391.13(B) is for single conductor cables, not with a 3 conductor cable. Am I missing something?
392.22 Number of Conductors or Cables.
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(C) Number of Type MV and Type MC Cables (2001
Volts or Over) in Cable Trays.
The number of cables rated 2001 volts or over permitted in a single cable tray shall not exceed the requirements of this section.
The sum of the diameters of single-conductor and multiconductor cables shall not exceed the cable tray width, and the cables shall be installed in a single layer. Where single conductor cables are triplexed, quadruplexed, or bound together in circuit groups, the sum of the diameters of the single conductors shall not exceed the cable tray width, and these groups shall be installed in single layer arrangement.
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So the above recognizes that the normal installation would be placing 3/c cables side-by-side and it defines the limits of that installation.
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(B) Ampacity of Type MV and Type MC Cables (2001 Volts or Over) in Cable Trays.
The ampacity of cables, rated 2001 volts, nominal, or over, installed according to 392.22(C) shall not exceed the requirements of this section.
(1) Multiconductor Cables (2001 Volts or Over).
The allowable ampacity of multiconductor cables shall be as given in Table 310.60(C)(75) and Table 310.60(C)(76), subject to the following provisions:
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The above tables can be used to determine the ampacity of the cables when you install them to the original provision of side-by-side, but it is subject to two provisions if the basic installation is modified:
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(a) Where cable trays are continuously covered for more than 1.8 m (6 ft) with solid unventilated covers, not more than 95 percent of the allowable ampacities of Table 310.60(C)(75) and Table 310.60(C)(76) shall be permitted for multiconductor cables.
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So if you have the cable tray covered, you need to reduce the ampacity as there will be more heat trapped,
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(b) Where multiconductor cables are installed in a single layer in uncovered cable trays, with maintained spacing of not less than one cable diameter between cables, the ampacity shall not exceed the allowable ampacities of Table 310.60(C)(71) and Table 310.60(C)(72).
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and if you choose to leave space between the cables then you can use the 71 and 72 tables, rather than multiplier modification in (a) for the 75/76 tables.
To summ it up, you can use tables 75 and 76 for the side-by-side installed MV cables.