Grounding of cable trays and metal conduits

Metallic cable trays and conduits must be properly grounded and bonded to ensure electrical safety, continuity, and compliance with NEC standards.

General Principles

All metallic cable trays and conduits are considered non-current-carrying parts of equipment that may become energized during a fault. They must be bonded and grounded to provide a low-impedance path for fault currents, protect personnel, and ensure proper operation of overcurrent devices . Grounding is required regardless of whether the tray or conduit is used as an Equipment Grounding Conductor (EGC) .

Equipment Grounding Conductor (EGC) Use

• Cable trays as EGC: Steel or aluminum trays can serve as the EGC if they meet NEC 392.60 criteria, including minimum cross-sectional area, UL classification, and installation in facilities where qualified personnel maintain the system .
• Separate EGC: If the tray is not rated as an EGC, a separate conductor must be installed in the tray or within the cables themselves, bonded to each tray section using grounding clamps or jumpers .
• Conductor sizing: EGC sizing should follow NEC Table 250.122, based on the largest overcurrent protection device in the circuit .

Bonding and Continuity

• Section-to-section bonding: Each tray or conduit section must be electrically continuous. This can be achieved using bonding jumpers, braided copper tape, or dedicated connectors at splice points, ensuring metal-to-metal contact by removing paint or insulation at connection points .
• Multi-point grounding: Trays and conduits should be connected to the building grounding system at both ends and at regular intervals (typically every 50–60 feet) to maintain low potential to ground and reduce electromagnetic interference .
• Mechanical integrity: Bonding clamps also provide mechanical anchoring to prevent EGC displacement under fault conditions .

Inspection and Compliance

• Pre-installation inspection: Grounding should be verified before cables are installed and energized to ensure compliance with NEC Sections 318 and 250 .
• Marking and verification: Trays used as EGC must be marked with UL classification and minimum cross-sectional area. Inspectors should confirm that all sections are properly bonded and continuous .
• Connection to facility ground: Trays and conduits should be bonded to structural steel, metallic supports, or other grounding electrodes to form a continuous equipotential network .

Special Considerations

• Material compatibility: Avoid bare copper in aluminum trays to prevent electrolytic corrosion; use insulated conductors with exposed bonding points .
• End connections: Conduits entering cabinets or equipment should overlap metal surfaces to maintain shielding and grounding continuity .
• Continuous path: Effective grounding must be permanent, continuous, and capable of safely conducting fault currents while maintaining low impedance . Proper grounding and bonding of cable trays and metal conduits not only ensures personnel safety but also enhances system reliability, reduces electromagnetic interference, and ensures compliance with NEC standards .