Cable tray fill for a plant room: a second tier

A cable tray looks full when you can't fit another cable on it. For the cable ratings it fills up sooner. AS/NZS 3008.1 gives grouping factors for cables in one layer on a tray, and only up to a set number of cables on each tier. This guide lays out one plant room tray with the cable tray fill tool, adds two cables, and carries the factor into the cable size. The ideas behind it are in Cable tray fill: one layer, tiers and grouping.

The plant room#

• A 300 mm perforated cable tray along the plant room wall.
• PVC insulated three core and earth cables: three 50 mm² chiller feeders, three 35 mm² air handling unit feeders and two 16 mm² pump circuits.
• The cables touch, side by side. A quarter of the width is kept free for later cables.
• New Zealand, so AS/NZS 3008.1.2.

Step 1. The tray as drawn#

The tool sets the cables side by side and adds their overall diameters. The eight cables take 197.2 mm. With 25 % kept free, a tier may take 225 mm, so they fit on one tier. They fill 65.7 % of the tray width, and weigh 11.41 kg/m.

Table 24 prints columns for 1, 2, 3, 4, 6 and 9 multicore cables. Eight cables fall between two columns, so the tool reads the 9 column, the lower factor. On one tier of perforated tray, touching, that is 0.73. Open the tray as drawn.

Step 2. Two more pumps#

The mechanical services add two more 16 mm² pump circuits. The run is now ten cables and 236.6 mm wide. That is past the 225 mm a tier may take, so the run needs 2 tiers.

The tool places the cables in the order you list them, each on the first tier with room. The chillers, the air handling units and three pumps fill the first tier. The last pump goes on the second tier.

The factor is read at the tiers used and the fullest tier: 2 tiers and 9 cables. On a perforated tray, touching, that is 0.68. The fill of the fullest tier is 72.3 %. Open the tray with two more pumps.

Stack the second tier at least 300 mm above the first. The factors of Table 24 hold only at that gap (Note 6).

Why not a wider tray#

A 450 mm tray takes all ten cables on one tier. But Table 24 prints no factor for more than nine touching multicore cables on a tier, so the tool gives none. Width alone does not fix it. Open the run on a 450 mm tray.

Step 3. Carry the factor into the cable size#

Each chiller has a design current of 115 A on a 40 m run. The cable is on a perforated tray, so in the cable size calculator pick Unenclosed spaced, the perforated tray as the support, 2 tiers and the cables on the fullest tier.

At 0.68 a 50 mm² cable carries 112.88 A, less than 115 A. The correction factors tool shows the shortfall against its 166 A tabulated rating. The cable size calculator moves the chillers up to 70 mm², which carries 143.48 A. Open the chiller cable at 0.68.

A 70 mm² cable is wider, so lay the tray out again. The three chillers, the air handling units and two pumps now fill the first tier, eight cables. Two pumps go on the second tier. It is still 2 tiers at 0.68, and the first tier now weighs 13.69 kg/m. Open the tray with 70 mm² chillers.

Step 4. Or share the tiers evenly#

The factor follows the fullest tier. Keep half of each tier free and the same ten cables split five and five. Five cables read the 6 column, and 2 tiers at 6 cables give 0.73. The fullest tier fills 44.4 % and weighs 8.46 kg/m. Open the tray with half of each tier free.

At 0.73 a 50 mm² chiller cable carries 121.18 A, enough for 115 A. Open the chiller cable at 0.73. The second tier goes in either way. Laying five cables on each tier costs nothing more, and keeps the three 40 m chiller feeders at 50 mm² instead of 70 mm². The tool fills the first tier first, so keeping half of each tier free is how to show that split in it.

The result#

The mass is the cable alone. Check the tray against its maker's load and span tables, with the fixings and the width you keep free. For cables in a conduit instead of on a tray, use the conduit fill calculator.