecting personnel and equipment from electrical hazards .