Calculation of grounding for cable tray supports

Metallic cable trays can serve as equipment grounding conductors (EGCs) when properly bonded and sized according to NEC standards, ensuring a continuous low-impedance path for fault currents.Using Cable Trays as Equipment Grounding Conductors

Metallic cable trays, such as steel or aluminum ladder, trough, or channel types, can function as the EGC if all sections and fittings are bonded together using listed connectors or bonding jumpers, creating a continuous electrical path for fault currents (NEC 392.60) . This eliminates the need for a separate grounding conductor in some installations, provided the tray meets the minimum metal cross-sectional area requirements specified in NEC Table 392.60(A) .

Grounding Conductor Options

There are three primary methods to provide grounding in a cable tray system :

1. Tray as EGC: The tray itself serves as the grounding path, with all sections bonded together.
2. EGC conductor in or on the tray: A separate copper or aluminum conductor runs along the tray and is bonded to each section.
3. Individual cable EGCs: Each multi-conductor cable includes its own equipment grounding conductor. When using a separate EGC in aluminum trays, insulated conductors are recommended to prevent electrolytic corrosion, with insulation removed only at bonding points using tin- or zinc-plated connectors .

Sizing the Grounding Conductor

The minimum size of the EGC is determined by NEC Table 250.122, based on the overcurrent protection rating of the circuit. For example, a 100 A circuit typically requires a 10 AWG copper EGC. When the tray itself is used as the EGC, the metal cross-sectional area of the tray must meet or exceed the minimum values in NEC Table 392.60(A), which considers the total area of side rails for ladder or trough trays or the minimum metal area for channel trays .

Bonding Practices

• Bonding clamps are used to connect EGCs to tray sections, ensuring both electrical continuity and mechanical stability under fault conditions .
• Alternate section bonding is acceptable, but continuous bonding is preferred for maximum safety.
• All metallic trays must be grounded according to NEC 250.96, regardless of whether the tray is used as an EGC .

Practical Considerations

• Ensure all tray sections, fittings, and supports are electrically continuous.
• Avoid placing bare copper EGCs in aluminum trays in moist environments; use insulated conductors instead.
• Verify that the tray material and bonding method provide a low-impedance path for fault currents to facilitate proper operation of overcurrent protection devices.
• Regular inspection and maintenance of bonding connections are recommended to maintain system safety and compliance. By following these guidelines, cable tray supports can be safely grounded, providing reliable fault protection and compliance with NEC standards .