Math Is Fun Forum

ninjaman

Member

Registered: 2013-10-15

Posts: 61

cable tray and trunking for electricians

hello,
i am trying to learn how to accurately measure and cut cable tray and trunking to be able to fabricate my own angles.
both of these items come in 3 metre lengths and can be cut with a hacksaw.
i want to be able to measure accurately the starting point, the cuts for the angles and the end points for my length.
i want to be able to make up sweeping 90 degree and less bends and sets.
i dont know how to insert pictures and the img thing that i read doesnt make much sense to me.
i have googled this and looked on youtube but there is not enough information on it.
the cable tray is 3 metres in length, this doesnt matter but i think the width does. it is 150mm across. i know that for a sweeping 90 degree bend there are two cuts and two bends. each bend is a 45 degree angle. but the length of the part in the centre is the concern as i have seen different lengths. i have one formula which is square root of 2 * width squared. so the width is already squared, multiply this by 2 and find the square root.
a set is two 45 degree bends, imagine a bar going horizontal then wanting to go up a step. it would do this at a 45 degree angle, the first going up and the second would level to horizontal and sit on the step. the space underneath the set would be a triangle but not nessysaraleigh right angled. any help would be great thanks
simon

Bob

Administrator

Registered: 2010-06-20

Posts: 10,630

Re: cable tray and trunking for electricians

Hi Simon,

Sorry but I'm not getting this at all yet.  Let's try to get the picture thing working. Years ago you could upload images but we lost that facility when we had to go to a new server.  The forum has minimal funds so had to accept this.  I joined imgur.com (it's free) and I upload pictures there and use the BC code to provide a link in a post.

If you take a look at one of my posts which has a picture you should be able to see the format.  Once I have an idea what you're trying to make I'll have a go at sorting the geometry.

Bob

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Children are not defined by school ...........The Fonz
You cannot teach a man anything;  you can only help him find it within himself..........Galileo Galilei
Sometimes I deliberately make mistakes, just to test you!  …………….Bob

George,Y

Member

Registered: 2006-03-12

Posts: 1,379

Re: cable tray and trunking for electricians

If your problem is to calculate length before cutting on rectangle objects,

knowledge on sines and cosines would help:

https://www.mathsisfun.com/algebra/trigonometry.html

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X'(y-Xβ)=0

ninjaman

Member

Registered: 2013-10-15

Posts: 61

Re: cable tray and trunking for electricians

Hello
i made a video that should help describe my problem. any help would be great thanks
if you know of any sites that show this sort of thing, let me know, that would be great
simon

https://www.youtube.com/watch?v=eQt8b-BLhX8&feature=youtu.be

Bob

Administrator

Registered: 2010-06-20

Posts: 10,630

Re: cable tray and trunking for electricians

hi ninjaman

Thanks for the video link.  That was most helpful.  I've had a go at making a diagram.  I made a strip like you and then folded it.  When folded the top will run from D to E and the bottom G to C to F to H.  The dotted lines show where the strip is folded.  Of course, the strip doesn't look like the diagram (as it is straight) but, hopefully, what I've done is how you want to construct your trunking.

When I started I thought the 182 mm corresponded to CA, but I've had a rethink and I believe CF is the correct amount.  As CDEF is a parallelogram DE = CF.  The fold angle is AEF which will be half of FCB.  Come to think of it, CB isn't right for the horizontal either.  Drop a perpendicular down from F to CB, let it cross CB at B' and CB' = 170mm. 

So the starting point for the calculation is CB' = 170 and FB' = 64.  For a new job you can obviously change those measurements.

What I would do is use a spreadsheet program like Excel by Microsoft to make up the right formulas.  Then you can easily plug in new values when you want. I'll show mine below.

So

and

The length left after the triangular sections are cut out is

You can calculate FF' by

My spreadsheet:

note.  Excel works in radians not degrees so to convert rads to degrees I have done x180/pi and to change back x pi/180

Bob

ps.  What material are you using?  You'll have to allow a small amount for the curvature of the folds (say 2mm maybe).

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Children are not defined by school ...........The Fonz
You cannot teach a man anything;  you can only help him find it within himself..........Galileo Galilei
Sometimes I deliberately make mistakes, just to test you!  …………….Bob

ninjaman

Member

Registered: 2013-10-15

Posts: 61

Re: cable tray and trunking for electricians

Hey Bob,
WOW! that's a lot more math than I thought would be involved. Thank you for the response and the picture as well. I think that I have made a mistake somewhere. the 182mm is the hypotenuse, 170mm adjacent and 64mm opposite. The trunking that I have made will sit on top of the 182mm. So CA should be 182mm. I think in my description I may have asked for where I put the centre of that fold (EA). What I meant was that the point A is where I start my measurements from as it is the only thing I know so far. From there I work back to get the centre line and then 10 degrees out the other way to get the other angle. The point A is the first angle. So F should fold back to A and FH should sit on top of the object I want to get over. I don't know if that will change the math much? I was hoping for something simple like the rise over run thing. Something that I could do on site quickly. Do you know where I could find more of this sort of thing?
Thanks again, you have been a big help
All the best, Simon

Bob

Administrator

Registered: 2010-06-20

Posts: 10,630

Re: cable tray and trunking for electricians

hi Simon,

Glad to help.  Let me know how it goes.

Bob

---

Children are not defined by school ...........The Fonz
You cannot teach a man anything;  you can only help him find it within himself..........Galileo Galilei
Sometimes I deliberately make mistakes, just to test you!  …………….Bob

NoMessCharley

Member

Registered: 2020-06-17

Posts: 1

Re: cable tray and trunking for electricians

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.

Last edited by NoMessCharley (2020-06-17 19:47:42)