An undersized cable tray does two kinds of damage. Cables piled several layers deep run hotter than their rating assumes, so they either have to be derated or they age early. And a tray that is full on the day of commissioning has no room for the next feeder, which means a second tray, new supports and a shutdown to install them. Sizing the tray properly is a half-hour exercise with the cable schedule; it is worth doing before the steelwork is ordered.
Step 1: build the cable schedule
List every cable that will use the tray, with its overall diameter taken from the manufacturer’s datasheet, not estimated. Group them as power, control and instrumentation, because the three are not treated the same way. Power cables are sized by heat; control and instrumentation cables are sized by space and by the separation they need from power.
Step 2: decide the layout for power cables
Power cables carrying significant load should be laid in a single layer, ideally with a gap of one cable diameter between circuits, so that each cable can shed heat to the air. Where they must touch or be bunched, the grouping factors in the cable derating factors chart apply and the conductor size may need to go up. For single-layer laying, the required width is simply the sum of the diameters plus the sum of the gaps. Ladder tray suits this arrangement because air circulates under the cables.
Step 3: apply a fill ratio to control and instrumentation cables
Small cables can be stacked, and here a fill ratio is used. The usable cross-section of the tray is its inside width multiplied by the side-rail height, and the cables are permitted to occupy a fraction of it. Common Indian practice, following the approach in IEC 60364 and most consultant specifications, limits fill to around 40 to 50 per cent of the cross-section for these cables, with the project specification governing where it states a figure. To compute the fill, take each cable’s diameter, square it, and add the results; that sum approximates the area the round cables occupy including the air between them. Divide by the permitted fill ratio to get the tray cross-section required.
Step 4: add spare capacity
A margin of 20 to 25 per cent over the calculated width is the usual allowance in industrial projects. It is cheap on the day and expensive to add later. Round up to the next standard tray width; perforated and ladder trays are commonly manufactured in widths of 50, 100, 150, 200, 300, 450, 600, 750 and 900 mm, with side-rail heights of 50 to 150 mm.
Step 5: check separation and support
Power and instrumentation cables belong on separate trays, or on either side of a metal divider, to keep interference out of the signal cables; the distance depends on the voltage and the sensitivity of the signal. Support spacing and load rating come from the tray manufacturer’s data to IEC 61537, and the tray must be earthed along its length so the metalwork never becomes a shock path. Finally, keep the tray route within the bending radius of the largest cable on it; a tray bend that looks tidy on a drawing can be tighter than the cable allows.
Worked example
A tray is to carry six LV XLPE armoured power cables of 45 mm diameter, laid single-layer with one-diameter spacing, and twelve control cables of 18 mm diameter. Power: six cables and five gaps of 45 mm gives 495 mm. Control at 40 per cent fill: twelve times 18 squared is 3888 sq mm, divided by 0.4 is 9720 sq mm; on a 50 mm rail height that is 195 mm of width. Total 690 mm; add 25 per cent and the next standard size is 900 mm, or, more practically, two trays: a 600 mm ladder for power and a 300 mm perforated tray for control, which also solves the separation question.