problems due to operating standard three conductor cables with standard sized EGCs in parallel. This has been a standard industrial practice for over 40 years with large numbers of such installations in service. This change was made without any safety or technical facts to justify this change.
To comply with Section 250.122, Three options are available: 1. Order special cables with increased sized EGCs which increases the cost and the delivery time. 2. Use three conductor cables without EGCs and install a single conductor EGC in the cable tray or use the cable tray as the EGC in qualifying installations. 3. Use standard cables but don’t utilize their EGCs, use a single conductor EGC or the cable tray as the EGC in qualifying installations.
Should industry be required to have special cables fabricated for such installations when there have been absolutely no safety problems for over 40 years? Each designer and engineer must make his own decision on this subject. If the installations are properly designed, quality materials are used, and quality workmanship is obtained, there is no safety reason for not following the past proven practice of paralleling the EGCs of standard three conductor cable.
392.8. Cable Installation. (E) Single Conductors.
This section states that single conductors in ladder or ventilated trough cable tray that are Nos. 1/0 through 4/0, must be installed in a single layer.
In addition to the fill information that is in Section 392.10(A)(4), an exception was added which allows the cables in a circuit group to be bound together rather than have the cables installed in a flat layer. The installation practice in the exception is desirable to help balance the reactance’s in the circuit group. This reduces the magnitudes of voltage unbalance in three phase circuits.
Where ladder or ventilated trough cable trays contain multiconductor power or lighting cables, or any mixture of multiconductor power, lighting, control, or signal cables, the maximum number of cables that can be installed in a cable tray are limited to the Table 392.9 allowable fill areas. The cable tray fill areas are related to the cable ampacities. Overfill of the cable tray with the conductors
Cooper B-Line, Inc
Compatibility Of Cable Tray Types And
Cable Trays Based On The NEC®
3", 4", & 6" Wide Solid or Ventilated Channel Cable Tray
Solid Bottom Cable Tray
Ventilated Trough Cable Tray
Ladder Cable Tray
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300 & 600 Volt * |
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Single Conductor |
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Cables - 600 Volt * |
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Type MV Multiconductor |
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Cables ** |
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Type MV Single Conductor |
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Cables ** |
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X - Indicates the Installations Allowed by Article 392
*- For cables rated up to 2000 volts.
**- For cables rated above 2000 volts.
***- For 1/0 - 4/0 AWG single conductor cables installed in ladder cable tray, maximum rung spacing is 9 inches.
operating at their maximum ampacities will result in cable heat dissipation problems with the possibility of conductor insulation and jacket damage.
392.9.Number of Multiconductor Cables. Rated 2000 Volts or less, in Cable Trays.
(A)Any Mixture of Cables. (1) 4/0 or Larger Cables
The ladder or ventilated trough cable tray must have an inside usable width equal to or greater than the sum of the diameters (Sd) of the cables to be installed in it. For an example of the procedure to use in selecting a cable tray width for the type of cable covered in this section
Sheet 3), [Example 392.9(A)(1)].
Increasing the cable tray side rail depth increases the strength of the cable tray but the greater side rail depth does not permit an increase in cable fill area for power or lighting cables or combinations of power, lighting, control and signal cables. The maximum allowable fill area for all cable tray with a 3 inch or greater loading depth side rail is limited to the 38.9 percent fill area for a 3 inch loading depth
Cable Tray Manual
32
