Cable trays for ground cabling

Cable trays can serve as a safe and effective grounding path when properly installed and bonded according to NEC standards.

Purpose of Grounding in Cable Trays

Grounding cable trays ensures that any stray electrical currents are safely directed into the earth, reducing the risk of electric shock, equipment damage, or fire hazards. The grounding system stabilizes the electrical network and provides a low-resistance path for fault currents, improving overall system safety and reliability .

NEC Guidelines for Cable Tray Grounding

The National Electrical Code (NEC) provides specific rules for grounding cable trays:

• Metallic trays as EGC: Metallic cable trays can serve as the equipment grounding conductor if they meet NEC requirements, including sufficient cross-sectional area and electrical continuity .
• Bonding: Each tray section should be bonded using grounding clamps to ensure mechanical stability and electrical continuity. This prevents the EGC from being displaced under fault conditions .
• Material limitations: Bare copper conductors should not be used in aluminum trays due to potential electrolytic corrosion. Insulated conductors with tin or zinc-plated connectors are recommended for aluminum trays .
• Ampere limits: Steel trays should not be used as EGCs for circuits with ground-fault protection above 600 A, and aluminum trays should not exceed 2000 A .
• Tray fill and separation: Proper fill limits must be observed (40% for power cables, 50% for control cables), and high-power and low-power cables should be separated to prevent electromagnetic interference .

EGC Options in Cable Trays

Designers have several options for grounding cable trays:

1. Using the tray itself as the EGC: Suitable for qualifying facilities where the tray meets NEC Section 318-3(c) requirements .
2. Single conductor EGC: A common grounding conductor can run along the tray to serve all circuits .
3. Individual EGCs: Each multiconductor cable can include its own EGC, or EGCs can be paralleled with the tray for enhanced safety .

Installation Best Practices

• Maintain accessibility for inspection and maintenance; trays should not be enclosed or placed in hoistways .
• Use sunlight-resistant cables and expansion splice plates for outdoor installations to accommodate thermal expansion .
• Ensure grounding resistance is low (ideally 4 ohms or less) to maximize fault current diversion and system stability .
• Follow all local and international electrical standards for materials, connection types, and resistance levels .

Summary

Properly installed cable trays provide both mechanical support and a reliable grounding path. Compliance with NEC standards, correct bonding, material selection, and adherence to fill and separation guidelines are essential to ensure safety, prevent corrosion, and maintain system performance over time .