Grounding requirements for cable trays connected to distribution boxes

Proper grounding of cable trays and distribution boxes ensures safety, prevents electrical faults, and complies with NEC standards.

Grounding Cable Trays

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. There are two main approaches: Using the cable tray as the EGC: The tray itself can serve as the grounding path if it is marked and rated for this purpose.

Each tray section should be electrically bonded to adjacent sections using grounding clamps or bonding jumpers to ensure continuity. This prevents disconnection under fault currents and maintains a reliable path to ground. Using a separate grounding conductor: If the tray is not rated as an EGC, a single conductor equipment ground wire should be installed along the tray and bonded to each tray section.

This conductor can be insulated or bare (for sizes #4 AWG or larger) and must comply with NEC Table 250-95 for sizing based on the highest overcurrent protection in the tray. Material considerations: Avoid placing bare copper conductors in aluminum trays to prevent electrolytic corrosion. Use insulated conductors with insulation removed only at bonding points, and employ tin or zinc-plated connectors for secure connections.

Grounding resistance: The grounding system should maintain a low resistance, ideally 4 ohms or less, to ensure effective fault current dissipation.

Grounding Distribution Boxes

Distribution boxes and junction boxes must be connected to the main grounding system. Key practices include: Grounding bolts on the box casing should be connected to the main grounding conductor. Metal sheaths and steel armor of cables entering the box must be bonded to these grounding bolts using copper conductors of equivalent cross-sectional area.

For high-voltage systems (110 kV and above), cable sheaths should be grounded via grounding switches, with resistance not exceeding 4 ohms, and separated from lightning protection grounding electrodes by 3–5 meters.

Inspection and Compliance

Grounding inspections should be performed before energizing the cables and ideally before cable installation to ensure all tray sections and boxes are properly bonded. Verify that all tray sections are marked as suitable for use as an EGC if the tray itself is used for grounding. Ensure compliance with NEC Sections 318-3, 318-7, and Table 318-7(b)(2) for cable tray grounding.

Best Practices

Run a continuous grounding conductor along the tray for maximum reliability. Use hot-dip galvanized steel, copper, or stainless steel for grounding wires depending on environmental conditions. Maintain proper separation between power and data cables to prevent interference.

Ensure mechanical stability of grounding connections to withstand magnetic forces during fault conditions. By following these practices, cable trays and distribution boxes will provide a safe, reliable path for fault currents, protect personnel and equipment, and comply with NEC standards.