Cable Tray as an Equipment Grounding Conductor: Standards and Compliance

What Using Cable Tray as an EGC Means

An Equipment Grounding Conductor provides the low-impedance fault return path between the metallic enclosures of electrical equipment and the source neutral or earth point. In a conventional installation, this function is served by a dedicated green-and-yellow insulated conductor run alongside the phase conductors. When cable tray is designed and installed to meet specific continuity criteria, it can replace or supplement this conductor, with the tray metalwork itself carrying the fault current in the event of an insulation failure.

The practice reduces conductor and labour cost on large installations where continuous tray runs span hundreds of metres of cable routes. It is more common in North American industrial practice (under NEC) than in Indian and European practice (under IS 3043 and IEC 60364), but IEC 61537 contains provisions for it and project-specific specifications increasingly reference it for large projects in the Middle East and Southeast Asia.

NEC 392.60: The US Standard for Cable Tray as EGC

NEC 392.60 permits steel cable tray to serve as an EGC provided: (1) the tray is identified as suitable for EGC use on the equipment approval listing, (2) the tray is continuous from the equipment to the grounding point without relying on sheet-metal screws or paint-coated fittings to maintain continuity, (3) the cross-sectional area of the tray steel meets the minimum values in NEC Table 392.60(A) based on the overcurrent protection rating of the circuit, and (4) all splices between tray sections use bonding jumpers or approved listed splice plates with the required contact area.

• NEC Table 392.60(A): minimum cross-sectional area of steel tray versus circuit breaker rating
• Tray marked suitable for use as Equipment Grounding Conductor on the manufacturer's listing
• Continuity must be maintained through all fittings, crosses, tees and bends
• Bonding jumpers required at every expansion joint and at connections to equipment enclosures
• Paint and galvanising must be removed or penetrated at bonding contact points
• EGC function does not replace separate grounding conductors where required by NEC for specific circuit types

IEC 61537 Provisions for Protective Earthing of the Tray

IEC 61537 clause 7 addresses the electrical continuity of metallic cable tray systems and states that where the tray is to serve as a protective conductor (PC), the system must be tested for continuity under the fault current conditions that could arise in the installation. The standard requires that the manufacturer declare the fault current capacity of the tray and that this capacity be verified against the prospective fault current at the installation point. IEC 61537 does not set a global minimum cross-section but requires that the designer confirm adequacy using the calculation method in IEC 60364-5-54 Annex A.

IS 3043 and Indian Practice

IS 3043:2018 does not explicitly address cable tray as an EGC in the manner of NEC 392.60. Indian practice generally treats cable tray as structural support only and specifies a separate earth continuity conductor (ECC) running alongside the cables in the tray. Many Indian project specifications, particularly for public sector clients and defence projects, require a dedicated ECC and do not permit tray-as-EGC even where IEC 61537 would allow it. For private industrial plants following IEC, the practice is more accepted when accompanied by documented continuity verification.

Continuity Testing for EGC-Rated Tray

Verifying that the installed tray system meets continuity requirements involves measuring the resistance of the tray between the farthest equipment connection point and the main earthing terminal. The measurement must be taken with the system isolated from the supply. The total resistance must be low enough that the fault current divided by that resistance creates sufficient voltage drop to operate the upstream overcurrent device within the required time. For typical LV circuits, a resistance below 0.5 ohm from the farthest tray point to the main earth bar is a reasonable design target.

Where Tray as EGC Is Not Suitable

Cable tray cannot serve as an EGC in circuits feeding medical equipment locations where additional protection requirements apply under IEC 60364-7-710. It is not suitable as the sole EGC for circuits above 600 V in most jurisdictions. Aluminium tray in EGC service requires separate analysis because aluminium oxidation at connections can raise contact resistance over time. GRP and fibreglass tray, being non-conductive, cannot serve this function.

In outdoor exposed installations where corrosion could reduce the tray cross-section over time, the EGC rating should be verified at the end of the installation's service life, not only at commissioning. For a 30-year plant life in a coastal environment, a GI tray specified for EGC duty should carry a corrosion allowance in its cross-section, or be specified in hot-dip galvanised construction to ASTM A123 with a minimum coating of 610 g/m2.

Using cable tray as an EGC is a legitimate and code-compliant practice in the right circumstances. The key is documentation: the tray must be listed for the duty, the continuity must be verified after installation, and the fault current calculation must confirm the tray section is adequate.