Cooper Bussmann CT02MAN - Manual (Page 30)

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Cooper B-Line, Inc Cable Tray Manual
392.7. Grounding. (B) Steel or Aluminum
Cable Tray Systems. (1) & (2)
Reprinted with permission from NFPA 70-1999, the National Electrical Code®,
Copyright© 1998, National Fire Protection Association, Quincy, MA 02269. This reprinted
material is not the complete and official position of the National Fire Protection Association,
on the referenced subject which is represented only by the standard in its entirety.
Table 392.7(B) "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. [See also Section 250-
120 for additional information]
The subject of using cable tray for equipment
grounding conductors was thoroughly investigated
by the 1973 NEC
® Technical Subcommittee on
Cable Tray. Many calculations were made and a
number of tests were performed by Monsanto
Company Engineers at the Bussman High Current
Laboratory. The test setup to verify the capability of
cable tray to be used as the EGC is shown in Figure
1 on page 29. The test amperes available were
forced through one cable tray side rail which had
three splice connections in series. No conductive
joint compound was used at the connections and the
bolts were wrench tight. Copper jumper cables were
used from the current source to the cable tray. The
cable tray was NEMA Class 12B. The test results
are shown on Page 45 (Appendix Sheet 1), Table I
for aluminum and Table II for steel cable tray.
One of the most interesting results of the tests was
for an aluminum cable tray with a corroded joint and
only two nylon bolts. 34,600 amperes for 14 cycles
produced only a 34° C temperature rise at the splice
plate area. If the protective devices work properly,
the temperature rises recorded at the cable tray
splices during these tests would not be sufficient to
damage the cables in the cable tray. Also note that
in these tests only one side rail was used, but in a
regular installation, both side rails would conduct
fault current and the temperature rise at the splice
plate areas would be even lower.
When the cable tray is used as the EGC,
consideration has to be given to the conduit or
ventilated channel cable tray connections to the
cable tray so that the electrical grounding continuity
is maintained from the cable tray to the equipment
utilizing the electricity. Conduit connections to the
cable tray were also tested. At that time, no
commercial fittings for connecting conduit to cable
tray were available, so right angle beam clamps
were used with very good results. There are now UL
Listed fittings for connecting and bonding conduit to
cable tray. This test setup and results are shown on
page 46 (Appendix Sheet 2).
Table 392.7(B).
Metal Area Requirements for Cable Trays
Used as Equipment Grounding Conductors
Maximum Fuse Ampere Rating,
Circuit Breaker Ampere Trip Minimum Cross-Sectional Area
Setting, or Circuit Breaker of Metal* In Square Inches
Protective Relay Ampere Trip
Setting for Ground-Fault
Protection of Any Cable Circuit Steel Aluminum
In the Cable Tray System Cable Trays Cable Trays
60 0.20 0.20
100 0.40 0.20
200 0.70 0.20
400 1.00 0.40
600 1.50** 0.40
1000 --- 0.60
1200 --- 1.00
1600 --- 1.50
2000 --- 2.00**
For SI units: one square inch = 645 square millimeters.
*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.