Cable Tray Size as per National Electrical Code-2002. Article 392


(I) No of Multi core Cables less than 2000 volts in the Cable Tray

 (A) 4/0 AWG/Kcmill (120 Sq.mm) Cable or Larger Cables:

  • The ladder cable tray: Tray must have an inside available width equal to or greater than the sum of the diameters of the cables, which installed in a single layer.
  • Solid bottom cable tray: the sum of the cable diameters is not to exceed 90% of the available cable tray width.

 (B) Cables Smaller Than 4/0 AWG/Kcmill (120 Sq.mm)

  •  Ladder Type Cable Tray: The total sum of the cross-sectional areas of all the cables to be installed in the cable tray must be equal to or less than the allowable cable area for the tray width, as per following Table.
  • Solid Bottom Cable Tray: The allowable cable area is reduced by 22%.
Inside width of Cable Tray Allowable Cable Area Sq.inch (Sq.mm)
6 inch (152.5mm) 7 Sq.inch (4516 Sq.mm)
9 inch (228.6mm) 10 Sq.inch (6451 Sq.mm)
12 inch (304.8mm) 14 Sq.inch (9032 Sq.mm)
18 inch (457.2mm) 21 Sq.inch (13548 Sq.mm)
24 inch (609.6mm) 28Sq.inch (18064 Sq.mm)

(C) 4/0 AWG (120 Sq.mm) or Larger Cables Installed with Cables Smaller than 4/0 AWG (120 Sq.mm)

  • Ladder Type Cable Tray: The ladder cable tray needs to be divided into two zones (a barrier or divider is not required but one can be used if desired) so that the No. 4/0 and larger cables have a dedicated zone, as they are to be placed in a single layer.
  • A direct method to determine the correct cable tray width is to figure the cable tray widths required for each of the cable combinations per steps (2) & (3). Then add the widths in order to select the proper cable tray width.

(D) Multi conductor Control and Signal Cables Only

  • Ladder Type Cable Tray: A ladder cable tray containing only control and/or signal cables may have 50% of its total available cable area filled with cable.
  • Solid Bottom Cable Tray: When using solid bottom cable tray, the allowable cable area is reduced from 50% to 40%.

(II) No of Single conductor Cables < 2000 volts in the Cable Tray (NEC-392.12)

  • All single conductor cables to be installed in the cable tray must be larger than 1/0 AWG (53.5 Sq.mm) and not to be installed with Solid Cable Tray.

 (A) 1000 Kcmill (500 Sq.mm) or Larger Cables

  • The sum of the diameters (Sd) for all single conductor cables to be installed shall not exceed the cable tray width as per following Table.
Inside width of Cable Tray Allowable Cable Area Sq.inch (Sq.mm)
6 inch (152.5mm) 6.5Sq.inch (4194 Sq.mm)
9 inch (228.6mm) 9.5 Sq.inch (6129 Sq.mm)
12 inch (304.8mm) 13 Sq.inch (8387 Sq.mm)
18 inch (457.2mm) 19 Sq.inch (12258 Sq.mm)
24 inch (609.6mm) 26Sq.inch (16774 Sq.mm)
30 inch (762mm) 32.5Sq.inch (20968 Sq.mm)
36 inch (914.5mm) 39Sq.inch (25161Sq.mm)

(2) 250 Kcmil (120 Sq.mm) to 1000 Kcmil (500 Sq.mm) Cables

  • The total sum of the cross-sectional areas of all the single conductor cables to be installed in the cable tray must be equal to or less than the allowable cable area for the tray width, as given in following Table
Inside width of Cable Tray Allowable Cable Area Sq.inch (Sq.mm)
6 inch (152.5mm) 6.5Sq.inch (4194 Sq.mm)
9 inch (228.6mm) 9.5 Sq.inch (6129 Sq.mm)
12 inch (304.8mm) 13 Sq.inch (8387 Sq.mm)
18 inch (457.2mm) 19 Sq.inch (12258 Sq.mm)
24 inch (609.6mm) 26Sq.inch (16774 Sq.mm)
30 inch (762mm) 32.5Sq.inch (20968 Sq.mm)
36 inch (914.5mm) 39Sq.inch (25161Sq.mm)

(3) 1000 Kcmil (500 Sq.mm) or Larger Cables Installed with Cables Smaller Than 1000 Kcmil (500 Sq.mm)

  • The total sum of the cross-sectional areas of all the single conductor cables to be installed in the cable tray must be equal to or less than the allowable cable area for the tray width, as given in following Table
Inside width of Cable Tray Allowable Cable Area Sq.inch (Sq.mm)
6 inch (152.5mm) 6.5Sq.inch (4194 Sq.mm)
9 inch (228.6mm) 9.5 Sq.inch (6129 Sq.mm)
12 inch (304.8mm) 13 Sq.inch (8387 Sq.mm)
18 inch (457.2mm) 19 Sq.inch (12258 Sq.mm)
24 inch (609.6mm) 26Sq.inch (16774 Sq.mm)
30 inch (762mm) 32.5Sq.inch (20968 Sq.mm)
36 inch (914.5mm) 39Sq.inch (25161Sq.mm)

(4) Single Conductor Cables 1/0 (50Sq.mm) to 4/0 (120Sq.mm)

  • These single conductors must be installed in a single layer.
  • Note: It is the opinion of some that this practice may cause problems with
  • To avoid these potential problems due to unbalanced voltages, the individual conductors for this type of cable tray wiring system should be bundled with ties. The bundle should contain all of the three phase conductors with the neutral if used.
Single conductor Size Cable Tray width
152mm 228mm 304mm 457mm 609mm
1/0AWG (50Sq.mm) 10 15 20 31
2/0AWG (70Sq.mm) 9 14 19 29
3/0AWG (9550Sq.mm) 8 13 17 26
4/0AWG (120Sq.mm) 8 12 16
250kcmill (120Sq.mm) 11 18 24
350kcmill (185Sq.mm) 9 14 19
500kcmill (240Sq.mm) 7 11 14
750kcmill (400Sq.mm) 5 8 10
1000kcmill (500Sq.mm) 4 6 8

(III) No of Cables more than 2.1 KV in the Cable Tray

  • The sum of the diameters of all cables rated 2001 volts or over, is not to exceed the cable tray width.

(IV) Barrier Requirements (NEC 392.6)

  • Barrier is used to separate cable systems, when Single Core cables of above and below 600 volts installed in the same cable tray. But when Multi Core type cables over 600 volts are installed in the same cable tray with cables rated 600 volts or less, no barriers are required.
  • The barriers should be made of the same material as the cable tray and height must equal to the depth of the cable tray.

Calculate Size of Conduit (Excel)


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  • Calculate Total Area of Multiple Cables.
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  • Calculate % Fill up area of Conduit.
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