Metallic Cable Trays
Cable tray may be used as the Equipment Grounding Conductor (EGC) in any installation where qualified persons will service the installed cable tray system. There is no restriction as to where the cable tray system is installed.
The metal in cable trays may be used as the EGC as per the limitations of table 392.60(A). All metallic cable trays shall be grounded as required in Article 250.96 regardless of whether or not the cable tray is being used as an equipment grounding conductor (EGC).
The EGC is the most important conductor in an electrical system as its function is electrical safety.
There are three wiring options for providing an EGC in a cable tray wiring system:
- An EGC conductor in or on the cable tray.
- Each multi-conductor cable with its individual EGC conductor.
- The cable tray itself is used as the EGC in qualifying facilities.
Correct bonding practices
To assure that the cable tray system is properly grounded
If an EGC cable is installed in or on a cable tray, it should be bonded to each or alternate cable tray sections via grounding clamps (this is not required by the NEC® but it is a desirable practice)
In addition to providing an electrical connection between the cable tray sections and the EGC, the grounding clamp mechanically anchors the EGC to the cable tray so that under fault current conditions the magnetic forces do not throw the EGC out of the cable tray.
A bare copper equipment grounding conductor should not be placed in an aluminum cable tray due to the potential for electrolytic corrosion of the aluminum cable tray in a moist environment.
For such installations, it is best to use an insulated conductor and to remove the insulation where bonding connections are made to the cable tray, raceways, equipment enclosures, etc. with tin or zinc plated connectors.
NEC Table 250.122 – Minimum size equipment grounding conductors for grounding raceway and equipment
Aluminum Cable Tray Systems
Table 392.60(A) – Metal area requirements for cable trays used as equipment grounding conductors
For Sl units: 1 square inch = 645
* Total cross-sectional area of both side rails for ladder or trough cable trays or the minimum cross-sectional area of metal in channel cable trays or cable trays of one-piece construction.
** Steel cable trays shall not be used as equipment grounding conductors for circuits with ground-fault protection above 600 amperes. Aluminum cable trays shall not be used as equipment grounding conductors for circuits with ground-fault protection above 2000 amperes.
Table 392.60(A) “Metal Area Requirements for Cable Trays used as Equipment Grounding Conductors” shows the minimum cross-sectional area of cable tray side rails (total of both side rails) required for the cable tray to be used as the Equipment Grounding Conductor (EGC) for a specific Fuse Rating, Circuit Breaker Ampere Trip Rating or Circuit Breaker Ground Fault Protective Relay Trip Setting.
These are the actual trip settings for the circuit breakers and not the maximum permissible trip settings which in many cases are the same as the circuit breaker frame size.
For specific areas requiring bonding for electrical continuity, refer to Figures 1-4.
Non-metallic cable trays do not serve as a conductor. It is also recommended that wire mesh cable trays not be used as an equipment grounding conductor.
Although permitted by the NEC, it is recommended due to the unique nature of the wire mesh, fittings are manufactured in the field from straight sections by cutting away the current carrying structural wires, reducing the current-carrying capability of the system. As such, the use of wire mesh cable trays as an equipment grounding conductor is not recommended.
If a wire mesh cable tray is supporting cable with a built-in equipment grounding conductor or control or signal cables, then the tray should have a low impedance path to a non-system ground to reduce noise and remove induced or stray currents. A separate grounding cable attached to the wire mesh cable tray is not usually required.
References:
- CABLE TRAY MANUAL Based on the 2011 National Electrical Code – EATON
- Cable Tray Installation Guidelines – NEMA Standards Publication VE 2-2006
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Edvard Csanyi
Hi, I'm an electrical engineer, programmer and founder of EEP - Electrical Engineering Portal. I worked twelve years at Schneider Electric in the position of technical support for low- and medium-voltage projects and the design of busbar trunking systems.I'm highly specialized in the design of LV/MV switchgear and low-voltage, high-power busbar trunking (<6300A) in substations, commercial buildings and industry facilities. I'm also a professional in AutoCAD programming.
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I have a question: this article states that it is best to use insulated cable, and to remove the insulation where bonding connections are made to the cable tray. What is the justification of using insulated cable? why not a bare copper grounding cable.?
can you put two bond jumpers on the same nut and bolt on cable trays or do we use separate nut and bolt for the second jumper.
Contractor installing TC Cables size #14’s in tray, the multi conductor their installing doesn’t have a green identifier. The contractor pulled a green single conductor #14 along with the multi-conductor cable. Does the NEC Code allow this application? The reason is – on TC Cable, having green as one of the conductors has to be special ordered, not a shelf item.
Dear Sir/Madame,
please advise me, now i install cables (multi-core power cables) on cable tray, can i install cables on to each other? due to the cable tray is limited of space.
Nui.
Greetings Edvard,
Less we not also forget the beneficial provisions of 250.122(F)(2) as well when the installer of traditional multiconductor cable assemblies are used, such as MC Cable or TC Cable to name a few. Traditionally the EGC in these cables are designed via their associated UL/ANSI Standard for use based on the OCPD protecting the single cable assembly so they aren’t intended for parallel use. However, where used as such the benefits of using 250.122(F)(2) just may save the day. So if you are using standard cable assemblies in a cable tray, in parallel, be sure to check out 250.122(F)(2) as it just may keep you from having to buy specially made cables.
How we have to select the earth bonding cable for instrument cable trays and electrical cable trays. i’m asking this because i’m working on site i don’t know which size of conductor i have select for instrument cable tray as well as electrical cable tray we have 15 to 20 instrument cables in instrument cable tray and 5(five)3c*240sqmm cables in electrical cable tray so which size will be suitable for both E & I trays..kindly explain me in detail so that it will be easy for everyone to select the cable for bonding as well as earthing
I have a question and can not seem to find it written anywhere. I am looking for an answer to verify that when grounding tray to earth on one cable tray run is the correct way and not having multiple tray to earth grounding which confuses the path in my opinion. I have an individual going against some engineered drawings and secretly having electricians add multiple tray to earth grounds on one cable tray run that’s 80 foot long. I’m looking for a NEC code stating anything about this so I can bring this up with written documents. Thanks
If you only use one bonding/ground point at one end of the tray or the other…you have the whole section unprotected and with resistance increasing with length…that one ground may not be enough. When installing tray for Comm, we (as per TIA/BICSI standard) would attach to bldg steel at beginning, the end and every 50ft. Most comm tray is not used as an EGC so I would think that would be the direction you would take…is your tray an EGC…if so…follow the NEC
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Regards.
Thanks for the giving important thing – Earthing which is very important in Electrical Installations
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