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#1 Re: Help Me ! » cable tray and trunking for electricians » 2020-06-17 19:38:55

I worked with cable tray about 40 years ago and remember I created a couple of simple formulae to work out how much triangular section of the cable tray to cut out to do various sets.  I have tried to explain them below.  The first one is when you know the angle you want to create and the second is when you want to make a parallel off-set.   

1. How to calculate size of cut-out section (D) for a pre-determined angle set 

Eg.  You have used your protractor and worked out you need to make a 22° angle in a 600mm cable tray.  By applying the following formula you can quickly find the size of cut out section that you need to cut out of the side of the cable tray, or gutter-type section to make that angle.

(A) = cable tray width (600mm) and B = Size of angle (22°)

First you have to find (C) which is found by dividing 90° by (B) 22° = 4.09 (C).

You can then calculate the size of the cut-out section by dividing the cable tray width (A) 600mm by 4.09 (C).

So the size of the section you need to cut out of a 600mm cable tray to make a 22° set = 600 ÷ 4.09 = 146.7mm

On a smaller scale for something like a gutter eg, if you needed to set a 148mm wide gutter to 36° the same formula applies where

C = 90° ÷ 36 = 2.5  Cut out section (D) = 148mm ÷ 2.5 = 59.2mm

To make a sweeping 90° bend in a 600mm cable tray just make 3 x 30° angles, ie. 3 x 200mmm cut-out sections, or 2 x 45° angles, ie. 2 x 300mm cut-out sections

How to make a parallel off-set in a cable tray

The other common calculation you need is when you have to work out the amount of set required to make a parallel off-set eg when you have to set a cable tray or gutter around a pillar or set to an off-set wall.

In this case, say you have to create an 350mm off-set in a 600mm cable tray to go around a pillar.  In order not to create too sharp a set you want the set to be over a distance of 1500mm (ie. in a straight line, not on the angle).

To calculate the size of the cut-out in the cable tray in this situation you divide the distance between sets by the width of the cable tray ie. 1500 ÷ 600 = 2.5, then divide the amount of off-set by 2.5, ie. 350mm ÷ 2.5 = 140mm.    To get the most accurate results, while the set will be close, it is always best to do the first set then calculate where to do your parallel set cut by positioning the tray.   

I hope this helps.

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