Provision No. 2: Where multiconductor cables are installed in a single layer in uncovered cable trays with a maintained spacing of not less than one cable diameter between cables, the ampacity shall not exceed the allowable ampacities of Table 310.71 and 310.72.
If the cable tray does not have covers and the conductors are installed in a single layer spaced not less than one cable diameter apart, the cable conductor ampacities can be 100 percent of the ambient temperature corrected capacities in Tables 310.71 or 310.72.
392.13.Ampacity of Type MV and Type MC Cables (2001 Volts or Over) in Cable Trays. (B) Single Conductor Cables (2001 Volts or Over).
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| Mult. |
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| Solid | Applicable | Amp. |
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Sec. | Cable | Unventilated | Ampacity | Table | Special |
No. | Sizes | Cable Tray | Tables | Values | Conditions |
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| Cover | (*) | By |
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| 1/0 AWG | No Cover | 310.69 |
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(1) | and | Allowed | and | 0.75 |
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| Larger | (**) | 310.70 |
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| 1/0 AWG |
| 310.69 |
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(1) | and | Yes | and | 0.70 |
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| Larger |
| 310.70 |
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| 1/0 AWG |
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| Maintained |
(2) | & Larger | No Cover | 310.69 | 1.00 | Spacing Of |
| In Single | Allowed | and |
| One Cable |
| Layer | (**) | 310.70 |
| Diameter |
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| Single |
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| Conductors |
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| Spacing Of |
(3) | In Triangle | No Cover | 310.67 |
| 2.15 x One |
| Config. | Allowed | and | 1.00 | Conductor |
| 1/0 AWG | (**) | 310.68 |
| O.D. Between |
| and Larger |
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| Cables(***) |
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(*) The ambient ampacity correction factors must be used.
(**)At a specific position, where it is determined that the tray cables require mechanical protection, a single cable tray cover of six feet or less in length can be installed.
The wording of Section 392.13(B)(3) states that a spacing of
2.15times one conductor diameter is to be maintained between circuits. Two interpretations of this statement are possible. Interpretation #1. - The 2.15 times one conductor diameter is the distance between the centerlines of the circuits (the center lines of the conductor bundles). Interpretation #2. - The 2.15 times one conductor diameter is the free air distance between the adjacent cable bundles. The use of the word “circuit” is unfortunate as its presence promotes Interpretation #1. An installation based on Interpretation #1 is not desirable as a free air space equal to 2.15 times one conductor diameter between the cable bundles should be maintained to promote cable heat dissipation.
Cooper
Spacing Between Conductors
(2.15 x O.D. of Conductor)
Technically Undesirable Installation
Interpretation #1
Spacing Between Conductors
(2.15 x O.D. of Conductor)
Technically Desirable Installation
Interpretation #2
CABLE TRAY WIRING SYSTEM DESIGN AND INSTALLATION HINTS.
Cable tray wiring systems should have a standardized cabling strategy. Standard cable types should be used for each circuit type. Most of the following circuits should be included; feeder circuits, branch circuits, control circuits, instrumentation circuits, programmable logic controller input and output (I/O) circuits, low level analog or digital signals, communication circuits and alarm circuits. Some cables may satisfy the requirements for several circuit types. Minimizing the number of different cables used on a project reduces installed costs. Some companies have cable standards based on volume usage to minimize the numbers of different cables used on a project. For example: if a 6 conductor No. 14 control cable is needed but 7 conductor No. 14 control cable is stocked, a 7 conductor control cable would be specified and the extra conductor would not be used. Following such a practice can reduce the number of different cables handled on a large project without increasing the cost since high volume cable purchases result in cost savings. Orderly record keeping also helps provide quality systems with lower installation costs. The following items should be included in the project's cable records:
Cable Tray Manual
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