Grounding and bonding of cable trays (on photo: Ground wire connected to cable tray; photo credit: solarprofessional.com)
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).
Grounding and bonding of cable trays
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
Table 2 – 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
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.
If the maximum ampere rating of the cable tray
is not sufficient for the protective device to be used, the cable tray
cannot be used as the EGC and a separate EGC must be included within
each cable assembly or a separate EGC has to be installed in or attached
to the cable tray.
For specific areas requiring bonding for electrical continuity, refer to
Figures 1-4.
Figure 1 left: Expansion splice plates; Figure 2 right: Horizontal adjustable plates
Figure 3 left: Discontinuos segments; Figure 4 right: Cable tray sections vertical adjustable splice plate
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 the wire mesh cable tray is to be used as an equipment grounding conductor, then the installation of a ground wire is 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.
